Reliability of a Lead-Free System: Grid Array Solder Joint Reliability, Part III


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Editor's Note: This column originally appeared in the February 2012 issue of SMT Magazine.For the past two months, this column has been devoted to a review of 2011 and an outlook for 2012. I want to now pick up where my November column left off and continue discussing the process parameters of six scenarios, best practices and the scientific rationale used to address each scenario.

To refresh and recap: These commonly occurring scenarios fall within the following combinations of BGA package solder ball and SMT assembly solder paste:

  • Scenario 1 - SAC305 solder ball + SnPb solder paste;
  • Scenario 2 - SnPb solder ball + SAC305 solder paste;
  • Scenario 3 - SAC305 solder ball + SAC305 solder paste;
  • Scenario 4 - SAC low-Ag solder ball + SAC305 solder paste;
  • Scenario 5 - SAC305 solder ball + SAC low-Ag solder paste; and
  • Scenario 6 - SAC low-Ag solder ball + SAC low-Ag solder paste.

We’ve talked about the insidious effects of heat damage in previous columns. But, when using SAC 305 solder paste, one has no choice but to use a higher reflow temperature. Again, this is because of that very fundamental principle and practice--solder paste dictates the process.

As well recognized for a convection-heat-transfer reflow process, the optimal reflow profile (temperature versus time), particularly the peak temperature and the two dwell times at peak temperature and above the liquidus, respectively, vary with specific packages and the overall assembly. The best practice to build high-yield, low-cost, and high-reliability products is to design and select the system (all components and materials) that can work with a required range of reflow profiles. In other words, a too-narrow reflow profile is bound for manufacturing “mishaps” sooner or later. It is a matter of record that many production defects have been either directly associated with or indirectly related to an improper reflow profile, which I have witnessed and solved during more than three decades in SMT manufacturing.

A marginal reflow profile, primarily the result of either a too low peak temperature or too narrow a process window, aggravates the likelihood of production defects, such as solder graping, head-on-pillow, etc. To avoid a marginal process window, the following ranges of process parameters are recommended, as a general guideline, for Scenario 2 (SnPb solder ball + SAC305 solder paste) and Scenario 3 (SAC305 solder ball + SAC305 solder paste).

Scenario 2 (SnPb Solder Ball + SAC305 Solder Paste)

  • Peak temperature: 239 to 259°C, preferably 240 to 250°C;
  • Dwell time above liquidus: 40 to 120 seconds, preferably 45 to 70 seconds; and
  • Dwell time at peak: 10 to 30 seconds, preferably 5 to 10 seconds.

The above range of the process parameters are also intended to accommodate the various sizes of solder balls in a given PCB assembly that contains different BGA packages. It is not a surprise that the smaller solder balls fall in the lower end of the temperature range, as well as the dwell time ranges, and the larger solder balls require the higher end of the ranges.

Since the higher reflow temperature is mandatory, BGA packages containing SnPb solder balls are vulnerable to higher process temperature. The probable consequences include solder ball drop defect, interfacial problem at solder ball and package substrate interface, as well as package interior damages. However, practically speaking, this combination of ball and paste is not commonly in demand.

Scenario 3 (SAC305 Solder Ball + SAC305 Solder Paste)

In this pure, lead-free SAC305 system, which is also called “forward compatibility,” the key process parameters are not much different from Scenario 2 in principle. However, the preferred ranges should be shifted toward the high end of the general ranges.

  • Peak temperature: 239 to 259°C, preferably 245 to 255°C;
  • Dwell time above liquidus: 40 to 120 seconds, preferably 60 to 90 seconds; and
  • Dwell time at peak: 10 to 30 seconds, preferably 10 to 15 seconds.

For this combination, the solder paste’s ability to promote the adequate wetting is the key. Keep in mind that an SAC305 solder paste requires a reflow temperature, at least, at 20°C above the liquidus temperature of SA305 alloy to achieve the good wetting and uniform wetting with all sizes of BGAs, thus quality solder joints across a PCB assembly.

Again, recognize that this discussion may not be in congruence with all opinions and beliefs. If not convinced, one way is to conduct tests, examine the solder joint microscopically, and perform the mechanical property evaluation. Trust and verify is always a wise practice.

Future columns will address the rest of questions related to the material, process and reliability of assembling BGA-type packages, including Scenarios 4, 5, and 6.

Dr. Hwang, a pioneer and long-standing contributor to SMT manufacturing since its inception as well as to the lead-free development, has helped improve production yield and solved challenging reliability issues. Among her many awards and honors, she has been inducted into the WIT International Hall of Fame, elected to the National Academy of Engineering and named an R&D Stars to Watch. Having held senior executive positions with Lockheed Martin Corporation, Sherwin Williams Co., SCM Corporation and IEM Corporation, she is currently CEO of H-Technologies Group providing business, technology and manufacturing solutions. She is a member of the U.S. Commerce Department’s Export Council, and serves on the board of Fortune 500 NYSE companies and civic and university boards. She is the author of 300+ publications and several textbooks and an international speaker and author on trade, business, education and social issues. Contact her at (216) 577-3284; e-mail JennieHwang@aol.com

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