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88 The PCB Magazine • February 2017 actual inspection. For reference, to help guide you, documents such as IPC-A-600: Acceptability of Printed Boards [1] and IPC-6012: Qualification and Performance Specification for Rigid Printed Boards [2] , are good guides for the good and the bad associated with printed circuit board con- structions. Definitions of various industry terms are contained within, along with visual examples of acceptable and unacceptable structures. A few additional tests that can be performed on incoming printed circuit board product in- clude solderability and thermal stress analyses. Solderability testing is just as it sounds and is performed to ensure that the metallic surfaces on a printed circuit board will wet with molten solder under standard test conditions. The test involves a commonly available soldering flux and a molten solder pot. A simple confirmation of whether the board can pass a solderability test, such as one called out in IPC-J-STD-003: Solderability Tests for Printed Boards [3] , is a good place to start. Thermal stress testing is also just as it sounds. A printed circuit board specimen is ex- posed to heat cycles via a molten solder pot to determine whether the board (and all its con- nections) can withstand the potential solder re- flow cycles that the board might see during the assembly process yet to come. After exposure of the test sample to the solder reflow cycles, cross- sectional samples are prepared, as described above when we discussed microsection analy- sis, for another evaluation of the internal board structures—this one after a stress event. For a guideline, IPC-TM-650, method 2.6.8: Thermal Stress, Plated-Through Holes [4] is a commonly utilized test for thermal stress analysis. So far, the testing mentioned in this column has dealt mostly with the "structures" associ- ated with printed circuit boards—traces, plated through-holes, via holes, etc. Although these aspects of printed circuit boards are clearly im- portant and relevant, there is another aspect of the board's construction that is equally as im- portant, yet commonly overlooked. That aspect relates to the material which is used to physi- cally construct the board, and its "structures"— the laminate. When designing and developing a printed circuit board, a laminate material is called out on the board's drawing. The material is chosen for various reasons, such as glass tran- sition temperature, thermal expansion capa- bilities, and delamination (or more specifically, lack-of-delamination) properties, just to name a few. To investigate these types of properties, one must call upon the use of thermal analysis test equipment. Of specific interest would be a differential scanning calorimeter (DSC) and a thermomechanical analyzer (TMA). A DSC measures heat flow in or out of a test specimen as a function of time or temperature. Determining the glass transition temperature (Tg) of a board's laminate material is a com - monly monitored material parameter, as the Tg value is an important factor when selecting the laminate material for the board's construc- tion, due to the reflow profile that will be used during the board' s assembly process. At the same time, degree of cure—a property which sheds light on whether a printed circuit board was cured properly—is also commonly investi- gated as an incompletely cured board can lead to board population issues during the assem- bly process. As a guideline, one can use IPC- TM-650, method 2.4.25: Glass Transition Tem- perature and Cure Factor by DSC [5] if interested in these properties. A TMA measures distance change of a test specimen as a function of time or temperature. Although TMA can also be used to determine Tg, like DSC, it's more generally used to mea- sure thermal expansion-related properties. Like the DSC testing described above, board design- ers also use thermal expansion properties of the board when designing the board's layout and when choosing the materials for the board's construction. A commonly used test method " A few additional tests that can be performed on incoming printed circuit board product include solderability and thermal stress analyses. " HOW STRONG IS YOUR FOUNDATION?