The request to define dewetting on surface mount pads is a good one. I frequently become involved in discussions related 
to the acceptability of solder on smt pads. Basically, how flat is flat and what exactly is the requirement. It has been my experience that  when using a vertical hot air level system, the solder level finish seems to be acceptable / usable  by most customers. However,  eventually you will have one customer that just cant seem to use the parts you are supplying because they are not flat or even enough.  This can lead to  debates as to are the parts in spec or not.  Because I have not really come across any clearly defined requirements ( photos would be nice) I have found it difficult to resolve these types of  issues to  the satisfaction of all concerned.  Rarely does the customer provide a defined objective requirement. IPC states "coverage and solderable" but that typically means two different things to person building the board and the person assembling the board. The answer to the problem is simple but sometimes costly.  A board supplier can either provide products with a surface acceptable to the buyer or not ... Regardless of specifications!  It is best if the issue of solder uniformity and flatness on smt pads are specifically addressed before product is ran. After the fact Interpretation of current specifications will most of the time end up in a dissatisfied customer who thinks you cant build a board right and a board supplier who believe the customer is too picky and must have assembly problems since no one else seems to have problems with their boards..... sound familiar to anyone?????


Just my two cents

Ed Cosper
GEI Circuits Inc. 

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