Electronics Assembly Knowledge, Vision & Wisdom
Impact of Low Silver Paste on Area Array Joint Quality
Impact of Low Silver Paste on Area Array Joint Quality
This paper provides analysis to "fill in the gaps" concerning the implementation for a wide variety of lead-free solder alloys.
Production Floor

Authored By:
Srinivasa Aravamudhan, Raiyo Aspandiar, Ph.D., Lilia May, Ph.D.,
Suddhasattwa Nad, Scott Mokler, and Dudi Amir
Intel Corporation
Hillsboro, OR USA

Narration and Analysis:
Jim Hall
Jim Hall, ITM Consulting
A Lean Six-Sigma Master Blackbelt, Jim has a wealth of knowledge in soldering, thermal technology, equipment and process basics. He is a pioneer in the science of reflow.
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This paper is one of many recent publications which provide test data and analysis to "fill in the gaps" in the published information concerning the full implementation of the wide variety Lead-Free solder alloys which are commercially available.

These authors specifically addressed the impact of low-silver alloys on the formation of solder joints on BGA components, primarily Head-on Pillow type defects.

The specific objectives were as follows:
  1. What is the impact of Low Ag alloy and paste flux system on the SMT failure rate and failure mode?
  2. What is the impact of reflow temperature on Low Ag solder joint formation?
The test parameters included:
  1. 4 low silver alloys and SAC 305 as a baseline
  2. 4 different flux formulations
  3. 2 peak reflow temperatures
  4. 25mil pitch, Flip Chip BGAs with 16 mil, SAC 405 balls.
The BGA's were reflowed onto test boards. Defects were evaluated by vertical pull tests and cross sections. Laboratory analysis of peak liquidus temperatures and intermetalics were also performed

  1. The flux system has the largest impact, one flux produced zero defects for both low silver and SAC 305.
  2. Solder alloy composition (% Silver and dopants) also impacts defect rates.
  3. Peak reflow temperature may need to be increased for low silver alloys to improve wetting and reduce defects. Unfortunately in this study, not all flux formulations were tested at the lower peak reflow temperature.
Reacting to increased silver metal costs in recent years,Personal Computer (PC) board assemblers are converting to low Ag SAC solder pastes, and transitioning away from the widely used SAC305 (Sn-3.0wt%Ag-0.5wt%Cu)composition. Previously, a few industry studies demonstrated similarities between low Ag SAC andSAC305 solder pastes in printability, slump, solder balling,and voiding [1-2]. However, no data has been published on the effect of using low Ag SAC solder paste on solder joint quality. Solder paste properties dictated by both flux composition and solder powder metallurgy are major determinants of solder joint defects such as head-on-pillow(HoP), non-wet open (NWO) and solder ball bridging(SBB) defects. In this study, flip chip ball grid array(FCBGA) packages were used to induce solder joint defects formation.

These packages were then assembled using low Ag SAC solder pastes as well as standard SAC305 solder pastes from various suppliers. To precipitate failures caused by the dynamic warpage of the packages, the solder paste printed volumes were maintained at the low end of acceptable ranges, and air atmosphere was employed in there flow oven. The surface mount technology (SMT)assembly solder joint yield was determined after reflow soldering from electrical testing of the daisy chain loops, as well as by failure analysis, which indicated the mode of solder joint defects. Analysis of the defects showed that the solder paste flux system was dominant over alloy metallurgy for determining solder joint yield. Moreover, to maximize solder joint yield when using low Ag SAC solder pastes, the peak reflow temperatures will need to be adjusted to higher levels than for that used for SAC305solder pastes.
Results from the study show that the selection of the right solder paste flux system is critical for low Ag or zero Ag solder alloy metallurgies to influence SMT process yields.Additionally, an increase in peak reflow temperature may be required to improve SMT yield and reduce solder joint defects. Due to the increase in liquidus temperature for low Ag and zero Ag, the minimum peak temperature should be increased based on the alloy used for the SMT assembly.However, an increase in minimum peak reflow temperature may result in an adverse impact to other components on the board that need additional evaluation.
Initially Published in the SMTA Proceedings
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