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SMT-Sept2017

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32 SMT Magazine • September 2017 in a sealed bag with moisture still on it for more than a few hours, you have the makings of some re- liability problems. If subsequent operations involve the applica- tion of coating, epoxy, encapsu- lation or others, moisture on the board can impede the process of curing. Moisture can also be a problem when you need to per- form an electrical test as it will skew the test results, especially in high-voltage or high-frequen- cy assemblies. The thermal profile of the wash process will impact the de- gree to which the PCBs will ab- sorb water. Normal process pro- files are such that the temper- ature of the PCB increases as it goes through the wash system process. This temperature in- crease leads to molecular expan- sion, which makes the board more hydrophobic, or less like- ly to absorb moisture. It is criti- cal to profile your wash systems such that board temperature ris- es steadily throughout the process by a few de- grees. Overbaking of PCBs after cleaning, post re- work, may cause subsequent PCB assembly de- fects. Heating of the boards will increase the rate of oxidation. You do not want to contin- ue to keep re-baking the PCBs as this oxidation may cause a problem in the wetting action of the solder. How to Measure PCB Water Absorption Electronic components, and in some cases PCB materials, are absorbing moisture in the re- work process. A good way to monitor the mois- ture level is by measuring the weight of the board before and post drying. Weigh a small board or sample coupon using a precision bal- ance. Follow this with a pre-bake of the board for two hours at 100°C, allowing it to cool. This is followed by re-weighing the parts, and noting the weight difference. You can use this weight difference to optimize pre-bake temperatures and times. In a thorough drying application, the assemblies should weigh less after cleaning than before cleaning. You need to allow enough time for any of the moisture adsorbed in the inner board layers to make their way out of the PCB. Depending on the board thickness, this can be more than four hours. Since you want the moisture (and other solvents, possibly) to escape, the best way to do this is to have the boards in a rack, verti- cally oriented with some space in between the PCBs. If they are stacked on top of one another or flat on the base of the oven, etc., then it can be more difficult for the moisture to escape. Once you have dialed in the drying process, make sure you keep the board dry by either put- ting it into a sealed moisture barrier bag or into a dry box. Make sure you seal the moisture bar- rier bag. If you don't vacuum seal your bag, you cannot count on them maintaining a less than 10% relative humidity inside the bag, regardless of how much desiccant you put in there. DRYING BOARDS AFTER REWORK CLEANING—TO DO OR NOT TO DO? Figure 1: Proper MSD handling prevents defects like this popcorned BGA.

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