Solder leveled - Hot Air Solder Leveled (HASL)
17 November 2008
The solder leveling process became popular in the early 80s and was still the most commonly specified finish for surface mount boards in the late 90s and early 2000.
Eliminating the solder coating under the solder resist reduced the possibility of the resist lifting during the assembly and soldering operations. Using the original etch resist, tin/lead coating was still a popular process in the 90s. The introduction of hot air solder leveling provided a guaranteed solderable surface from the PCB manufacture. It also provided a further hidden benefit to the assembler of stressing the board. If the solder resist coating was poor or the lamination of a multilayer circuit was questionable then it would generally show up during exposure to the molten solder bath prior to shipment to the customer. As the coating process was conducted after solder mask/resist application, part of the process was also referred to as SMOBC (solder mask over bare copper). Originally the coating was more expensive than traditional tin/lead plated finish but this is not true today.
Introduction of the solder leveling process also eliminated significant mismatch between circuitry and the resist apertures. This was due to only limited temperature being applied to the laminate prior to resist application. With the tin/lead reflow processes the laminate is exposed to soldering temperatures which exceed the laminate's glass transition temperature. This causes expansion and contraction due to the stress in the laminate which is no longer held by the copper foil.
The leveling of PCB panels consists of a pre-clean, fluxing, hot air leveling, and a post-clean. Pre-cleaning is usually done with a micro-etch of the copper surface. The panels are either lowered into a solder bath vertically or transported horizontally. As the panels exit the solder air-knives ‘squeegee’ off the excess solder from the surface of the pads or out of the through holes. Key to successful leveling is making sure the supplier sets-up his process to get the correct coverage and this takes time to get the best, consistent coverage.
The quality and consistency of the solder leveling process could be far better than some of the examples in the industry. Often the variation on coatings is down to the time spent on setting up the process for different designs. The PCB industry could have done better even with vertical leveling systems as has been demonstrated by sub-contracting leveling services like CEMCO. The leveling process over the last 10 years has been demonstrating satisfactory coatings for lead-free in recent trials on tin/copper/nickel boards for the SMART Group workshops and production lines.
As a guide the following specification may be used or modified for solder coated printed boards:
If specified, all exposed outer copper surfaces shall be coated with solderable finish of tin/lead or lead-free on the surface of mounting pads, test points and plated through hole. The coating should provide a minimum of 12 months shelf life and meet the solderability requirements of the IPC or IEC standards using a wetting balance. The coating thickness should average be between 10 – 15um on specified pads. The coverage in the plated through hole should be with a minimum of 3um on the knee of the plated through hole. The solder leveling process should not affect the minimum hole size requirements.
The thickness of the coating should be tested using XRF measurement for thickness and coating composition. Regular solder samples should be taken to control the build up of copper in the solder bath which will affect the solder reflow temperature. Panels should not be leveled twice due to the possible impact on copper dissolution on through hole plating.
Assembly trails on different finishes were conducted by SMART Group to demonstrate the PPM Yield comparing solder leveled boards with other finishes. The results show the defects associated with finishes and is in-line with many other evaluations conducted over the years. As a basic guide, the solderable life for a correctly leveled board should exceed:
Tin/Lead reflowed 12 months minimum
Hot air levelled 12 months minimum
Lead-Free Hot air leveled 12 months minimum
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