Reliability Study of Low Silver Alloy Solder Pastes

Estimated reading time: 17 minutes

Sn3.0Ag0.5Cu solder paste is currently the common alloy for lead-free solder paste in the industry. However, the price of silver has kept increasing over the last several years. This drives the desire for alternative low/no silver alloy materials and leads to the development of many alternative alloys. Today, many alternative low/no silver alloy solder pastes are available in the market. There are publications on the alternative lead-free alloys [1-3]. However, most of the studies focus on the alternative alloys of the BGA solder balls and their reliability. There is very limited publication on the thermal reliability performance of low/no silver alloy solder pastes. In our previous publication [4], alternative low/no silver alloy solder paste performed well in the process evaluation. Many alternative solder pastes had good printability and wettability, as shown in Figure 1 and Figure 2.

Figure 1: Printability of low/no silver alloy solder pastes. Paste J was SAC305 solder paste. Other materials were low Ag alloy solder pastes. It showed that many alternative alloy materials had equivalent printing performance as SAC305.

Figure 2: Wetting Test Results of Various Lead-Free Alloy Solder Pastes. The images of Wetting Test for a SAC305, SAC0307 and SnCuNi alloy solder paste were shown. These three solder pastes had on average a similar spreading diameter.

From the process point of view, the use of alternative alloy solder paste is feasible. However, there is a lack of information on their reliability. In this paper, we will discuss the microstructure, reliability, and failure analysis of lead-free solder joints reflowed using alternative low Ag solder pastes.

Experimental Details

Lead-Free Solder Pastes

Six different lead-free solder pastes were investigated in the study, as shown in Table 1. Type 3 no clean solder pastes were used. Material A, SAC305, was used as the control. Material B, SAC0307, is an alternative low-Ag solder paste without other alloy additions. Material C, SAC0307, contains some micro-alloying additions which can impact the Sn grain coarsening. Material D (SnCuBiCo) and Material E (SnCuNi) have no Ag in their compositions. In addition, a near-eutectic SnBi alloy with a small Ag addition was investigated (Material F). This alloy has a liquidus temperature of around 138°C, which is significantly lower than other tested lead-free alloys. Sn3.0Ag0.5Cu alloy melts at approximately 217 ºC. Alternative low silver high temperature alloys have melting points of around 227-228 ºC. Low temperature Sn-58Bi eutectic is known to be brittle. The small Ag addition is necessary in order to refine the microstructure of the Sn/Bi eutectic, promoting creep deformation by grain boundary sliding over brittle fracture [5]. The alloy compositions of the solder paste materials are listed in Table 1.

Table 1. Solder Paste Materials and their Alloy Compositions.

Test Vehicle and Components

The Company Multi-Function Test Vehicle is used in the study (Figure 3). The board dimension is 225mm x 150mm x 1.67mm. The board surface finish is OSP. The test vehicle has many different SMD component types such as BGAs (0.8mm and 1.0 mm pitch), CSP (0.5mm pitch, 0.4mm, 0.3mm), QFN component (0.5mm pitch and 0.4mm pitch), leaded components (SOIC, QFN100, QFN208, etc.), chip components (0201,0402, 0603, 0805), through hole components, etc… In addition, the test vehicle has different areas designed for printability test, slump test, wettability test, solder ball test, pin testability, etc.

Figure 3: Company Multi-Function Test Vehicle, Rev 1.0.

Components of different types and sizes were assembled for the reliability testing, as shown in Table 3. Daisy chained components were monitored during the thermal cycle test.

Table 2: Components in the Reliability Testing.

Assembly Process and Testing Conditions

All the samples using solder pastes A to E were reflowed using a typical lead-free profile with a peak temperature of around 245 °C (Figure 4). SnBiAg paste (material F) was reflowed using a low-temperature reflow profile with a peak temperature of about 170 °C (Figure 5). The reflow was performed in an air atmosphere environment.

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