Authored By:
Cheng-Hao Chin, Gnyaneshwar Ramakrishna
Cisco Systems Inc.
ON, Canada
Summary
High performance computing application in recent years drives the development of Ball Grid Array (BGA) components to higher I/O count and greater power level. With demand on higher power consumption raise, to mitigate joule heating, current crowding and even to meet more strict power integrity requirement, usage of trace that has same width as non-solder mask defined (NSMD) pad or even usage of copper pour have gradually become viable design options. This change turns some of NSMD pads in the BGA footprint into partial solder mask defined (SMD) pads due to their wider BGA escaping trace and their exposure of greater solderable areas. It is important to understand how this kind of partial SMD would affect solder joint reliability when used in real application.
In this paper, a comprehensive study is presented to demonstrate the impact of different BGA escaping trace design cases on solder joint reliability by performing board level thermal cycling test on 3.2-mm 16-layer thick test vehicles. Various PWB layout variations such as pure SMD and pure NSMD pads with different trace width, trace count, routing direction and mix of different variations are studied. Two different daisy-chained components with 0.5 mm pitch and 0.8 mm pitch were used to understand their correlation with the type of package.
The results showed design using wider trace would have longer solder joint fatigue life and this phenomenon becomes more significant when using component with smaller pad size. In addition, design using SMD pad showed the least performance among this study. Design using pad connecting to single wide trace showed similar characteristic life to those connecting to two wide traces. Moreover, it seems design using two wide traces have the benefit of extending its time to failure of first few wearouts. Finally, the outcome of design using mixing of 2 variations is also analyzed and discussed in this study.
Conclusions
A comprehensive study is presented to demonstrate the impact of different BGA escape trace design cases on solder joint reliability. The result showed longer characteristic life for design using wider trace and this phenomenon becomes more significant when using component with smaller pads. It is also believed that the effect of wide trace will be less effective on design using component with pads greater than 16 mils. Besides, design using SMD pad gives the worst solder joint reliability performance among all the cases.
When comparing design using single trace and design using two traces, the latter has less than 5% of improvement on characteristic life and it’s time to failure of first few failures has an improvement in cycle by about 30%. It is believed that design using this kind of partial SMD pad would still get completely different reliability characteristic from that of a full SMD pad.
Routing perpendicularly has slightly negative effect on the performance of solder joint reliability compared to routing diagonally. The cause of this phenomenon requires further investigation. Thus, it is suggested to keep the same fanout direction as the real application when design TCoB test vehicle.
Design using mixing of two variations in a single footprint would have its reliability performance bounded by those of design using each variation alone. The detailed mechanism should rely on finite element analysis for more insight.
From the failure analysis perspective, the inspected failure modes show a good level of consistency of the weakest link of a solder joint. Besides, most of the cracks are found on the PWB side in this study and it supports the existence of the correlation between the reliability of the solder joints and how their underneath land pattern is designed.
Initially Published in the SMTA Proceedings
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