**1. Introduction**

Designing electro-mechanical systems in the aerospace industry is a challenging task for many reasons. First, the programs may last decade, so when the design phase starts the design team must envisage how the product will be sustained and maintained in 20 or 30 years on. Second: reliability is a *must* in this sector. They cannot be taken for granted or worse of all avoided by the design team. Possibly, this is the most important feature a design team should address. All these features are typically summarized in what is defined as quality management system (QMS) that are the company's processes that overlook the design and production phases trying to guarantee the respect of such important requests. All this does not come for free,

#### *Design and Manufacturing*

on the contrary. Guaranteeing and satisfying all these aspects leads to high costs of the engineering and production phase. Nevertheless, engineering and design teams are constantly pressed by the executive board to deliver cost-effective solutions, in time and in-spec.

In this context, the role of industrial engineering teams inside an aerospace company can play a decisive role in delivering the targeted requirements (time-cost-quality).

In order to do so, the industrial engineering team needs to be part of the design team from the beginning, even during offer proposition if needed. Moreover, its requirements, suggestions and strategies must not be seen as secondary or expendable to meet selected electrical or technical specification. On the contrary, if a particular feature needs to be sacrificed during design phase, this should be a technical performance that is not directly requested by the customer or end-user.

During the design flow, industrial engineering can be engaged in two possible ways:


In essence, design for manufacturing (DFM) is a development & design issue, not a manufacturing topic. "D" stands for design and therefore "DFM" is a design challenge".

The following sections contain indication on how the industrial engineering team can be effective during the design phase (i.e. implementing best practices for DFM) and in the subsequent production phase in order to proactively sustain and improve the manufacturing processes.

The following terms are often referred to in the rest of the chapter:


**5**

**Figure 1.**

*Design for Manufacturing of Electro-Mechanical Assemblies in the Aerospace Industry*

**2. Design for manufacturing production technologies and best practices**

The design team should treat manufacturing requests and constraints as a requirement in the same way it tackles the technical requirements posed upon the item under development. Therefore, manufacturing aspects require a design

DFM strategies can be summarized as best practices or design rule. In general,

An important feature of designing and producing parts in the aerospace industry is that large quantities of the same part to be produced are seldom encountered, as occurs in the consumer market or semiconductor industry. Apart for very specific components, for example, transmit/receive modules inside a phased array, most other parts that compose an electro-mechanical system are usually produced in a

Trade studies are very important in the aerospace industry. They should be carried out at the beginning of the design phase to identify the most viable solution. It is important to emphasize that the Producibility requirements have the same dignity as the electromechanical requirements expressed technical specifications and the team's objective must be to respect ALL requirements, or identify the most balanced

There are multiple ways to implement a project that fulfills the given requirements and conditions. *N* alternatives can be identified (a minimum of three is suggested). The decision on which design solution is" best" can be taken using a radar chart type diagram, as depicted in **Figure 1**, which shows the specific requirements and the constraints to be considered (industrial, growth capability, business opportunities and so on). It is useful to subdivide the requirements and constraints

A typical case study is here provided with the aid of **Figure 1**. The goal of the team is to design a microwave electromechanical assembly fulfilling some electromechanical requirements listed in technical specification. Moreover, the part shell be produced within a maximum cost figure (expense of components and labor) and

rules can be strict and often are associated with the concept of violation and penalty. An alternative way of implementing the process can be obtained by giving guidelines. The latter are less strict and provide a design philosophy rather than

*DOI: http://dx.doi.org/10.5772/intechopen.90098*

strategy and a verification method.

scale of a few parts per month or even less.

solution among a set of proposed viable solutions.

into NEED-TO-HAVE and GOOD-TO-HAVE categories.

the design cycle shall be less than 12 months long.

*Radar chart helps understanding design trade options.*

giving strict indications.

*Design for Manufacturing of Electro-Mechanical Assemblies in the Aerospace Industry DOI: http://dx.doi.org/10.5772/intechopen.90098*

### **2. Design for manufacturing production technologies and best practices**

The design team should treat manufacturing requests and constraints as a requirement in the same way it tackles the technical requirements posed upon the item under development. Therefore, manufacturing aspects require a design strategy and a verification method.

DFM strategies can be summarized as best practices or design rule. In general, rules can be strict and often are associated with the concept of violation and penalty. An alternative way of implementing the process can be obtained by giving guidelines. The latter are less strict and provide a design philosophy rather than giving strict indications.

An important feature of designing and producing parts in the aerospace industry is that large quantities of the same part to be produced are seldom encountered, as occurs in the consumer market or semiconductor industry. Apart for very specific components, for example, transmit/receive modules inside a phased array, most other parts that compose an electro-mechanical system are usually produced in a scale of a few parts per month or even less.

