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J Weld Join > Volume 40(3); 2022 > Article
Kim, Hong, Lee, and Kim: Effects of Solder Powder Particle Size and Substrate Surface Finish on Degradation Properties of Solder Joints

Abstract

In this study, the degradation characteristics of solder joints composed of powder-size Sn-3.0Ag-0.5Cu and the substrate surface finish are compared. Reflow is performed at a N2 ambient to apply an organic solderability preservative (OSP), electroless nickel/immersion gold (ENIG) substrates, and solder pastes of Type 6 (5-15 ㎛) and Type 7 (2-11 ㎛). Subsequently, a thermal shock test (TST) is performed for 1,500 cycles at a temperature of -40 °C to 125 °C (dwell time of 10 min) to analyze the deterioration characteristics. After N2 reflow is performed, the void content of the OSP substrate/solder joints is higher than that of the ENIG substrate/solder joints, and the shear strength of the ENIG substrate/solder joints is higher than that of the OSP substrate/solder joints. After 500 TST cycles, the shear strength at the OSP substrate/Type 7 solder joints does not decrease, whereas the shear strength at the ENIG substrate/Type 7 solder joints decreases by approximately 25%. Cross-sectional analysis shows that Cu6Sn5 and (Ni,Cu)3Sn4 intermetallic compounds (IMCs) emerged in the OSP and ENIG interfaces, respectively. The increasing rate of IMC thickness at the Type 7 solder joints is higher than that at the Type 6 solder joints. Fracture analysis shows that the OSP and ENIG joints fractured in the solder and solder/IMC interfaces, respectively. The IMC of the OSP and ENIG joints exhibit scallop and needle shapes, respectively. The ENIG joints fractured at the solder/IMC interface because of a stress concentration in the needle-type IMC. After 1500 TST cycles, the shear strength of all samples decreases by more than 50%. Cross-sectional analysis shows a crack at the solder joints of all samples after 1,000 cycles. The decrease in shear strength is caused by these cracks.

1. Introduction

Since the miniaturization and densification of electronic products have reduced the size of components, the size of solder joints has also decreased, requiring an understanding of fine solder pastes. Currently, the most commonly used solder paste is Type 4 with a solder powder size of 20-38 ㎛. Table 1 shows the types depending on the particle size of solder pastes outlined in the IPC J-STD-005A specifications1).
Table 1
Solder power size with solder Type1)
Solder Type Less than 0.5% larger than (μm) 10% Max. between (μm) 80% Min. between (μm) 10% Max. less (μm)
3 60 45-60 25-45 25
4 50 38-50 20-38 20
5 40 25-40 15-25 15
6 25 15-25 5-15 5
7 15 11-15 2-11 2
As the solder powder size decreases, a technology to prevent oxidation during the manufacturing process becomes very important. Under the assumption that an oxide film is formed at the same thickness regardless of the solder powder size, the entire surface area increases as the particle size decreases, and the oxygen content of the entire solder increases. Also, it has been reported that the reflow joining property may be degraded as the powder particle size decreases since the amount of unmelted residual solder powder increases in the presence of an oxide film2). As a result, a nitrogen reflow or vacuum reflow process that can lower the oxygen concentration during the process is applied to prevent oxidation3-5). Since the nitrogen reflow process may be applied by injecting nitrogen into the existing reflow equipment, the facility limitation is relatively small, and the vacuum reflow process has the advantage of effectively reducing the oxygen concentration and voids in a joint2-6).
The types of surface finishes for printed circuit boards (PCBs) include organic solderability preservative (OSP), electroless nickel/immersion gold (ENIG), immersion tin, immersion silver, and hot air solder leveling, (HASL). The OSP surface finish is the cheapest and simplest but is disadvantageous in that the organic film is burned during the reflow, making PCBs vulnerable to high-temperature and high-humidity environments. On the other hand, the ENIG surface finish, which involves the electroless nickel/immersion gold plating method, provides resistance to high temperature and high humidity and is mainly applied to electronics in severe environments7).
The voids formed in the joint after the reflow directly affect the joining strength and the reliability of the joint8), and the void characteristics according to the surface finish vary with the level of surface oxidation of PCB pads. The largest number of voids is formed in an OSP joint with a high oxidation level during the process, and the voids merge into a large void while the solder is melted. On the other hand, it has been reported that voids in an ENIG joint do not merge but remain as small voids after the solder is melted since the oxidation level and reaction rate of the ENIG joint is lower than that of the OSP joint9).
Different intermetallic compounds (IMCs) are formed at each joint interface of the OSP and ENIG surfaces. Cu6Sn5 and Cu3Sn IMC are often formed with OSP, whereas (Cu,Ni)₆Sn₅ and (Ni,Cu)3Sn4 IMC are formed with ENIG, depending on the extent of copper diffusion10). An IMC formed at the joint interface has a significant effect on reliability, and previous studies have reported that Kirkendall voids formed with the growth of the Cu3Sn IMC and the Ni-Sn-P layer formed between the (Ni,Cu)3Sn4 IMC and (Ni,Cu)3Sn4/ Ni layers lead to brittle failures and degrade reliability7,11). Also, it has been reported that the needle- shaped (Ni,Cu)3Sn4 IMC concentrates stress at the joint more than the scallop-shaped Cu6Sn5 IMC, and a Ni- based IMC results in a larger difference in thermal expansion coefficient, increasing the internal stress and reducing the fatigue life12,13).
In this study, chip resistors were joined through the nitrogen reflow process by applying Type 6 and Type 7 solder pastes and OSP and ENIG surface-finished PCBs, and 1,500 cycles of the thermal shock test (TST) were conducted in the temperature range of -40-125°C to compare the degradation properties of the joints. After the process, the void content and shear strength of the joints were measured, and the shear strength and the cross-sectional microstructure were compared before and after TST.

