In the past ten years, the number of through-hole components on printed circuit boards (PCBs) has dropped significantly. The miniaturization of electronic products has led to fewer and fewer applications of through-hole technology (THT), and the lead pitch of through-hole components is getting finer. Therefore, the soldering of these components has also changed from wave soldering to point-to-point selective soldering. When surface mount components (SMD) are located very close to through-hole (THT) components on the PCB layout, soldering these small, fine-pitch components is a challenge. This research was done in collaboration with a large automotive EMS customer, and it defined the process window for through-hole soldering of fine-pitch components. It determines the feasibility of welding and defines the layout design parameters, making it possible to weld on components with SMD and other components.
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Point-to-point selective soldering has three main goals: good through-hole filling, no bridges, and minimal solder balls. There are many parameters in the circuit board design that will affect the welding effect. The diameter of the through hole is related to the lead size of the component. If the diameter of the through hole is too small, the lead cannot be inserted smoothly and may be damaged. When the diameter of the through hole is too large, the gap is too large to hold the solder, and the solder may flow out.
The bridge accepts the influence of pad and via diameter, lead extension length, lead diameter/size, and lead pitch. The smaller the pitch, the greater the challenge to achieve bridge-free welding. Some leads are round and some are square. Whether the design will produce bridging depends on the capillary action of the solder. This is the ability of the solder to flow in a narrow space with the help of external forces such as gravity. The properties of solder, such as temperature and fluidity, have an effect on capillary action.
Tin ball is another challenge. In any case, if the solder ball does not adhere to the solder mask when leaving the solder wave, this problem can basically be eliminated. Choosing the best solder mask is the best solution to make the circuit board design more robust.
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