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DECEMBER 2018 I SMT007 MAGAZINE 89 ic acids and give false results. The test runs for no less than 72 hours with most running it to 168 hours. Measurements are taken every 20 minutes to monitor any effect the elevated heat and humidity have when combined with the residues present on the sample. Much like the IC test, the idea is to determine if the res- idues present will facilitate electrical leakage and/or electrochemical migration. The accep- tance criteria details for this test are found in the J-STD-004 standard—section While SIR is a good test to look at the assem- bly house's ability to process the chosen set of materials with their equipment, it does not necessarily translate to the actual product being built. For that test, I recommend temper- ature, humidity, and bias (THB) testing. This test is designed to accelerate electrical leakage or dendrite growth, particularly on device die surfaces that would never be tested with stan- dard SIR testing. One standard set of parame- ters is 85°C and 85% RH for up to 1,000 hours, but that can vary. THB testing is done on actual assemblies and not test coupons, which means there are no comb patterns to judge the results and you will use active components instead of dieless. There isn't a related IPC test method for this, but it is commonly done in many sectors of the industry. You will need to create a test fixture to apply normal operating power and cycles with a monitor of expected feedback for a mea- surement point. While there isn't an associat- ed IPC test method, it is certainly a good idea for first article assemblies. THB testing is done on actual assemblies and not test coupons, which means there are no comb patterns to judge the results and you will use active components instead of dieless. handling, etc. When you know the process or material that is causing elevated ionic res- idues, you can then address that specific pro- cess and optimize it to a point where it leaves minimal active residues. The standard extrac- tion method is usually done on a full board where you must calculate the total surface area of the sample to include a population factor of 10% if components are present. This means the results are an average of contamination if they were evenly spread across the entire sam- ple—front and back. While this information is good, it leaves a few questions unanswered. There is a grow- ing understanding in the industry that to bet- ter determine the risk of electrical leakage or other contamination related issues, you need to look at much smaller areas of the sample. This allows you to look at specific processes like wave solder, hand solder, localized clean- ing effectiveness, etc. This type of extraction can be done in several ways—both manual and automated. When the IC is processed on the effluent, the number is much more mean- ingful than an average cleanliness number. When elevated ionics are found after a local- ized extraction process, you can then go back and compare to data from the PC fabrication, component, or flux residues. From my perspec- tive, IC analysis is generally the best tool to use for determining the possible impact processing residues will have on reliability. The next test method I want to highlight is TM-650 Surface Insulation Resis- tance (SIR). This test is used for material and process qualification utilizing unpopulated test coupons with known spacing comb pat- terns in an elevated heat (40°C) and humidi- ty (90%) chamber with constant voltage appli- cation. The test method was originally written to use the B-24 test boards, but in the past few years, more and more companies are using IPC B-52 test boards. This is a much better option because it incorporates components common- ly used in today's manufacturing. There is another SIR test option—TM-650—which is similar but uses test param- eters of 85°C and 85% RH, which can alter the chemical composition of weak organ-

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