2. Experimental Method

2.1 Sample Preparation for Solder Paste Evaluation

The substrates used in the experiment included OSP and ENIG surface-finished substrates with a size of 130×130 mm. Fig. 1 shows the substrate design. The resistor chip sizes used were 3216 (3.2×1.6 mm), 2012 (2.0×1.2 mm), 1608 (1.6×0.8 mm), and 1005 (1.0×0.5 mm). Fig. 2 shows the size of each joint pad. The pads were prepared with a mask aperture of 100% and a thickness of 100 ㎛. Type 6 and Type 7 solder pastes (Senju, Japan) were used as joining materials. Fig. 3 shows the particle size of each solder powder.
Fig. 1
Optical image of ENIG finish substrate
jwj-40-3-207gf1.jpg
Fig. 2
Design rule of the PCB pad size (mm) for various chip resistor: (a) 3216 resistor, (b) 2012 resistor, (c) 1608 resistor, (d) 1005 resistor
jwj-40-3-207gf2.jpg
Fig. 3
SEM micrograph of (a,b) surface and (c,d) cross-sectional images of solder powder : (a,c) Type 6 solder paste, and (b,d) Type 7 solder paste
jwj-40-3-207gf3.jpg
To prepare samples for the solder paste evaluation, the screen printing (MK-878Mx, MINAM), chip mounting (CP-45FV NEO, Samsung Techwin), and nitrogen reflow (1809UL, HELLER) were performed using the surface mount technology (SMT) process. Fig. 4 shows the temperature profile. After the process, a visual analysis, X-ray void analysis, and shear strength measurement were performed, and the microstructure of the joints was observed through cross-sectional analysis. The shear test was conducted at a rate of 167 ㎛/s and a height of 100 ㎛.
Fig. 4
Reflow soldering profile in the nitrogen atmosphere
jwj-40-3-207gf4.jpg

2.2 Thermal Shock Test (TST)

A thermal shock test (NT-1531W, ETAC) was conducted to evaluate the degradation properties of the joints according to the solder particle size and substrate surface finish. In the thermal shock test, 1,500 cycles (about 30 minutes per cycle) were performed by maintaining the temperature for 10 minutes and varying the temperature within five minutes over the -40°C-125°C range, and the shear strength measurement and cross- sectional analysis were made every 500 cycles. Also, after 500 cycles, the fracture mechanism was analyzed by observing the fracture surface of the shear strength sample joints.

3. Experimental Results

3.1 Visual Analysis

Fig. 5 shows the visual analysis results of the 2012 resistor. The analysis results indicated no significant difference in the shape of the solder fillet while slight discoloration was observed with OSP due to oxidation in the exposed Cu pad at the end that was not filled in with the solder. On the other hand, no such discoloration was observed with ENIG.
Fig. 5
Optical micrographs after nitrogen reflow: (a) OSP finish/Type 6 solder joints, (b) OSP finish/Type 7 solder joints, (c) ENIG finish/Type 6 solder joints and (d) ENIG finish/Type 7 solder joints
jwj-40-3-207gf5.jpg