Trade studies are very important in the aerospace industry. They should be carried out at the beginning of the design phase to identify the most viable solution. It is important to emphasize that the Producibility requirements have the same dignity as the electromechanical requirements expressed technical specifications and the team's objective must be to respect ALL requirements, or identify the most balanced solution among a set of proposed viable solutions.

There are multiple ways to implement a project that fulfills the given requirements and conditions. *N* alternatives can be identified (a minimum of three is suggested). The decision on which design solution is" best" can be taken using a radar chart type diagram, as depicted in **Figure 1**, which shows the specific requirements and the constraints to be considered (industrial, growth capability, business opportunities and so on). It is useful to subdivide the requirements and constraints into NEED-TO-HAVE and GOOD-TO-HAVE categories.

A typical case study is here provided with the aid of **Figure 1**. The goal of the team is to design a microwave electromechanical assembly fulfilling some electromechanical requirements listed in technical specification. Moreover, the part shell be produced within a maximum cost figure (expense of components and labor) and the design cycle shall be less than 12 months long.

**Figure 1.** *Radar chart helps understanding design trade options.*

*Design and Manufacturing*

time and in-spec.

(time-cost-quality).

the customer or end-user.

design phase.

challenge".

"purely" engineering team.

improve the manufacturing processes.

the part during the entire production time

ways:

on the contrary. Guaranteeing and satisfying all these aspects leads to high costs of the engineering and production phase. Nevertheless, engineering and design teams are constantly pressed by the executive board to deliver cost-effective solutions, in

In this context, the role of industrial engineering teams inside an aerospace company can play a decisive role in delivering the targeted requirements

In order to do so, the industrial engineering team needs to be part of the design team from the beginning, even during offer proposition if needed. Moreover, its requirements, suggestions and strategies must not be seen as secondary or expendable to meet selected electrical or technical specification. On the contrary, if a particular feature needs to be sacrificed during design phase, this should be a technical performance that is not directly requested by

During the design flow, industrial engineering can be engaged in two possible

1.In the final design stages to verify that the part designed by the electrical or electronic engineering team fulfills several conditions regarding physical dimensions, materials employed, interconnects, and so on. In practice, the role of the industrial engineering team is to give a "go ahead" or "modify" decision based on the outcome of a specific checklist compilation and knowhow of the manufacturing process. In this context the industrial engineering members act as review body rather than participant of the design team. This approach often leads to difficulties when the production of the part rampsup since some aspects related to manufacturing were overlooked during the

2.Early on the design stage to recommend manufacturing related views, propose suggestions and identify solutions that would have been probably rejected by a

In essence, design for manufacturing (DFM) is a development & design issue, not a manufacturing topic. "D" stands for design and therefore "DFM" is a design

The following sections contain indication on how the industrial engineering team can be effective during the design phase (i.e. implementing best practices for DFM) and in the subsequent production phase in order to proactively sustain and

• Industrial engineering: a team of people, or a better a division of the company, which is constantly involved in both engineering and manufacturing activities. Its essence is to act as the *trait-d'union* between development engineering and production & operations areas. The team is responsible for representing production requirements and needs in the design team, designing the manufacturing work-flow of the part (work-cell and work-cycle design) and sustaining

• Producible/Producibility: the attribute of a part that can be manufactured in a given time and cost constraint thorough industrial repeatable processes featur-

ing a level of quality, for example, compliant with ISO9100 standards.

The following terms are often referred to in the rest of the chapter:

**4**

Electrical requirements such as gain, noise, signal linearity and DC power consumption can be summarized in REQ \_1. Thermo-mechanical requirements, such as maximum temperature of operation and the capability of withstanding certain shocks and accelerations, can be associated to REQ \_2. Reliability specifications are considered in REQ \_3. The term *space de-rating* is often referred to in aerospace industry to recall the concept that an electrical component can be operated in a suboptimum electrical condition that reduces the probability of component failure. Finally, cost related aspects are accounted for in REQ \_4. Automated assembly greatly influences overall labor cost and shall be also considered when computing REQ \_4. On the other hand, automation is a feasible solution when a large quantity of components is to be produced. This is due to the non-recurring expenses associated with the development of automated programs for the specific part. Design time can be considered in CONST\_2 while the use of certain component and material to satisfy safety or export prescriptions are considered in CONST\_1. An example of safety prescription is REACH requirement applicable in the EU to improve the protection of human health and the environment through the better and earlier identification of the intrinsic properties of chemical substance. The U.S.A. applies a limitation on the export of components only to specific and approved end-user countries (ITAR, ECCN or EAR99).

