Flux Residues In Lead Free Wave Soldeing Process

Hi Samir

I totally agree with you on the customer issue. We have all of them. Where we have free hands we supose to use the flux as specified (one fits all) by our technical guy at the head office in europe. I realy start to question the desicion since we have problems with more than one board. Sure will be digging in the archive. Thanks for all the tips.

Smartasp

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Samir - You and I need to go have a beer.

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Gentlemen I have managed to pull myself together after banging my head. I am not sure the flux volume is the total solution here. All spray processes deposit flux in a similar manner, that is to say the vast majority of material sprayed onto the PCB surface is not sprayed directly on the centerline of a PTH. If this were the case topside wetting would not be a problem, there would be ample flux on the topside of the PCB to obtain picture perfect fillets. As the spray head traverses the PCB the flux is forced into an inverted cone or v shaped deposit pattern. The flux is then impinged into the hole at a moderately acute angle and may only wet the bottom part of the PTH. You can tape a sheet of thermal paper over the top of the PCB and run it through your fluxer be sure to leave the pre-heaters and solder pot off, the flux that reaches the paper will cause a reaction that turns the paper black. This will allow you to fine tune your spray pattern to get the most even flux deposition and may allow you to reduce the volume you use and make your customer happy. Most fluxes are loaded with surfactants that allow them to wet nearly any surface, if the flux is applied in proper volume it will wet to the top of the PTH if given time and provide good filleting. This wetting depends also on the condition of the flux, if you are using a VOC flux you may not want to use forced convection preheats this will drive off the carriers (alcohol) and limit the spread of flux up the barrel. If you are using a VOC free material run your first preheater as low as possible without sacrificing your profile. You may also want to try a faster conveyor speed, remember you only have 1 to 2 percent activators in your flux. These are usually dicarboxilic acids, usually succinic or adipic and the melt at approximately 150 to 189� C. Once these temperatures are reached the flux will complex very quickly and become benign. If this happens prior to the top fillet being formed you will not form the fillet. Oxidation rates double with every 10� C rise in temperature and at or near soldering temps the metal surfaces will reoxidize very quickly. Using a chip wave can be extremely hard on the flux get it through the chip and into the laminar wave as quickly as possible and you may see better results. If this product does not have bottom side SMT parts turn off your chip altogher. Hope this helps.

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