3.2 Analysis of Void Content

The void content analysis of the joints was conducted by measuring the void area relative to the total joint area. Fig. 6 shows the results. The average void content depending on the substrate surface finish is indicated by a dotted line in Fig. 6. The average void content of the ENIG joints was found to be smaller than that of the OSP joints. As seen in the visual analysis, the OSP pad was easily oxidized during the reflow process, and the oxidation level of the pad affected the formation of voids.
Fig. 6
Void content comparison of the chip resistor solder joints: (a) OSP finish and (b) ENIG finish
jwj-40-3-207gf6.jpg

3.3 Evaluation of Shear strength

Fig. 7 shows the shear strength results before and after TST. While the difference in the bonding strength of the initial resistor was not significant, the average strength value was 3.34 kgf for OSP and 3.38 kgf for ENIG, indicating ENIG was slightly higher. The void content of all samples was relatively small at 15% or lower and was deemed to have an insignificant effect on the reduction of the shear strength. The average shear strength of the Type 6 and Type 7 samples was 3.28 kgf and 3.44 kgf, respectively, indicating that Type 7 was higher.
Fig. 7
Shear strength after nitrogen reflow and thermal shock test: (a) OSP finish/Type 6 solder joints, (b) OSP finish/ Type 7 solder joints, (c) ENIG finish/Type 6 solder joints and (d) ENIG finish/Type 7 solder joints
jwj-40-3-207gf7.jpg
After 500 cycles of TST, the rate of decrease in shear strength was about 18% in the 3216 resistor and about 14% in the 2012 resistor for the OSP/Type 6 samples, whereas there was little decrease in shear strength in the 1608 and 1005 resistors. In the case of the OSP/ Type 7 samples, the rate of decrease was about 17% in the 3216 resistor, while there was no decrease in shear strength in the 2012, 1608, and 1005 resistors. For the ENIG/Type 6 samples, the shear strength was decreased by 12% in the 3216 resistor, about 7% in the 2012 resistor, and about 15% in the 1608 resistor while there was no decrease in shear strength in the 1005 resistor. For the ENIG/Type 7 samples, the shear strength was decreased by about 29% in the 3216 resistor and by 25% in the 2012 resistor while there was no significant decrease in shear strength in the 1608 and 1005 resistors. After 1,000 cycles of TST, the shear strength of both the 3216 and 2012 resistors was reduced by more than 40%, and that of all resistors was reduced by more than 50% after 1,500 cycles.
Based on these results, it was found that the larger the resistor size, the larger the decrease in shear strength. The decrease in shear strength of the solder joints is attributed to the difference in the thermal expansion coefficient of each structure, and the corresponding thermal strain may be expressed as Εt=α(△T) (Εt: thermal strain, α: thermal expansion coefficient, △T: temperature change). The linear expansion strain according to the thermal strain can be expressed as δt=Εt·L (L: length), which implies that, in the presence of a thermal strain, the linear expansion strain increases as the length increases. Therefore, under the experimental condition where the structure and material were the same but only different in size, the rate of decrease in shear strength was confirmed to be large since the linear expansion strain increased with the resistor size.

3.4 Analysis of Fracture Surface

In the shear strength test, while the strength was found to decrease at a similar level in all samples after 1,000 cycles, the fracture surface of the 2012 resistor was analyzed because the rate of decrease was different in the Type 7 samples with OSP and ENIG after 500 cycles. As shown in Fig. 8(a), on the fracture surface of the OSP/Type 7 sample, a ductile fracture was observed inside the solder, and some fracture was observed at the solder/lower IMC interface. A high-magnification analysis of the IMC fracture surface indicated a scallop-shaped Cu6Sn5 IMC. On the other hand, the fracture surface of the ENIG/Type 7 sample was mostly observed to be an interface fracture between the solder and lower IMC, and the high-magnification analysis of the IMC fracture surface indicated a needle-shaped (Ni,Cu)3Sn4 IMC. It is believed that the rate of decrease in shear strength of the ENIG/Type 7 sample was large because this needle-shaped (Ni,Cu)3Sn4 IMC concentrates the stress more in the joint. Since the fractures mostly occurred in the solder and at the solder/lower IMC interface, the thickness of the lower IMC was measured following the cross-sectional analysis.
Fig. 8
(a-d) SEM micrograph and (e-f) EDS analysis of fracture surface after TST 500 cycles: (a,b,c) OSP finish/Type 7 solder joints, (d,e,f) ENIG finish/Type 7 solder joints
jwj-40-3-207gf8.jpg