The yellow line, in **Figure 1**, appears to be a solution featuring high technical merit but requiring the use of some component that is not compliant with safety constraints or export limitations. This is quantified by the low value expressed in CONST\_1. On the contrary, the blue line represents a solution that complies with time and material/component prescription but features low technical merit. The green and burgundy curve represent solutions that suitably trade-off between all requirements and constraints. Some requirements may be in contrast against each other. For example, higher electrical performance may be obtained at the expense of poorer reliability or vice versa. Similarly, demanding thermomechanical requirements can be fulfilled if accepting the higher costs of using advanced materials and extra labor time. Moreover, even within the same set of requirements, for example electrical performance expressed as REQ \_1 there might be some conflict. Higher gain and linearity is obtained at the expense of greater power consumption.

Typically, the identified solution will cover most of the requirements leaving unsatisfied only a minimal part. Therefore, the best solution is the one having the largest area in conjunction with no points close to the origin of the radar chart, consequently the burgundy curve in **Figure 1**.

The project manager must work to manage the lifetime risk of the product/ program linked to the failure to meet these requirements. In the event of conflict, a trade-off must be made between the electromechanical requirements and those of producibility, privileging the latter especially for series production (items with multiplicity ≥5 for one system).

Finally, design guidelines are particularly useful in contexts where most of the assembly is performed manually, whereas rules apply where the process is highly automated and product performance is obtained by-design rather by manufacturing tuning.

#### **2.1 Production technologies and processes**

Production of electrical assemblies operating at high frequency requires a set of manufacturing technologies that ranges from packaging to adhesion up to interconnects. The topic is very broad and some aspects are covered in [1]. What is important for this chapter is that several of these processes are manual. While,

**7**

**Table 1.**

*Attributes of several alloys for brazing.*

**Alloy Family**

*Design for Manufacturing of Electro-Mechanical Assemblies in the Aerospace Industry*

effects, and therefore must be taken into account during design phase.

on one side, manual assembly can help obtain desired product performance on the other it increases tuning time since the "starting point" can be quite far apart due to the larger variability of manual processes. Moreover, at microwave frequencies, interconnects and adhesives influence electrical performance due to the parasitic

A best practice that greatly aids design for manufacturing topics is the manufacturing organization meeting with design engineers to discuss the latest developments in manufacturing technology. Moreover, the Industrial engineering team should periodically provide a report containing investments and improvements foreseen in manufacturing over the following 2–3 years. In this way, the company and the engineering team are well aware of advances in manufacturing and can

**Packaging**: Kovar is used to match the expansion of alumina or similar ceramics, and is typically used as a carrier for microwave integrated circuit substrates of these materials. If it forms part of the ground plane it is usually plated, or it may be plated to allow soldering or brazing to the ceramic. Kovar is used for small carriers since its density is higher than Aluminum and therefore not advised for large packaging where the overall weight can become too large. On the contrary, Aluminum, thanks

**Adhesion**: plays in important role in microelectronics since it provides simultaneously electrical grounding and mechanical bonding. Adhesion at integrated circuit (IC) level can be performed thorough epoxy attach (gluing) or eutectic die

Eutectic die attach (brazing) is a highly controlled die attach process for high reliability, high accuracy, and high performance devices. To achieve high yield, sophisticated heating and cooling mechanisms are employed. This means controlling that the device heats and cools according to a very strict parameter line. The essence of a eutectic reaction is going from liquid to solid, using eutectic heating and cooling. Eutectic alloys for soldering are composed of Sn (tin), Pb (lead), Ag (silver) and Au (gold). When different metals are combined into alloys, a range of melting temperatures are created with varying proportions of each metal used: AuSi@363°C, AuSn@280°C. The advantage is a very high conductive (thermal and electrical) adhesion obtained at the expense of a manual and very complicate processes (a few seconds or degrees difference in the brazing oven could mean success or failure of the process). **Table 1** reports key attributes of alloys for brazing

**Features Composition Melt** 

**temp. [°C]**

Sn63Pb36.7Sb0.3 183 Sn60Pb39.7Sb0.3 183–188 Sn62Pb36Ag2 179

In50Pb50 180–209

Au80Sn20 280

*DOI: http://dx.doi.org/10.5772/intechopen.90098*

profitably orient design choices in the future.

to its smaller density is used for the overall packaging.

attach (brazing). Let us analyze pros and cons of each method.

SnPb Typically used in surface mount assembly. High bond

AuSn Strong bond strength. Excellent thermal and electrical

reliability.

conductivity.

In Elastic interconnect In100 156.7

*2.1.1 Packaging, adhesions and interconnects*

#### *Design for Manufacturing of Electro-Mechanical Assemblies in the Aerospace Industry DOI: http://dx.doi.org/10.5772/intechopen.90098*

on one side, manual assembly can help obtain desired product performance on the other it increases tuning time since the "starting point" can be quite far apart due to the larger variability of manual processes. Moreover, at microwave frequencies, interconnects and adhesives influence electrical performance due to the parasitic effects, and therefore must be taken into account during design phase.

