PCBs manufacturing is a lengthy and complex process and on the other hand they demand reliability and usability. The manufactured PCB products are also cost sensitive and they are even costly when produced in small quantity. The reason is the manufacturing setup cost or the Non-recurring engineering cost that increases price when produced small quantity.
The PCBs after etching go through the PCB finishing stage. It is one of the very important manufacturing aspects of the finalized PCBs. In some case it is very important and has the prime importance that is dictated from the application of the assembly. It deals with the soldering and copper stickiness strength to the PCB resin layer. It protects the copper from oxidation and metal migration. There are different types of finishes and have own performance and cost factor. The process selected is a trade-off between performance, cost and application where PCB is to be installed.
In PCB industry, the PCB designer or product developer and PCB manufacturer both have very close concern to copper surface finishes. The designer requests this feature due to two main reasons: it protects the exposed copper tracks, vias, and pads from corrosion and oxidation, secondly, it forms a thin layer of solder over the copper that helps to make solder joint easy and stiff during the manual or automated soldering process. A PCB soldered without surface finishing is prone to air-bubble in the joint that later on may cause dry and broken joint. On the other hand an exposed copper PCB or without surface finish may cause the copper parts to be corroded and produce resistive contacts. The HASL process which stands for Hot Air Surface Leveling finish method is a process that consists of passing manufactured PCB through a tub of molten tin/ lead i.e., PbSn alloy. The alloy adheres to the PCB surfaces, the excessive solder alloy is removed through the air knives. An air knife is a hot and fast air blow that removes the excessive solder from the PCB surface. The advantages of HASL are it’s low cost, easy availability from the PCB manufacturer, increasing rework times and good shelf life comparative to other finishing methods. However, the disadvantages are uneven surface, poor quality for fine pitch devices, and inclined to solder bridging.
Fig. 1 shows the PCB finished using the HASL method, the Fig. 2, shows the PCB after dipped into the molten alloy of Sn/Pb. The Fig. 3, shows the different steps of HASL process.
Fig. 1:- Final fabricated PCB finished using HASL method
Fig. 2:- PCB dipped into molten Sn/tin alloy
Fig. 3:- HASL method block diagram that shows process after PCB etching (Step 1)
It is evident from the studies that a PCB finishing process should be finalized from product usage environment and capabilities of the manufacturer.