Issue link: https://iconnect007.uberflip.com/i/457216
February 2015 • The PCB Magazine 25 The HASL Surface Finish This article will deal with the second point, HASL coatings onto electroless nickel surfaces. HASL is probably the oldest PCB surface finish that is still available today. This involves taking a fluxed copper track PCB panel and dipping it into a pot of molten solder for a short period of time as shown in Figure 2(a). Upon removal hot air knifes on either side of the PCB panel blow off the excess solder. The result is that the exposed copper is covered in a thin layer of sol - der ranging from 3 to 15 µm depending on the surface area and orientation of the PCB panel. An example of a HASL soldered PCB panel is shown in Figure 2(b). The HASL surface finish is recognized for being highly solderable, as the surface is normally very similar in composition to that of the solder being used. An example image of the cross-section is shown in Figure 4(a) where a thin 1–2 µm intermetallic layer has formed between the solder and the copper substrate. A New Technology From Existing Processes The HASLEN surface finish follows the same process, with the exception that a PCB panel that is coated in electroless nickel is used rather than copper. It is the unique solvation environment provided by these DES fluxes that enables the oxide removal from electroless nickel, meaning that this process is possible. The structure of a HASLEN finished PCB is demonstrated in Figure 3(a) where the copper has a 6–7 µm layer of electroless Ni that is used as standard in ENIG PCBs followed by a layer of sol- der that ranges from 3–15 µm in thickness. As the method of removal of the excess solder in HASL is physical (i.e., by air knifes) there is a thickness variation dependent on the size of the pad as well as orientation. Figure 3(b) shows an image of a PCB that has been prepared with a HASLEN coating. The surface pads show uniform coverage right across their whole surface area and in addi - tion both vias and plated through holes are coat- ed right through the whole thickness of the PCB. Figure 2: (a) diagram of hASL coating setup and (b) image of hASL coated PCB panel. HASLEN: A NEW HIGH-RELIABILITy SURFACE FINISH FOR PCBS continues feature