A best practice that greatly aids design for manufacturing topics is the manufacturing organization meeting with design engineers to discuss the latest developments in manufacturing technology. Moreover, the Industrial engineering team should periodically provide a report containing investments and improvements foreseen in manufacturing over the following 2–3 years. In this way, the company and the engineering team are well aware of advances in manufacturing and can profitably orient design choices in the future.

#### *2.1.1 Packaging, adhesions and interconnects*

*Design and Manufacturing*

countries (ITAR, ECCN or EAR99).

power consumption.

consequently the burgundy curve in **Figure 1**.

**2.1 Production technologies and processes**

multiplicity ≥5 for one system).

Electrical requirements such as gain, noise, signal linearity and DC power consumption can be summarized in REQ \_1. Thermo-mechanical requirements, such as maximum temperature of operation and the capability of withstanding certain shocks and accelerations, can be associated to REQ \_2. Reliability specifications are considered in REQ \_3. The term *space de-rating* is often referred to in aerospace industry to recall the concept that an electrical component can be operated in a suboptimum electrical condition that reduces the probability of component failure. Finally, cost related aspects are accounted for in REQ \_4. Automated assembly greatly influences overall labor cost and shall be also considered when computing REQ \_4. On the other hand, automation is a feasible solution when a large quantity of components is to be produced. This is due to the non-recurring expenses associated with the development of automated programs for the specific part. Design time can be considered in CONST\_2 while the use of certain component and material to satisfy safety or export prescriptions are considered in CONST\_1. An example of safety prescription is REACH requirement applicable in the EU to improve the protection of human health and the environment through the better and earlier identification of the intrinsic properties of chemical substance. The U.S.A. applies a limitation on the export of components only to specific and approved end-user

The yellow line, in **Figure 1**, appears to be a solution featuring high technical merit but requiring the use of some component that is not compliant with safety constraints or export limitations. This is quantified by the low value expressed in CONST\_1. On the contrary, the blue line represents a solution that complies with time and material/component prescription but features low technical merit. The green and burgundy curve represent solutions that suitably trade-off between all requirements and constraints. Some requirements may be in contrast against each other. For example, higher electrical performance may be obtained at the expense of poorer reliability or vice versa. Similarly, demanding thermomechanical requirements can be fulfilled if accepting the higher costs of using advanced materials and extra labor time. Moreover, even within the same set of requirements, for example electrical performance expressed as REQ \_1 there might be some conflict. Higher gain and linearity is obtained at the expense of greater

Typically, the identified solution will cover most of the requirements leaving unsatisfied only a minimal part. Therefore, the best solution is the one having the largest area in conjunction with no points close to the origin of the radar chart,

The project manager must work to manage the lifetime risk of the product/ program linked to the failure to meet these requirements. In the event of conflict, a trade-off must be made between the electromechanical requirements and those of producibility, privileging the latter especially for series production (items with

Finally, design guidelines are particularly useful in contexts where most of the assembly is performed manually, whereas rules apply where the process is highly automated and product performance is obtained by-design rather by manufactur-

Production of electrical assemblies operating at high frequency requires a set of manufacturing technologies that ranges from packaging to adhesion up to interconnects. The topic is very broad and some aspects are covered in [1]. What is important for this chapter is that several of these processes are manual. While,

**6**

ing tuning.

**Packaging**: Kovar is used to match the expansion of alumina or similar ceramics, and is typically used as a carrier for microwave integrated circuit substrates of these materials. If it forms part of the ground plane it is usually plated, or it may be plated to allow soldering or brazing to the ceramic. Kovar is used for small carriers since its density is higher than Aluminum and therefore not advised for large packaging where the overall weight can become too large. On the contrary, Aluminum, thanks to its smaller density is used for the overall packaging.

**Adhesion**: plays in important role in microelectronics since it provides simultaneously electrical grounding and mechanical bonding. Adhesion at integrated circuit (IC) level can be performed thorough epoxy attach (gluing) or eutectic die attach (brazing). Let us analyze pros and cons of each method.

Eutectic die attach (brazing) is a highly controlled die attach process for high reliability, high accuracy, and high performance devices. To achieve high yield, sophisticated heating and cooling mechanisms are employed. This means controlling that the device heats and cools according to a very strict parameter line. The essence of a eutectic reaction is going from liquid to solid, using eutectic heating and cooling. Eutectic alloys for soldering are composed of Sn (tin), Pb (lead), Ag (silver) and Au (gold). When different metals are combined into alloys, a range of melting temperatures are created with varying proportions of each metal used: AuSi@363°C, AuSn@280°C. The advantage is a very high conductive (thermal and electrical) adhesion obtained at the expense of a manual and very complicate processes (a few seconds or degrees difference in the brazing oven could mean success or failure of the process). **Table 1** reports key attributes of alloys for brazing


#### **Table 1.** *Attributes of several alloys for brazing.*

microelectronic parts. Important parameters to drive the choice in microelectronic components are the electro & thermal conductivity (to determine in-package device electro-thermal performance) and melt temperature (that implies manufacturing complexity). Gold-Tin alloys (Au/Sn) are typically employed in assembly of microwave devices while Tin-Lead (Sn/Pb) is preferred for the production of digital boards.

