3. Results and discussion

#### 3.1 Effects of resin modulus on solder joint cracks

Figure 9 shows the thermal mechanical properties of typical adhesives films as a function of temperatures. Compared with conventional epoxies by imidazole and cationic curing types with higher Tg (over 100°C), acrylic adhesives showed lower modulus, because it was a low Tg material (45°C). Especially at 200°C when hot-bar releasing bonded samples at the end of TC bonding for Sn-58Bi solder ACFs applications, even cured acrylic adhesives showed 0.4 Mpa storage modulus, and cationic epoxy and imidazole epoxy showed 7.1 and 5.7 Mpa, respectively.

Figure 10 shows Sn-58Bi solder ACFs joints cracks using this low Tg and low modulus acrylic adhesives regardless of bonding pressures from 1 to 3 Mpa. However, there are no solder joint cracks when using other higher storage modulus adhesive materials under the same bonding pressures.

In terms of solder joints cracks, there are two assumptions on it: one is CTE mismatch between shrink adhesives and solders during cooling process, the 2nd is higher elasticity of adhesives than molten solders when bonding pressure is removed, and then liquid solder joints are cracked under high temperature.

Figure 11 shows endothermic and exothermic behaviors of Sn-58Bi solder materials in a DSC analysis. In a heating period, Sn-58Bi solder melts at 139°C and stay at a liquid state until cooling to 125°C, and Sn-58Bi solders were totally solid below 90°C. As a result, adhesives shrinkage or rebound is able to cause liquid Sn-58Bi solder joints cracks only above 90°C, because solid Sn-58Bi joint is with 30.9 Gpa young's modulus [32] which cannot be cracked. Figure 12 gives the thermal expanded behaviors of three typical adhesives. The shrinkage percentages of three adhesives films were summarized in Table 2 in cooling process. 90°C is lower than the imidazole epoxy's Tg and cation epoxy's Tg, as a result, there was a huge shrinked amount when cooling process was lower than glass transition temperature

at two epoxies. �11.2, �13.2 and �5.2% dimension shrinkage were shown for acrylic resin, imidazole epoxy and cation epoxy in the cooling region from 200 to 90°C. However, good solder joint morphologies were found at imidazole epoxy based Sn-58Bi solder ACFs joints in Figure 10. As a result, solder ACFs joint cracks were not related with compressive stress by adhesives shrinkages in the cooling process, but

Solder joint morphologies of Sn-Bi58 ACFs joints by various bonding pressures. (a, d, g) Acrylic resins bonded by 1, 2 and 3 MPa, respectively; (b, e, h) imidazole epoxy bonded by 1, 2 and 3 MPa, respectively; (c, f, i)

related with resin elasticity, especially for lower modulus acrylic adhesives.

DSC behaviors of Sn-58Bi solder during heating and cooling process.

Figure 10.

Figure 11.

71

cationic epoxy bonded by 1, 2 and 3 MPa, respectively.

A Review: Solder Joint Cracks at Sn-Bi58 Solder ACFs Joints

DOI: http://dx.doi.org/10.5772/intechopen.83298

Figure 9.

Storage modulus of typical adhesives films as function of temperature. (a) 30–250°C (b) specific region at 200–250°C.

A Review: Solder Joint Cracks at Sn-Bi58 Solder ACFs Joints DOI: http://dx.doi.org/10.5772/intechopen.83298

#### Figure 10.

SiO2 fillers addition were compared in this thermal cycling test. What is more, the dwell time of -45°C was remained for 15 minutes and it rapidly increased to 125°C for 15 minutes dwelling time. Joint contact resistances in this thermal cycling test were designed to be recorded every 200 cycles. Until 1000 cycles, joint morphologies and each failure mode of Sn-Bi58 solder ACF joints with or without silica

Figure 9 shows the thermal mechanical properties of typical adhesives films as a function of temperatures. Compared with conventional epoxies by imidazole and cationic curing types with higher Tg (over 100°C), acrylic adhesives showed lower modulus, because it was a low Tg material (45°C). Especially at 200°C when hot-bar releasing bonded samples at the end of TC bonding for Sn-58Bi solder ACFs applications, even cured acrylic adhesives showed 0.4 Mpa storage modulus, and cat-

Figure 10 shows Sn-58Bi solder ACFs joints cracks using this low Tg and low modulus acrylic adhesives regardless of bonding pressures from 1 to 3 Mpa. However, there are no solder joint cracks when using other higher storage modulus

In terms of solder joints cracks, there are two assumptions on it: one is CTE mismatch between shrink adhesives and solders during cooling process, the 2nd is higher elasticity of adhesives than molten solders when bonding pressure is removed, and then liquid solder joints are cracked under high temperature.

