What is HASL LF AND HASL in PCB?

Hot Air Solder Leveling (HASL) is the most common type of PCB surface finish used in the industry. HASL finishes are composed of solder, with proportions of approximately 63% tin and 37% lead, commonly referred to as a 60/40 split. The process for applying this finish is begun by dipping the circuit board into a molten pot of the tin/lead alloy after the solder mask has been applied. Next, a Hot Air Leveler (HAL) removes the excess solder,using hot air knives to leave behind only the thinnest possible layer. This remaining layer of solder protects the traces underneath it from corrosion, while easing the task of soldering components to the board by pre-tinning the whole pad. HASL is a very cost-effective surface finish compared to other types of finishes and thus is considered a great choice for general purpose boards.

Lead-Free Hot Air Solder Leveling (LF-HASL) is similar to HASL in appearance and usage; however, the solder, in this case, contains a mix of 99.3% Tin and 0.6% Copper. This alloy results in a higher melting point for lead-free solder, when compared with leaded solder. LF-HASL is a replacement for leaded solder, used when a lead-free or RoHS compliant PCB is required. Please note that a laminate with higher temperature tolerance is needed for applying this finish; otherwise, the process is identical. 

In the past, HASL was one of the most popular surface finish choices due to its qualities as a low cost and robust solution. See below figure 1,Recent fundamental changes in the PCB industry, such as new, more complex surface mount technology (SMT), have revealed HASL’s shortcomings. HASL is not suitable for use with SMT due to uneven surfaces not being compatible with fine pitch components. Recently, lead-free LF-HASL became available, but now there are other lead-free options more suitable for a high-reliability product.

Figure 1: Example of a HASL Surface Finish