Tin/Lead (Sn/Pb) based alloys are the most commonly used alloys for welding on copper, nickel or silver surfaces. The addition (optional) of a small percentage of antimony prevents the transformation of the tin (beta) phase into a tin (alpha) phase called "tin plague", with a reduction in the volume of the alloy mass and a drastic decrease in the mechanical strength of the welded joint. Silver is added to allow soldering on silver surfaces without causing the alloy to over-dissolve the plating metal. All tin-based alloys are strongly discouraged for welding gold surfaces, due to the rapid dissolution of gold in the alloy (scavenging).

Indium-based alloys are particularly useful due to their great ductility, which attenuates or eliminates failure problems resulting from fatigue failure of welded joints, and by the lower solubility of gold in such alloys. About 1% of gold must dissolve in an indium/lead based alloy before the AuIn2 solid phase can be formed, which is stable in equilibrium with lead up to 319°C and acts as a barrier, limiting the further dissolution of gold: a thin film of gold can withstand for 15 minutes in an In50Pb50 alloy bath.

Gold/Tin (Au/Sn) alloy is specifically used to weld gold surfaces without having to use flux, due to the high gold content it contains. It is normally sufficient to use a nitrogen-based inert atmosphere during the process. This alloy is able to dissolve gold in considerable proportions (up to 1–2 microns in thickness) during a normal welding cycle lasting a few minutes, which requires that the surfaces to be gilded have a thicker plating, i.e. at least 3–4 μm.

Epoxy attach (gluing), on the other hand, is a far more easier manufacturing process than brazing. It can be very often automated and the time constraints/ temperature constraints of the process are much less critical than brazing. Usually the devices is cured for 30 minutes inside a curing oven at 120°C. Nowadays, silverloaded epoxy adhesive with high thermal and electrical conductivity are available whose electrical and thermal performance are not far from the ones obtainable with chip brazing.

**Interconnects**: are the electrical connections between semiconductor devices such as integrated circuits or transistors and the first level of packaging. The most familiar and widely used First-Level Interconnect (FLI) is the wire bond. Wire bonds are available in several types, such as ball bonds, wedge bonds, and ribbon bonds, each with unique variations. The typical wire bond for high-end applications is a wire bond is formed using a gold wire that is typically 25 μm diameter, though high-volume commercial systems at lower frequencies use aluminum wire bonds with diameters as large as 54 μm.

The purpose of the wire bond is to create an electrical connection between an IC and some type of conductor, typically a metal trace. At lower frequencies the wire bond performs as a simple electrical contact between points and is specified at a maximum current handling. However, as frequency increases, wire bonds begin to perform as inductors. The requirements on the wire bond increase as frequency is increased. Typically, the length of the wire is limited to reduce inductance. Also, the shape of the wire bond is specified and in some cases manual accomplishment becomes unavoidable. **Figure 2** depicts the equivalent electric circuit and the corresponding parasitic reactance and resistance as a function of frequency of a 1 mm/25 μm diameter wire bond. As frequency increases, the parasitic effects

**9**

**Figure 3.**

**Figure 2.**

*Design for Manufacturing of Electro-Mechanical Assemblies in the Aerospace Industry*

*Bond wire simplified geometry and equivalent circuit (left) and impedance vs. frequency (right).*

become large and can be compensated only by decreasing wire length, and some-

is demonstrated in **Figure 3**. A *simple* RF chain, composed by a cascade of two amplifying stages, is considered. The two amplifiers, in Monolithic Microwave Integrated Circuit (MMIC) technology, are connected to rest of the circuit trough a pair of wires at the I/O ports respectively. The length of each wire is controlled by a

The effect of wire length, and therefore inductance, on a high frequency circuit

The gain is rather flat for LEN = 300 μm (highest curve, marker P1), while it becomes quite rippled and gain drops for LEN = 800 μm (lowest curve, marker P6). Consequently, length of bond wires should be carefully controlled. Occasionally

Wire bonds can be connected using ultrasonic bonding, thermos-compression bonding, and thermosonic bonding [2]. Ultrasonic bonding uses pressure and ultrasonic vibrations from a bonding tool to create the bond between the wire and the metal surface. Thermo-compression uses pressure from the bonding tool and high temperature to create the bond. Thermosonic bonding combines ultrasonic and

operator ability is essential to obtain the desired electrical performance.

times operator skill becomes mandatory.

