There are generally 2 main types of through hole components - axial leaded through hole components and dual in line package components. Sometimes, these through hole components may fail to solder correctly down onto the PCB even soldered under the suggested soldering temperature.
There are a number of reasons why a good solder joint can fail to be made on the PCB. Here are some common problems that we may sometime miss.
Component’s lead temperature is too cold and cannot form a good solder join.
Some components have larger thermal mass, which absorbs heat energy from its leads into the large resistor elements inside the molded package. If the leads do not get enough heat from exposure, the leads near the package will still be too cold. It will fail to activate the flux and form a good solder join.
The below figure shows MP900 and MP9000 series Kool-Pak power film resistors from Caddock with larger exposed mounting surfaces.
To solder this kind of components, you should increase the dwell time of the soldering wave, or increase the pre-heating stage of the components prior to soldering.
Through-Hole Dimensions on the PCB are insufficient (too small)
Solder requires space to flow up the leads of a through-hole component, through the PCB mounting hole and to the top of the PCB. If the PCB mounting hole for your through-hole component is too small compared to the lead you’re attempting to solder to, then solder cannot flow properly and you’ll get a bad solder joint. Engineers should be aware of the PCB through-hole dimensions on their board and check the manufacturer’s recommendations.
Physical damage to the solder joint after soldering
It’s hard to classify this as “soldering failure”, but sometimes a physical stress is carelessly applied to the solder joint. After soldering, some processes may cause physical stress and fracturing, such as installing a heat sink on the surface, or installing unfitted unfitted plastic case molding. This may cause damage to the solder joint, which some engineers may mis-understand as “soldering failure”. This isn’t a failure of your soldering process - it’s a failure in the overall assembly process. if you see damage like this, you should check other steps of your process that involve physical stresses or manhandling of the boards.