B. Gumpert, B. Fox, L. Woody
The move to lead free (Pb-free) electronics by the commercial industry has resulted in an increasing number of ball grid array components (BGAs) which are only available with Pb-free solder balls. The reliability of these devices is not well established when assembled using a standard tin-lead (SnPb) solder paste and reflow profile, known as a backward compatible process. Previous studies in processing mixed alloy solder joints have demonstrated the importance of using a reflow temperature high enough to achieve complete mixing of the SnPb solder paste with the Pb-free solder ball.
Research has indicated that complete mixing can occur below the melting point of the Pb-free alloy and is dependent on a number of factors including solder ball composition, solder ball to solder paste ratio, and peak reflow times and temperatures. Increasing the lead content in the system enables full mixing of the solder joint with a reduced peak reflow temperature, however, previous research is conflicting regarding the effect that lead percentage has on solder joint reliability in this mixed alloy solder joint.
Previous work by the authors established a protocol for soldering Pb-free BGAs with SnPb solder paste based on solder ball size and target lead content in the final solder joint. The units from that testing were subjected to thermal cycling between -55C and 125C and compared to a SnPb baseline assembly. Results showed that mixed alloy joints performed as well as or better than standard SnPb joints under these conditions.
This study continues the previous work with evaluation of reliability in a production design. Functional assemblies were built using Pb-free BGAs in a SnPb solder process and subjected to life testing including accelerated aging and highly accelerated life testing (HALT). Results from this testing are compared to SnPb baseline units and previous product development test results.
BGAs with SAC305 balls were inserted into an existing military design and installed using a backward compatible process (SnPb solder). The assembly process was modified slightly (solder paste application and reflow profile) according to guidelines developed in previous studies to accommodate these Pb-free components, but kept within a window suitable for typical SnPb assembly. Cross-section evaluations of the base-line units validate that the process used resulted in complete mixing of the solder joints.
These assemblies were subjected to HALT and accelerated aging environments similar to those used in production development evaluations. The units survived nearly all functional testing through these environments, demonstrating survivability of the mixed alloy solder joints under exposure to humidity, temperature, vibration, and thermal cycling. The only failures exhibited came after a significant amount of aging and testing was complete, and were attributed to component failure, not solder joint failure.
This study shows that when the parameters of a backward compatible process are appropriately controlled and applied, the resulting BGA solder joint can have acceptable reliability in a military product for certain expected environmental conditions. We must keep in mind, however, that this relies on certain assumptions about the behavior of a low-Pb solder joint. Testing of Pb-free solder has demonstrated that reliability can be dependent on the parameters of the test and the order of certain testing. More studies should be performed to further investigate the point or range at which the Pb-content of a solder joint results in Pb-free-like behavior or standard SnPb-like behavior.
One observation of unique interest in this study is the lack of change in the morphology of the mixed-alloy solder joints. The Sn63Pb37 solder joints show a marked change in structure, with significant grain growth through both thermal cycling and accelerated aging in temperature and humidity. The mixed-allow joints do not show the same amount of change in shape of the lead-rich areas.
Initially Published in the IPC Proceedings