*Bond wire length effect on a RF chain around 30 GHz.*

variable "LEN" and is swept from 300 to 800 μm.

thermos-compression methods to create the bonds.

*DOI: http://dx.doi.org/10.5772/intechopen.90098*

*Design for Manufacturing of Electro-Mechanical Assemblies in the Aerospace Industry DOI: http://dx.doi.org/10.5772/intechopen.90098*

#### **Figure 2.**

*Design and Manufacturing*

boards.

(scavenging).

chip brazing.

an In50Pb50 alloy bath.

have a thicker plating, i.e. at least 3–4 μm.

with diameters as large as 54 μm.

microelectronic parts. Important parameters to drive the choice in microelectronic components are the electro & thermal conductivity (to determine in-package device electro-thermal performance) and melt temperature (that implies manufacturing complexity). Gold-Tin alloys (Au/Sn) are typically employed in assembly of microwave devices while Tin-Lead (Sn/Pb) is preferred for the production of digital

Tin/Lead (Sn/Pb) based alloys are the most commonly used alloys for welding on copper, nickel or silver surfaces. The addition (optional) of a small percentage of antimony prevents the transformation of the tin (beta) phase into a tin (alpha) phase called "tin plague", with a reduction in the volume of the alloy mass and a drastic decrease in the mechanical strength of the welded joint. Silver is added to allow soldering on silver surfaces without causing the alloy to over-dissolve the plating metal. All tin-based alloys are strongly discouraged for welding gold surfaces, due to the rapid dissolution of gold in the alloy

Indium-based alloys are particularly useful due to their great ductility, which attenuates or eliminates failure problems resulting from fatigue failure of welded joints, and by the lower solubility of gold in such alloys. About 1% of gold must dissolve in an indium/lead based alloy before the AuIn2 solid phase can be formed, which is stable in equilibrium with lead up to 319°C and acts as a barrier, limiting the further dissolution of gold: a thin film of gold can withstand for 15 minutes in

Gold/Tin (Au/Sn) alloy is specifically used to weld gold surfaces without having to use flux, due to the high gold content it contains. It is normally sufficient to use a nitrogen-based inert atmosphere during the process. This alloy is able to dissolve gold in considerable proportions (up to 1–2 microns in thickness) during a normal welding cycle lasting a few minutes, which requires that the surfaces to be gilded

Epoxy attach (gluing), on the other hand, is a far more easier manufacturing process than brazing. It can be very often automated and the time constraints/ temperature constraints of the process are much less critical than brazing. Usually the devices is cured for 30 minutes inside a curing oven at 120°C. Nowadays, silverloaded epoxy adhesive with high thermal and electrical conductivity are available whose electrical and thermal performance are not far from the ones obtainable with

**Interconnects**: are the electrical connections between semiconductor devices such as integrated circuits or transistors and the first level of packaging. The most familiar and widely used First-Level Interconnect (FLI) is the wire bond. Wire bonds are available in several types, such as ball bonds, wedge bonds, and ribbon bonds, each with unique variations. The typical wire bond for high-end applications is a wire bond is formed using a gold wire that is typically 25 μm diameter, though high-volume commercial systems at lower frequencies use aluminum wire bonds

The purpose of the wire bond is to create an electrical connection between an IC and some type of conductor, typically a metal trace. At lower frequencies the wire bond performs as a simple electrical contact between points and is specified at a maximum current handling. However, as frequency increases, wire bonds begin to perform as inductors. The requirements on the wire bond increase as frequency is increased. Typically, the length of the wire is limited to reduce inductance. Also, the shape of the wire bond is specified and in some cases manual accomplishment becomes unavoidable. **Figure 2** depicts the equivalent electric circuit and the corresponding parasitic reactance and resistance as a function of frequency of a 1 mm/25 μm diameter wire bond. As frequency increases, the parasitic effects

**8**

*Bond wire simplified geometry and equivalent circuit (left) and impedance vs. frequency (right).*

**Figure 3.**

*Bond wire length effect on a RF chain around 30 GHz.*

become large and can be compensated only by decreasing wire length, and sometimes operator skill becomes mandatory.

The effect of wire length, and therefore inductance, on a high frequency circuit is demonstrated in **Figure 3**. A *simple* RF chain, composed by a cascade of two amplifying stages, is considered. The two amplifiers, in Monolithic Microwave Integrated Circuit (MMIC) technology, are connected to rest of the circuit trough a pair of wires at the I/O ports respectively. The length of each wire is controlled by a variable "LEN" and is swept from 300 to 800 μm.

The gain is rather flat for LEN = 300 μm (highest curve, marker P1), while it becomes quite rippled and gain drops for LEN = 800 μm (lowest curve, marker P6). Consequently, length of bond wires should be carefully controlled. Occasionally operator ability is essential to obtain the desired electrical performance.

