Selecting an EMS Company for Aerospace Products December, 2017


www.us- tech.com Continued from previous page


failure can stem from are cleaning and soldering. Washing a PCB after the soldering process


typically produces a board that sparkles and looks ready for its journey to the end user. However, ar- eas that are not visible contain the details that spoil this sparkling picture. For example, a board with hidden residual flux contamination may pass QC and operate properly. In its operating environ- ment, there may be high humidity and tempera- ture changes that generate condensation, causing residual flux to grow. This can eventually result in leakage paths


that ultimately cause failure. High-impedance, high-power, analog, and power-on-board circuits in today’s micropower electronics are even easier to disrupt with stray voltage sources. IPC, the global trade association serving the


printed board and electronics assembly indus- tries, has a task group devoted to ad- dressing the cleanliness levels of un- populated (bare) PCBs, and has es- tablished a base standard for cleanli- ness. The standard has acceptable contamination levels based on the end application. High-reliability boards, such as those used in mili- tary/aerospace applications, require more stringent levels of purity than do circuit boards for general electron- ics use. The IPC-TM-650 standard sets


an acceptable range of 65 to 2 micro- grams/in.2 of sodium chloride (NaCl) for general applications, but 10 to 2 micrograms/in.2 for military applica- tions. Is this standard enough to pre- vent failures and can present clean- ing methods truly clean the boards that are being produced today? Digicom Electronics, an EMS


company based in Oakland, Califor- nia, spent two years researching cleaning methods, processes and equipment. The company found that it is not enough to put the boards through a wash cycle. As a result, it implemented a highly-efficient, in- line cleaning system aimed at miti- gating failures caused by contaminat- ed PCBs. This innovative process uses a


special combination of chemicals, temperature, wash cycles, timing, and equipment that turns out clean, high-quality PCBs. Deionized water from polishing tanks is used and re- cycled. Filters catch the solids while powerful blowers ensure that harsh chemicals are isolated and confined in a dedicated wash area. A refrac- tometer checks the stability of the tank to make sure it’s not compro- mised. The process is completely “green.” All drained solids are envi- ronmentally friendly.


Nitrogen and Soldering Many major failures result from


a weakness in the solder joint that connects the wirebond to the PCB or the solder connecting the device or package to the board. While using ni- trogen is not an absolute require- ment, nitrogen can help to strength- en the bond and improve solder adhe- sion.


Nitrogen improves soldering


quality by reducing oxide formation and creating higher surface tension across the solder surface. The wetted surface area can increase by 20 to 30 percent, creating a stronger joint. Higher surface tension minimizes solder balling when using fine-pitch solder paste, and greater surface ten- sion can hold larger and heavier com- ponents on the bottom of a PCB when performing double-sided reflow. Less oxidation improves the wetting on nickel-gold (NiAu) and


PCB before and after Digicom’s cleaning process.


bare copper solder leads. Nitrogen also improves the visual appearance of the solder joint surface, leaving it smoother and shinier, especially for


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lead-free assemblies, simplifying inspection and rework. In reflow soldering, flux activity, residues and


cleanliness are all significant. According to data cited by TEquipment.net, longitudinal studies pro- vide enough reliable data to confirm that all these factors are improved by using nitrogen. Rework de- fects were monitored over a two-year period, one year prior to a switch to nitrogen reflow and one year after the switch was completed. With the in- troduction of nitrogen being the only significant change, the proportion of defective joints fell from 82 to 37 DPMO, a decrease of nearly half. Other operations have shown improvements in first-pass yield from 5 to 7 percent, which translates into a reduction in defect levels of 50 to 60 percent. Many manufacturers of reflow and selective soldering equipment incorporate the capability of


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