SMT007 Magazine

SMT007-Apr2020

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APRIL 2020 I SMT007 MAGAZINE 77 The reflow profiles on most of the tech data- sheets, and the correlating analytical data, are based on bare test boards that don't represent the thermal mass or complexity of your prod- uct. When determining the profile parameters, you must use an actual assembly—most often, a first article or prototype unit. The thermo- couples need to be placed at locations with the highest areas of thermal mass and areas with components that are difficult to process like QFNs and other bottom-terminated com- ponents. Another good idea is to keep up on machine maintenance. If the exhaust ducts are clogged, the airflow will be restricted, and flux activa- tors won't be drawn out of the machine and can collect on the inner surfaces. When enough of this material collects, it can drip back onto the surface of the board in a form that is basi- cally pure activator and extremely conductive and corrosive. Now, we move on to the plated through-hole process, where applicable, of course. Wave sol- dering can be the dirtiest of all solder processes, and this is normally because of the liquid flux application process. The standard process is an indexing spray flux at the opening of a wave solder machine. The problems we see with this process is excess flux being drawn to the top side of the assembly through the vias and up the part leads. The issue with this is that the flux may or may not reach the required tem- perature to render it near benign as designed by the manufacturer. This is especially a prob- lem if a selective solder pallet is used. When flux is drawn into areas that are designed to keep flux out, it is shielded by the pallet from the proper temperatures. The same can be said for other types of selective solder using robotics or humans. Humans are nor- mally the worst of the worst when it comes to causing electrical leakage and electrochemical migration. This is usually associated with the use of a bottle of liquid flux for hand solder of PTH parts. Generally, extra flux will make a nice shiny solder joint, but it will also leave a lot of active flux residues causing issues. The next assembly category is what I con- sider most of the miscellaneous processes that aren't standard. This is where the conformal coating, various epoxies for staking, under- fill, etc., are applied. Each of these processes have their own inherent problems, and the preceding soldering processes can have a huge impact if not properly done. Excessive flux res- idues will impact the adhesion of any subse- quent application on top of the electrical leak- age issues. Then, the parts need to be installed into their housings in the case of facilities doing a full box build or into boxes for shipping to the next sup- plier. Yes, even this process can impart enough contamination and handling residues to facili- tate the dreaded dendrite growth. The simple tip I have for this process is to be sure to only handle the assemblies by the edges. I assume you've always only handled them by the edges, right? RIGHT? The other thing to keep at the top of your mind is to only use new, or prov- ably clean, ESD bags for packaging. Those are just a few simple things that can be done to help reduce the number of detri- mental residues on your assemblies, and none of them cost a dime. I do apologize if you came to my column this month, hoping to see some detailed dive on the implementation of a digi- tal factory. I'd have been interested in reading what I had to say about that as well. Maybe next time this topic rolls around. SMT007 Eric Camden is a lead investiga- tor at Foresite Inc. To read past columns or contact Camden, click here. Excessive flux residues will impact the adhesion of any subsequent application on top of the electrical leakage issues.

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