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Solder Short
Wave soldering devices below 0.050" pitch should be avoided and questioned during Design for Manufacture reviews of new designs. Yes, it can be done but it takes a lot more effort from machine and process engineers. Soldering 0.032" pitch can be achieved, 0.025" is problematic and using 0.020" on the base of the board requires more rework staff. This example of shorting is seen at the top of the pins on a QFP device and may be improved by increasing the solid of the flux. It is often caused by incorrect pre heat or limited time in the wave. The thermal effect of the device can tend to cool the solder, slowing down drainage. On some occasions when the shorts are at the top of the lead form it is a solderability issue. If a lead area close to the plastic body is slow to wet it is also slow to drain, hence a short. As with SOIC devices QFPs benefit from solder thiefs on the trailing edge of the parts but only if the solder shorts are always on the last two pins. The solder thief pad should always be a minimum of three times the length of the last pad and on the same pitch. With QFPs the device is also positioned at 45odeg to the direction of travel through the wave. Try glueing some components to the base of a glass plate; this will assist you convince other engineers and design engineers that design for manufacture is a must.