Preheat problems with mixed technology process

In our process we are running a mixed technology line that includes both SMT and thru-hole components. The board contains many power transistors on heat sinks and electrolitic capacitors. We are having difficulty:

Getting a good solder fillet on our power transistor leads and also on our heat sink post.
Setting our preheat due to heat sinks and bulk capacitor labels and wires.

What can you recommend in determining what the preheat should be? Our current preheat is 3 stage force convection at the bottom and 1 IR on the 3rd zone only.

Anthony Kaw Boon

Expert Panel Responses

There are many factors in the wave process that can affect the soldering of components. I doubt that preheat is the issue here but the normal preheat temperature measured on the top of the laminate should be 210 - 230 degres F. Most likely you have a solderability and thermal issue as well the type of flux you are using. The solderability of power transistor leads and heat sink posts are not at the top of the list for components and also present a thermal drain that slows the wetting process. You may need to increase your dwell time at the wave, increase the amount of flux on the board or change to a more active flux or even pre-tin the components that present a solderability problem. It should also be noted that if you are getting 75% hole fill and the product is class 1 or 2 then you meet the IPC requirements for a good solder joint.

Greg Hueste
Senior Applications Engineer
Speedline Technologies

Greg joined Electrovert in February 1984. Based out of the Electrovert applications laboratory in Camdenton Missouri, Greg has been in the process applications support role since 2000. His primary responsibilities include providing process and machine applications support for the wave soldering lines as well as process, machine and operations training. He also provides applications support for the reflow and cleaner lines. Greg is a PBET certified trainer and holds two patents on wave solder nozzle design.

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