Wire bonds can be connected using ultrasonic bonding, thermos-compression bonding, and thermosonic bonding [2]. Ultrasonic bonding uses pressure and ultrasonic vibrations from a bonding tool to create the bond between the wire and the metal surface. Thermo-compression uses pressure from the bonding tool and high temperature to create the bond. Thermosonic bonding combines ultrasonic and thermos-compression methods to create the bonds.

#### *2.1.2 Automatic vs. manual manufacturing*

The choice of manual or automatic assembly is driven by a some parameters. First is the electrical and thermal requirements. In some cases, the requirements could be so stringent that only a manual process is capable of performing a very fine-tuning. For example, when temperature and heat dissipation are critical, then brazing can become the only acceptable solution. The effect of interconnect parasitic were also discussed, in the previous Section 2.1.1, and how operator support can become decisive to obtain acceptable performance, especially at GHz frequencies.

Another parameter to be accounted for is the number of parts to be produced in 1 week, 1 month or 1 year. This number plays a crucial role. If a mass production is foreseen, then manual assembly is not advised due to the lengthy and costly process associated with it. On the contrary, when very few parts are to be produced then manual process is acceptable, also because automatic assembly requires the development of programs and codes with the consequent Non Recurring Expenses (NRE) for developing them.

#### *2.1.3 Additive manufacturing in the aerospace sector*

The paradigm of design for manufacturing can be found in Additive Manufacturing (AM) technology. AM represents a key example where an advancement in production technologies enables new engineering concepts that can come to life *only* with this technology. In this sense, it is quintessentially a design *enabled by* manufacturing.

In the aerospace sector, AM is applied mostly on metallic parts (Aluminum, Steel, Titanium and related alloys) rather than composites (plastics) as occurs in the consumer industry. In fact, the initial investment in terms of machinery and training is very high and must be carefully accounted for in the business model.

AM in aerospace has been happening for some time now with many applications, covering everything from the creation of aircraft or helicopter parts, making lighter and more efficient engines, 3D printed turbines etc. 3D technologies generally save on time, money and create stronger, lighter, and more efficient finished products [3].

An example of AM technology and process applied to the aerospace industry is shown in **Figure 4**.

The part itself is not very complex, but is proves how AM can be gainfully exploited to create lighter or more complex structures than the ones previously realized with "prior" technologies.

One of the challenges of the market is the restriction of the volume of construction and the size of the product. An aircraft is made up of very large components and additive manufacturing is today limited to the volume offered by the 3D printer. Most technologies offer solutions with limited print volume, making 3D printing applicable only to small components. So, this constraint that could slow down the growth of the market. Even if so, today's 3D technologies have already made it possible to create and qualify fairly large (approx. 30 cm) components for space [3, 4] and aviation [5]. Finally, the latest available machines (SLM500, Concept Laser Xline 20000R, EOS M 400) are capable of building even larger pieces.

#### **2.2 Design rules and design guidelines**

Design rules can be seen as a set of physical, geometrical, chemical, mechanical limitations. They are very useful when the manufacturing process is constant and

**11**

*Design for Manufacturing of Electro-Mechanical Assemblies in the Aerospace Industry*

repetitive as happens in the semiconductor industries or in large scale production. This paradigm however is less stringent in the aerospace industry since there is not a mass production of items, but on the contrary, a production of a large quantity of different parts each one characterized by very small multiplicity. Moreover, while digital board assemblies can follow rules developed for the consumer market, high frequency microwave assemblies (operating at 100 MHz–30 GHz) are typical of the aerospace industry and suffer from less standardization. Consequently, for the

Anyhow, rules and guidelines should address the following features that are

1.Designing parts for "modularity", i.e. a module is a self-contained component that is equipped with standard interfaces that allow it to be integrated into a larger system. Modularity has several benefits: the product is easy to assemble/ re-assemble and most of all, in complex systems, it aids to detect quality prob-

2.Designing parts to compensate for process statistics and yield, component and

3.Ensure the product can be assembled and manufactured using standards processes, i.e. identifiable and written in a production document or drawing without requiring ultra-specialized capabilities or different production approaches

Design rules are written to suit a specific production technology. In the electronics for aerospace industry important production technologies are microwave

Digital board production uses rules similar to the ones developed for consumer and telecom products, always taking into account that aerospace industry produces a relatively small amount of high-performance products as opposed to consumer market. Anyhow, well known standards can be applied, for example the IPC-2291 "Design Guideline for Printed Electronics" or IPC-2252 "Design Guide for RF/ Microwave Circuit Boards" considering class 3 for the aerospace industry.

