Application of EDS Technique for OSP Film Thickness Measurement

Nowadays, there are numbers of pad finishing types in market for ball grid array package (BGA). OSP (organic solderability preservatives) pad finishing technology is one of the most widely used for the purpose of increasing the joint strength and achieving more excellence in heat-resistance. Especially, to develop the ICs products to have compatibility with non-clean soldering fluxes and solder paste as well as lead-free solder, OSP becomes one most suggested. Spansion with the commitment to customer requirements, has to implement OSP pad in our products i.e. FBGA packages. OSP pad is one of pad finishing technologies that Cu pad is coated with the thin film of a kind of organic material to protect Cu from oxidation during storage and many processes in IC manufacturing. The thickness the OSP film coated over Cu layer is in order of micrometer. Normally, during solder ball attachment process OSP will be cleaned out by soldering flux and heat to allow Cu to expose and attach to solder ball. The point is that if the thickness of OSP is too thin (less than 0.12 micron), the film cannot prevent copper from oxidation during storage and heat cycles. On the other hand, if the OSP film is too thick (more than 0.4 micron), OSP will not be perfectly cleaned out by flux during soldering reflow process. Hence this will bring to the bad solder joint because the presence of some OSP residue can result in bad solder wetting. This can affect on the attachment of solder ball and finally lead to significantly weak solder joint. Therefore, thickness of OSP film is one of the most critical factors that we have to consider and control in order to achieve desirable joint strength. Without OSP thickness information we will be unable to be confident in the quality of our product and unable to control incoming quality of substrate from supplier site. So, it is highly necessary to measure the OSP thickness or at least for monitoring and analysis purposes