The specific reason of PCB pad refusing to solder is PCB pad pollution or oxidation. The temperature is not enough during SMT or wave soldering. Solder paste for SMT or flux for wave soldering is invalid. The solder paste used in SMT has poor compatibility with PCB.
There are many reasons for poor welding. You can find a bad physical board of a serious PC to do EDX analysis to see what components are on the surface of the bad tin pad. If there are other factors besides PCB and SMT production, then look for the source from other elements.
Common pad:
Square pad-it is often used when there are many components on the printed board and the printed circuit is simple. This kind of pad is easy to realize when making PCB by hand.
Circular pad-widely used in single-sided and double-sided printed boards with regularly arranged components. If the density of the board allows, the pad can be larger to avoid falling off during welding.
Island pad-the connecting line between the pad and the pad is integral. Often used for vertical irregular arrangement installation. For example, such pads are often used in tape recorders.
Polygonal pad-used to distinguish pads with similar outer diameters but different apertures, which is convenient for processing and assembly.
Oval pad-This pad has enough area to enhance the anti-stripping ability, and is often used for dual in-line devices.
Torx pads are usually used where large vias are grounded. There are several reasons for this design:
The fixing holes need to be metallized and connected to GND. If the fixed hole is fully metallized, it is easy to block the hole during reflow soldering.
The use of internal metal screw holes may lead to poor grounding due to installation or repeated disassembly. The plum blossom hole pad can ensure good grounding no matter how the stress changes.
Extended data:
Pads and pads in PCB pads can often be used alternately; Functionally, however, for surface mountable components, pads are two-dimensional surface features, while for pluggable components, pads are three-dimensional features.
Generally speaking, pads do not include plated through holes. Bypass holes are plated through holes connecting different circuit layers. Blind bypass hole is a bypass hole buried by connecting the outermost layer with one or more inner layers, and only connects the inner layers.
As mentioned earlier, pads generally do not include plated through holes (PTH). PTH in the pad will take away a lot of solder during the welding process, and in many cases, there will be solder joints with insufficient solder.
References:
Baidu encyclopedia -pad