I have a Class 3 medical customer where we are seeing solder balls adjacent to the fine pitch device leads post reflow. We are processing these with OA so small amounts of solder balls usually wash of at clean. These however, are being mechanically trapped along the edge of the mask and are present after wash. This customer has several different board types and we have similar problems on all. My question is: Is here a specific solder mask that is prone to this sort of problem? If not, how can I tell if the cure is correct on the mask?
Knock off some balls and you may find it has a flat bottom to it were it has mechanically stuck to the solder resist usually seen under 60X magnification. Common with Liquid photoimageable resist (I know again) Best way to check resist cure is to pay for chemical extraction with IR analysis unfortunately. Some Stencil cleaner wipes may give you an indication all is not well if you rub the surface of the resist and get discolouration off.But is not a fault free test. Methylene Chloride is no good as these resist are designed to withstand these harsh chemicals but will not withstand most stencil cleaning solvents used such as methoxy-2-propanol, no way near as strong as Methylene Chloride but is used to thin resist in its application so has a good chance of showing a discolouration it if not properly cured etc. Hope it helps. Thanks Greg BLT Circuit Services Ltd England
MAke a new stencile and have the aperatures reduced by 20% in X and Y. This has solved 90% of all solder balling issues I have ever dealt with. Be careful with your fine pitch and micro-BGA's. If you are not having problems with fine pitch and 1:1 aperature, you may want to leave those alone.