Storage modulus of typical adhesives films as function of temperature. (a) 30–250°C (b) specific region at

Figure 11 shows endothermic and exothermic behaviors of Sn-58Bi solder materials in a DSC analysis. In a heating period, Sn-58Bi solder melts at 139°C and stay at a liquid state until cooling to 125°C, and Sn-58Bi solders were totally solid below 90°C. As a result, adhesives shrinkage or rebound is able to cause liquid Sn-58Bi solder joints cracks only above 90°C, because solid Sn-58Bi joint is with 30.9 Gpa young's modulus [32] which cannot be cracked. Figure 12 gives the thermal expanded behaviors of three typical adhesives. The shrinkage percentages of three adhesives films were summarized in Table 2 in cooling process. 90°C is lower than the imidazole epoxy's Tg and cation epoxy's Tg, as a result, there was a huge shrinked amount when cooling process was lower than glass transition temperature

modified were compared through the observation of SEM images.

ionic epoxy and imidazole epoxy showed 7.1 and 5.7 Mpa, respectively.

3.1 Effects of resin modulus on solder joint cracks

adhesive materials under the same bonding pressures.

3. Results and discussion

Lead Free Solders

Figure 9.

70

200–250°C.

Solder joint morphologies of Sn-Bi58 ACFs joints by various bonding pressures. (a, d, g) Acrylic resins bonded by 1, 2 and 3 MPa, respectively; (b, e, h) imidazole epoxy bonded by 1, 2 and 3 MPa, respectively; (c, f, i) cationic epoxy bonded by 1, 2 and 3 MPa, respectively.

Figure 11. DSC behaviors of Sn-58Bi solder during heating and cooling process.

at two epoxies. �11.2, �13.2 and �5.2% dimension shrinkage were shown for acrylic resin, imidazole epoxy and cation epoxy in the cooling region from 200 to 90°C. However, good solder joint morphologies were found at imidazole epoxy based Sn-58Bi solder ACFs joints in Figure 10. As a result, solder ACFs joint cracks were not related with compressive stress by adhesives shrinkages in the cooling process, but related with resin elasticity, especially for lower modulus acrylic adhesives.

Figure 12. Thermal expansion properties of typical adhesives films.

Adhesive modulus is divided by storage and loss modulus in viscoelastic materials [33]. Storage modulus is to measure the stored energy and represent the elastic property, and loss modulus is to measure the energy dissipated as heat and represent the viscous property [34]. In this study, solder joint cracks were due to adhesive rebound, which is elastic properties, and not due to heat dissipation and adhesive viscos property, so storage modulus is used.

Figure 13 illustrates the strain changes of acrylic based Sn-58Bi solder ACFs with respect to the 50 mN tensile sinusoidal load with 10 mN amplitude and 0.1 Hz frequency in DMA analysis as a function of temperatures. In details, strain changes in Figure 13 were consisted of two parts, one part is due to thermal expansion and the other is elastic strain due to plastic deformation. In this study, the plastic deformation, which is the dimension recover of deformed polymer when mechanical loading disappeared, is used to estimate adhesive rebound. According to the following equation, storage modulus is the ratio of applied tensile stress to elastic strain. In other words, adhesive rebound amounts are reversely proportional to its storage modulus in the following Eq. (2).

$$\text{Storage modulus} = \text{Stress} / (\text{Elastic strain}) \tag{2}$$

3.2 Effects of US bonding on enhancing resin modulus

A Review: Solder Joint Cracks at Sn-Bi58 Solder ACFs Joints

DOI: http://dx.doi.org/10.5772/intechopen.83298

Figure 13.

Figure 14.

73

function of temperatures.

30 seconds remaining pressure time is too long for assembly.

3.3 Effects of silica filler on enhancing resin modulus

Increased storage modulus of acrylic solder ACFs during cooling process.

