When working with larger joints such as those on the inputs of the SOIC-10 package, the correct choice of solder is important for best performance. Texas Instruments recommends the use of a no-clean style paste, such as AIM M8 SAC Type 4 for best results. Water soluble variants can typically lead to increased voiding, which over time can reduce thermal performance due to these voids present in the joint. Figure 2-4 and Figure 2-5 show an example of no clean chemistry performance, while Figure 2-6 and Figure 2-7 demonstrate an example of water soluble chemistry performance on larger solder joints.

Figure 2-4 No Clean Solder Paste X-ray, Left Input Lead

Figure 2-5 No Clean Solder Paste X-ray, Right Input Lead

Figure 2-6 Time Zero X-rays, Left Input Lead

Figure 2-7 Time Zero X-rays, Right Input Lead

The solder paste chosen for this experiment is a water soluble SAC305 type 4 solder paste, and this choice was purposefully made to subject the devices under test to the worst case solder voiding scenario of an SOIC-10 package on an SOIC-16 footprint.

The solder integrity of the devices was again checked after the 500 temperature cycles outlined in Figure 2-3. Note that the device shown in Figure 2-8 and Figure 2-9 are a random sample from the tested cards, and is not a continuation of the solder voiding shown in Figure 2-6 and Figure 2-7.

Figure 2-8 500 Temperature Cycles X-rays, Left Lead

Figure 2-9 500 Temperature Cycles X-rays, Right Lead

The x-rays demonstrate that while there is solder voiding present on the fused leads, the quantity does not deviate from the time zero sample in any significant way. The solder voiding is comparable to the voiding that was seen before cycling, and resulted in no failures even after 500 temperature cycles under constant DC load. This shows that temperature cycling does not cause solder voiding to get worse, regardless of presence at initial manufacturing. In general, however, a no-clean style of solder is still recommended for use in application, as this helps mitigate any additional failures due to this issue in production.