Issue link: https://iconnect007.uberflip.com/i/1058015
88 SMT007 MAGAZINE I DECEMBER 2018 Some of the current test derivatives of mili- tary specifications and others are original test methods designed by professionals from differ- ent corners of our industry based on specific materials or processes. The test methods are the backbone of the J-STD series, which are the assembly documents of choice that cover bare boards, assemblies, flux, paste, solder, etc. If you have any questions about how to assem- ble a product, you can find a section in one of those standards; within that, you will more than likely find a reference to a test method. There are even standards that apply specifi- cally to space and automotive industries among others with demands outside of most electron- ics. These standards are used in most assem- bly houses around the world to ensure quality and reliability. On any print, you should find a reference to what specifications to build to and the order of precedent as agreed upon between user and supplier (AABUS). IPC specs are also typically called out. That is a normal default that in lieu of direct specifications from the customer, an assembler should use IPC stan- dards as assembly guidance documentation that will include tests from TM-650. The test methods I want to discuss in this col- umn are mostly related to cleanliness and dif- ferent ways to determine if the process is clean enough for the intended end-use environment. Knowing the effect of residual ionic content is among the most important data points when looking at reliability. It is so critical because the ionic content is directly related to electrical leakage and electrochemical migration-related issues in a normal field service environment. When you have the fire triangle—contamina- tion, available atmospheric moisture, and bias differential—the risk of electrical leakage is greatly increased. There are no industry limits regarding allowable amounts of ionic content determined by ion chromatography because there isn't a single set of cleanliness limits that apply to all electronics. It is important for each assembler to determine if the ionic content present will be detrimental to the field perfor- mance of your specific product. The single test used by the industry the lon- gest to test cleanliness is TM-650 2.3.25: Resis- tivity of Solvent Extract. The problem with using this test for acceptance is that it was designed in the 1970s and was never intend- ed to be used for this purpose. You can find a very detailed white paper—IPC WP-019— that describes the history of this test method and why it should no longer be used as it most commonly is today. In short, the product being built when this test was introduced did not use bottom-terminated surface-mounted parts using no-clean solder paste among dozens of other material differences. When addressing cleanliness, the most accurate method to determine the ionic con- tent is ion chromatography. The test method for that is TM-650 2.3.28: Ionic Analysis of Circuit Boards, Ion Chromatography Method. This method requires subjecting the sample to a mixture of isopropyl alcohol and deion- ized water at 80°C for one hour. This will bring surface contaminants that can become soluble into a solution that will be processed through the integrated circuit (IC) equipment. After an IC test is complete, you will have the exact type and amounts of each anion, cation, and a general weak organic acid total. Each of the materials that go into the assembly pro- cess will have a chemical signature that can be matched back to the IC results. This information is crucial in determining the risk of electrical leakage. If you find elevated levels of ionics, you can look at the IC results and compare them to flux activators, plating chemistries, wash signatures, bare board fab, If you have any questions about how to assemble a product, you can find a section in one of those standards; within that, you will more than likely find a reference to a test method.