Figure 15 shows the effects of delaying ultrasonic horn lift-up time on the acrylic based Sn-58Bi solder ACFs joint morphologies during an US bonding method. A crack-free Sn-58Bi solder ACFs joint can be successfully obtained by maintaining pressure below 45°C the acrylic adhesives Tg during a cooling process, because high storage modulus was established at low temperatures. Because 90°C is the complete point of Sn-58Bi solder joint solidification in Figure 11, there was a still solder joint crack when removing bonding pressures at 100°C in Figure 15. However, over

Strain of acrylic based Sn-58Bi solder ACFs in respect to a sinusoidal load applied in the DMA test as a

The faster approach of removing the solder joint crack is to increase resin storage modulus over Tg by adding silica fillers into acrylic polymer resins.

Because Tg of acrylic resin is 45°C, adhesive showed a relatively higher modulus at room temperatures below 45°C and lower modulus above 45°C. As a result, the elastic strain is smaller below 45°C and larger at above 45°C. In addition, 30 wt% Sn-58Bi solder particles melting behavior at 139°C will enlarge the measured sample dimension in Figure 13. Figure 14 shows storage modulus of acrylic adhesives were increased during cooling process and Table 3 summarizes specific storage modulus of acrylic adhesive during cooling process.


Table 2. Shrinkage percentages of typical adhesives films during cooling. A Review: Solder Joint Cracks at Sn-Bi58 Solder ACFs Joints DOI: http://dx.doi.org/10.5772/intechopen.83298

Figure 13. Strain of acrylic based Sn-58Bi solder ACFs in respect to a sinusoidal load applied in the DMA test as a function of temperatures.

#### 3.2 Effects of US bonding on enhancing resin modulus

Figure 15 shows the effects of delaying ultrasonic horn lift-up time on the acrylic based Sn-58Bi solder ACFs joint morphologies during an US bonding method. A crack-free Sn-58Bi solder ACFs joint can be successfully obtained by maintaining pressure below 45°C the acrylic adhesives Tg during a cooling process, because high storage modulus was established at low temperatures. Because 90°C is the complete point of Sn-58Bi solder joint solidification in Figure 11, there was a still solder joint crack when removing bonding pressures at 100°C in Figure 15. However, over 30 seconds remaining pressure time is too long for assembly.

#### 3.3 Effects of silica filler on enhancing resin modulus

The faster approach of removing the solder joint crack is to increase resin storage modulus over Tg by adding silica fillers into acrylic polymer resins.

Figure 14. Increased storage modulus of acrylic solder ACFs during cooling process.

Adhesive modulus is divided by storage and loss modulus in viscoelastic materials [33]. Storage modulus is to measure the stored energy and represent the elastic property, and loss modulus is to measure the energy dissipated as heat and represent the viscous property [34]. In this study, solder joint cracks were due to adhesive rebound, which is elastic properties, and not due to heat dissipation and

Figure 13 illustrates the strain changes of acrylic based Sn-58Bi solder ACFs with

Because Tg of acrylic resin is 45°C, adhesive showed a relatively higher modulus at room temperatures below 45°C and lower modulus above 45°C. As a result, the elastic strain is smaller below 45°C and larger at above 45°C. In addition, 30 wt% Sn-58Bi solder particles melting behavior at 139°C will enlarge the measured sample dimension in Figure 13. Figure 14 shows storage modulus of acrylic adhesives were increased during cooling process and Table 3 summarizes specific storage modulus

Dimension change Acrylic Imidazole epoxy Cation epoxy 200°C 21% 14% 6% 90°C 9.8% 0.8% 0.8 200 Cooling to 90°C �11.2% �13.2% �5.2%

Storage modulus ¼ Stress=ð Þ Elastic strain (2)

respect to the 50 mN tensile sinusoidal load with 10 mN amplitude and 0.1 Hz frequency in DMA analysis as a function of temperatures. In details, strain changes in Figure 13 were consisted of two parts, one part is due to thermal expansion and the other is elastic strain due to plastic deformation. In this study, the plastic deformation, which is the dimension recover of deformed polymer when mechanical loading disappeared, is used to estimate adhesive rebound. According to the following equation, storage modulus is the ratio of applied tensile stress to elastic strain. In other words, adhesive rebound amounts are reversely proportional to its

adhesive viscos property, so storage modulus is used.

Thermal expansion properties of typical adhesives films.

storage modulus in the following Eq. (2).

Figure 12.

Lead Free Solders

Table 2.

72

of acrylic adhesive during cooling process.

Shrinkage percentages of typical adhesives films during cooling.
