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PCB007-Aug2018

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AUGUST 2018 I PCB007 MAGAZINE 55 stated, "Circuit board manufacturers are ex- tremely hesitant at providing instructions on drying their circuit boards. Information from the ZVEI [2] should also be regarded critically. The cardinal problem is the high temperature which is recommended for tempering. If this is applied, the result is often delamination and distortion of the circuit boards. Corrosion and the formation of intermetallic phases of the metallic surfaces are also to be expected." The research investigated "whether gentle drying at 45°C or 60°C and at low relative hu- midity achieves the same result as tempering at high temperatures." They began first with QFP components which were saturated and then dried in seven different environments, referencing J-STD-033 standards in their re- port. These were charted in comparison of the various drying processes and are outlined in Figure 1. The researchers then selected four PCB types and repeated the same procedures of saturation then drying and weighing to 0.1% water weight. This was done using 60°C at <1%, 45°C at <1%, and 125°C at 5%. Their summary results were that "125°C demon - strates the shortest drying time; however, oxi- dation of the soldering pads and board warp- age make it unsuitable for the particular board types tested." These comparisons are charted in Figure 2. Conclusion Research by SMT & Hybrid GmbH (now SMT Elektronik) was conducted just prior to the original publication of IPC-1601, which now provides detailed guidelines for the packaging and storage of PCBs, both from the PCB manu- facturer and at the assembler's manufacturing floor. It also describes the solderabilty risks as- sociated with high-temperature baking. Ultra- low RH and low temperatures can significantly mitigate those risks while preventing moisture damage during reflow. Examples of such PCB damage are illustrated in Figure 3. PCB007 References G. Schubert, Th. Schonfeld, A. Friedrich, SMT & Hybrid Gmbh, "Drying Printed Circuit Boards," Production of Printed Circuit Boards and Systems, September 2009. Richtwerte/Empfehlung des ZVEI, Fachver- band der Leiterplattenindustrie. Rich Heimsch is director at Super Dry. Figure 2: Comparison of four PCB types at various drying conditions. Figure 3: In addition to extending the safe storage time, defects and damage such as popcorning and delamina- tion during the reflow process caused by moisture can be avoided.

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