Frequently Asked Questions
Work on lap joints indicates that for tin/lead solder (60/40), the ideal joint thickness for maximum shear strength is 75-100 microns. This range was established using practical tests and gives joint strengths that are higher than those expected from the solder alloy in isolation. In very narrow gaps, the trapping of fluxes and voids reduces the wetting area of the joint, and thus reduces the strength. Wider gaps overcome this particular problem but suffer from reduced wetting because the capillary action of the solder, which fills the joint gap, is reduced. This means that the shear strength of wider joints tends to be similar to that of the solder alloy itself.
The temperature at which the joint is made also has an effect, higher temperatures tending to decrease the recommended joint thickness for maximum strength. This is because higher temperatures increase the molten alloy and flux mobilities between the joint surfaces, leading to a reduction in the trapping of fluxes and voids. This effect occurs until a temperature is reached where increased alloy oxidation overcomes flux activity.
Further information
Some information is only available to TWI Industrial Members.
Best practice guides for the electronics and electrical industries:
Soldering - processes
Soldering - process control
Soldering - materials and design