3.5 Cross-sectional Microstructure Analysis

Fig. 9 and Fig. 10 show the cross-sectional analysis results of the 2012 resistor microstructure in the OSP and ENIG samples, respectively. The OSP samples exhibited a flat-shaped (Cu,Ni)₆Sn₅ IMC at the top and a scallop-shaped Cu6Sn5 IMC at the bottom, whereas the ENIG samples exhibited a needle-shaped (Ni,Cu)3Sn4 IMC both at the top and bottom. The IMC layer was observed to have grown after 500 cycles of TST. Fig. 11 shows the thickness measurements of the IMC. The IMC thickness was increased more in the Type 7 sample than in the Type 6 sample with both OSP and ENIG. Since the heat capacity of the Type 7 solder powder is much smaller than that of the Type 6 solder ball, the melting process likely occurred faster during the reflow, and the diffusion in the pad layer (Cu or Ni) was more active. Therefore, it is believed that more IMC particles were present in the solder and near the IMC of the Type 6 joint compared to the Type 7 joint, and the increase in the IMC thickness was greater. This phenomenon can be also observed with the large IMC formed inside the solder of the initial Type 7 joint, as shown in Fig. 12(b). The IMC formed inside the solder improves the joint properties by preventing the propagation of cracks, as shown in Fig. 12(c), which is the reason why the shear strength of the OSP/Type 7 joint did not decrease ever after 1,000 cycles of TST. As shown in Fig. 13, localized cracks were observed after 1,000 cycles of TST, and the cracks were found to propagate more preferentially towards the lower IMC in the ENIG samples than in the OSP samples. Thus, it is believed that the IMC formation inside the solder affected the joint properties in the case of OSP since the fracture occurred inside the solder, whereas the growth rate of the lower IMC affected the joint properties in the case of ENIG since the fracture occurred at the interface of the lower IMC. After 1,500 cycles of TST, cracks were observed over the entire surface in all samples, and these cracks were found to reduce the shear strength.
Fig. 9
(a,b,d,e) Cross-sectional SEM micrograph and (c,f) EDS analysis after (a,d) nitrogen reflow and (b,e) TST 500 cycles: (a-c) OSP finish/Type 6 solder joints and (d-f) OSP finish/Type 7 solder joints
jwj-40-3-207gf9.jpg
Fig. 10
(a,b,d,e) Cross-sectional SEM micrograph and (c,f) EDS analysis after (a,d) nitrogen reflow and (b,e) TST 500 cycles: (a-c) ENIG finish/Type 6 solder joints and (d-f) ENIG finish/Type 7 solder joints
jwj-40-3-207gf10.jpg
Fig. 11
IMC thickness of Type 6 and 7 solder joints with surface finish of PCB
jwj-40-3-207gf11.jpg
Fig. 12
  Cross-sectional SEM micrographs: (a) OSP finish/Type 6 solder joints after nitrogen reflow, (b) OSP finish/Type 7 solder joints after nitrogen reflow and (c) ENIG finish/Type 7 solder joints after TST 1,000 cycles
jwj-40-3-207gf12.jpg
Fig. 13
Cross-sectional SEM micrographs after TST 1,000 cycles: (a) OSP finish/Type 6 solder joints, (b) OSP finish/ Type 7 solder joints, (c) ENIG finish/Type 6 solder joints and (d) ENIG finish/Type 7 solder joints
jwj-40-3-207gf13.jpg

4. Conclusions

In this study, the nitrogen reflow process characteristics and the thermal degradation properties of the solder joints were compared according to the solder particle size and substrate surface finish using Type 6 and Type 7 Sn-3.0Ag-0.5Cu solder pastes and OSP and ENIG surface-finished substrates.
  • 1) After the nitrogen reflow, the void content was higher in the OSP joints than in the ENIG joints, and the varying level of oxidation during the process with the substrate surface finish was found to affect the void content. As a result of this difference in void content, the initial shear strength of the ENIG samples was measured higher than that of the OSP samples.

  • 2) After 500 cycles of TST, there was no decrease in shear strength in the OSP/Type 7 joint sample of the 2012 resistor, whereas there was a decrease of about 25% in the ENIG/Type 7 joint sample. As a result of the fracture surface analysis, it was found that the (Ni,Cu)3Sn4 IMC formed in a needle shape in the ENIG joint concentrated the stress in the joint, therefore reducing the shear strength. The cross-sectional analysis results indicated that a large amount of Cu6Sn5 IMC was formed inside the solder of the OSP/Type 7 joint, and this IMC improved the joint properties by interfering with the propagation of cracks.

  • 3) After 1,000 cycles of TST, cracks were observed in all joints, and the shear strength was reduced as a result.

Acknowledgment

This study was conducted with the support of the Materials and Components Technology Development Project (No. 20017409, 20017419) of the Ministry of Trade, Industry and Energy.

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