Circuits Assembly - December 2007 - (Page 24)

Solder Materials A Test Comparison of SAC and Non-SAC Pb-Free Solders Howard P. Stevens SAC 305 evolved into the default Pb-free choice, but many groups have failed to run the alloy through the complete battery of tests. nexpensive tests regularly referred to and performed by electronics manufacturers might include the following. A pull test, for one, may be performed to ascertain at what force level a solder joint system will fail by pulling the component out of the joint, destroying the component without destroying the joint, or pulling the component, joint and pad off the board. It is a test to failure. The best outcome would be for the component, joint and pad to pull off the board as the entire joint assembly would then be stronger than the underlying board. The worst outcome would be for the component to pull out of the solder joint. No pass/fail criteria exist for this test, so one is left with the theory that the greater the force exerted to cause failure, the stronger the solder joint. Likewise, a drop test may be performed to determine whether joint failure could occur when a board is dropped from a predetermined height. Thermal cycling with subsequent tests for shock and vibration are routinely performed on Sn63. Thermal cycling provides accelerated aging information useful in predicting joint I reliability. Each solder manufacturer shows test results on their alloys. One thousand thermal cycles (accelerated aging) is an acceptable number of cycles, which loosely represents about 3.5 years of PCB service. The boards are then tested for shock and vibration, again to provide joint reliability data. Testing of the intermetallic layer is beneficial, since an inadequate intermetallic thickness may indicate insufficient bond formation between the component lead and solder, and the solder and pad. An either very thin or excessively thick intermetallic layer connotes greater solder attachment failure potential. A desired inter-metallic thickness is 1 to 5 µm. Over time, the intermetallic layer may grow. Thermal cycling should show accelerated aging growth of the intermetallic thickness. Too great an intermetallic thickness is detrimental to joint reliability because the joint may become more brittle. Shear testing may be performed to see at what force a joint component can be “sheared”, thus indicating joint strength. It is thought that the harder the solder alloy, the greater the shear force necessary to shear the component. Since lead is malleable and SnPb solder is softer than Pb-free solder, it stands to reason Pb-free solders would have higher shear strength than SnPb solders.Other studies have confirmed this. 1 However, an examination of IPC standards (IPC-A-610D, J-STD-001, etc.) failed to turn up any performance standards (or pass/fail criteria) for pull, drop or shear testing. The most that can be said about a circuitsassembly.com 24 Circuits Assembly DECEMBER 2007 http://circuitsassembly.com

Table of Contents Feed for the Digital Edition of Circuits Assembly - December 2007

Circuits Assembly - December 2007 Contents Caveat Lector Industry News Market Watch Talking Heads Global Sourcing Screen Printing Better Manufacturing Metalization Options for COB Assembly A Test Comparison of SAC and Non-SAC Pb-Free Solders An A-to-Z Guide to X-Ray Inspection BEST: A ‘Funky Chicken’ with an EMS Niche Tech Tips Wave Soldering Process Doctor Pb-Free Lessons Learned Getting Lean Eastern Approaches Product Spotlight Ad Index Assembly Insider Technical Abstracts

Circuits Assembly - December 2007

For optimal viewing of this digital publication, please enable JavaScript and then refresh the page. If you would like to try to load the digital publication without using Flash Player detection, please click here.