SMTA International Conference Proceedings

Transient Solder Separation of BGA Solder Joint During Second Reflow Cycle Phase III – The Impact of Back Drill

Authors: Steven Perng and Weidong Xie
Company: Cisco Systems, Inc.
Date Published: 9/22/2019   Conference: SMTA International

Abstract: Via In Pad Plated Over (VIPPO) has become an inevitable via-pad structure in high density routing design. The benefits of adopting the VIPPO design includes, for example, reducing signal path lengths and increases board functionality density. However, the long Cu barrel of the via structure has an adverse antenna effect on signal integrity. There are two common practices to minimize the antenna effect. First is to mix VIPPO with the dog-bone via design. The other is to reduce the Cu barrel length by back drilling.

For the VIPPO and dog bone mixed design, due to the CTE mismatch between the Cu and PCB material, solder separations are found between the bulk solder and IMC on the package side. The root cause analysis can be found in Phase I1 of this study. The proof of primary driving force, the timing of separation, and the duration of separation can be found in Phase II2 of this study.

This study (Phase III) focuses on the impact of back drill. The idea is to minimize the impact of antenna effect on signal integrity by reducing the length of the Cu barrel. To an extent, it won't induce the solder separation. In the other words, it is to find the “safe” back drill length without causing the solder separation.

A 4”x5” daisy chained test vehicle in 93mil and 125mil board thickness with OSP surface finishing is used. For the memory device on the board, there is one daisy chain connected to each of the 24 pairs of VIPPO and VIPPO pad with back drill design. And, the 25th daisy chain connect to the rest of 30 VIPPO pad with back drill design. With this paired daisy chain design, the continuity of each VIPPO and VIPPO with back drill pair can be monitored in-situ and in real time throughout the reflow process.

Seven back drill depth are incorporated, including 50%, 64%, 69%, 75%, 80%, 86%, and 92%. The continuity and resistance values, of each VIPPO and VIPPO with back drill pair, are collected in real time through data logger. There are two folds of objectives. One is to find the trend of percent of solder separation pins vs. back drill depth. The other is, based on the trending data, to find the “safe” back drill length (or percentage) without causing solder separation.

Key Words: 

VIPPO, VIPPO Mixed Design, Back Dill, Solder separation, Micro Via, CTE mismatch

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