On the other hand, production of complex microwave parts is very typical to the aerospace & defense sector and seldom finds application elsewhere. This is related to the high cost involved in development and production. Design rules for these objects often end up as a few set of geometrical rules. An example of design rules

*DOI: http://dx.doi.org/10.5772/intechopen.90098*

latter guidelines rather than rules should be applied.

critical in any industrial manufacturing process:

lems or non-conformities

*Metallic part optimization thanks to AM.*

for each realized component.

material deviations

modules and digital boards.

*2.2.1 Design rules*

**Figure 4.**

*Design for Manufacturing of Electro-Mechanical Assemblies in the Aerospace Industry DOI: http://dx.doi.org/10.5772/intechopen.90098*

**Figure 4.**

*Design and Manufacturing*

frequencies.

for developing them.

manufacturing.

products [3].

shown in **Figure 4**.

realized with "prior" technologies.

**2.2 Design rules and design guidelines**

*2.1.2 Automatic vs. manual manufacturing*

*2.1.3 Additive manufacturing in the aerospace sector*

The choice of manual or automatic assembly is driven by a some parameters. First is the electrical and thermal requirements. In some cases, the requirements could be so stringent that only a manual process is capable of performing a very fine-tuning. For example, when temperature and heat dissipation are critical, then brazing can become the only acceptable solution. The effect of interconnect parasitic were also discussed, in the previous Section 2.1.1, and how operator support can become decisive to obtain acceptable performance, especially at GHz

Another parameter to be accounted for is the number of parts to be produced in 1 week, 1 month or 1 year. This number plays a crucial role. If a mass production is foreseen, then manual assembly is not advised due to the lengthy and costly process associated with it. On the contrary, when very few parts are to be produced then manual process is acceptable, also because automatic assembly requires the development of programs and codes with the consequent Non Recurring Expenses (NRE)

The paradigm of design for manufacturing can be found in Additive

ing is very high and must be carefully accounted for in the business model.

Manufacturing (AM) technology. AM represents a key example where an advancement in production technologies enables new engineering concepts that can come to life *only* with this technology. In this sense, it is quintessentially a design *enabled by*

In the aerospace sector, AM is applied mostly on metallic parts (Aluminum, Steel, Titanium and related alloys) rather than composites (plastics) as occurs in the consumer industry. In fact, the initial investment in terms of machinery and train-

AM in aerospace has been happening for some time now with many applications, covering everything from the creation of aircraft or helicopter parts, making lighter and more efficient engines, 3D printed turbines etc. 3D technologies generally save on time, money and create stronger, lighter, and more efficient finished

An example of AM technology and process applied to the aerospace industry is

One of the challenges of the market is the restriction of the volume of construction and the size of the product. An aircraft is made up of very large components and additive manufacturing is today limited to the volume offered by the 3D printer. Most technologies offer solutions with limited print volume, making 3D printing applicable only to small components. So, this constraint that could slow down the growth of the market. Even if so, today's 3D technologies have already made it possible to create and qualify fairly large (approx. 30 cm) components for space [3, 4] and aviation [5]. Finally, the latest available machines (SLM500, Concept Laser Xline

Design rules can be seen as a set of physical, geometrical, chemical, mechanical limitations. They are very useful when the manufacturing process is constant and

The part itself is not very complex, but is proves how AM can be gainfully exploited to create lighter or more complex structures than the ones previously

20000R, EOS M 400) are capable of building even larger pieces.

**10**

*Metallic part optimization thanks to AM.*

repetitive as happens in the semiconductor industries or in large scale production. This paradigm however is less stringent in the aerospace industry since there is not a mass production of items, but on the contrary, a production of a large quantity of different parts each one characterized by very small multiplicity. Moreover, while digital board assemblies can follow rules developed for the consumer market, high frequency microwave assemblies (operating at 100 MHz–30 GHz) are typical of the aerospace industry and suffer from less standardization. Consequently, for the latter guidelines rather than rules should be applied.

Anyhow, rules and guidelines should address the following features that are critical in any industrial manufacturing process:


#### *2.2.1 Design rules*

Design rules are written to suit a specific production technology. In the electronics for aerospace industry important production technologies are microwave modules and digital boards.

Digital board production uses rules similar to the ones developed for consumer and telecom products, always taking into account that aerospace industry produces a relatively small amount of high-performance products as opposed to consumer market. Anyhow, well known standards can be applied, for example the IPC-2291 "Design Guideline for Printed Electronics" or IPC-2252 "Design Guide for RF/ Microwave Circuit Boards" considering class 3 for the aerospace industry.

On the other hand, production of complex microwave parts is very typical to the aerospace & defense sector and seldom finds application elsewhere. This is related to the high cost involved in development and production. Design rules for these objects often end up as a few set of geometrical rules. An example of design rules

applicable to hybrid microwave modules or hybrid microwave integrated circuit is given in the following:

