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SMTA International Conference Proceedings

Systematic Investigation of Impact of SMT Parameters, Isothermal Aging and Alloy Microstructure on Lead-Free BGA Solder Joint Reliability

Authors: Weidong Xie, Tae-Kyu Lee, and Steven Perng
Company: Cisco Systems, Inc.
Date Published: 10/14/2012   Conference: SMTA International

Abstract: Solder joint reliability of lead-free BGA packages has been studied extensively in last decade, it has been widely accepted that lead-free solder alloys, compared to the SnPb counterpart they intend to replace, have a lot unique characteristics either from microstructure or SMT assembly point of view. To ensure better long term reliability of the products, it is essential to understand how lead-free solder alloys evolve and behavior under different assembly and qualification testing conditions. Aims to better understand the lead-free solder behavior under different environmental conditions and to identify the major factors thereafter to improve or enhance the products’ long term reliability, a systematic investigation has been conducted to capture the impact from SMT parameters and isothermal aging on lead-free solder joint reliability.

In this paper, the statistical methodology has been applied to investigate the impact of SMT and other packaging parameters on lead-free solder joint fatigue life. To validate the methodology, a test vehicle was designed on an 8”x15” double-sided printed circuit board (PCB) with three different thicknesses (62mil, 93mil, and 125mil) to represent the thermal mass requirements in assembling the real products. There were multiple test sites on both sides of the PCB, which were populated with daisy-chained components. To reflect the real situation, the components were selected to include different package types (FCBGA, PBGA, CSP, QFN, etc), different pitches (0.4-1.0 mm), and different package size (6-50mm). Assembly parameters were collected and analyzed, a step-down stencil had to be used to accommodate the ultra-fine pitch BGA components. The assembled test vehicles then went through 0/100°C accelerated thermal cycling (ATC) with nominal of 10-minute hot and cold dwell. Some samples were isothermally treated before cycling to study the impact of isothermal aging onto solder alloy microstructure and solder joint reliability. Memory components were assembled on both sides of some TVs with different spacing to verify the effectiveness of staggering as a mitigation of clam-shelled configuration commonly adopted on high density boards. The study showed that, among other insightful findings, PCB thickness, die size of the package, standoff, and SMT configuration (single-side versus double-sided) were found to be the top 4 factors in terms of lead-free BGA joint reliability.

Note: This paper was presented as part of the Lead Free Soldering Technology Symposium during SMTA International.
Click here to download complete video recordings of the entire symposium.

Key Words: 

solder joint, reliability, Pb-free, BGA, packaging

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