Ask the Experts
April 16, 2007 - Updated
July 4, 2007 - Originally Posted

HASL and solder mask process?

On older circuit boards where HASL was used to heavily plate the circuit board, how did they print the solder mask onto the unevenly plated surface?

Perry Babin

Expert Panel Responses

HASL was a process to apply solder to the exposed copper on the boards, especially the pad areas and plated through holes. Prior to HASL, hot air leveling, there was a process called electroplated tin/lead which was used as an etch resist during the board fabrication process. Once the board was etched the electroplated tin/lead coating was fused, or stated differently was reflowed. It was after this point in the process when the soldermask was applied to the board. This was the standard board process for plated through hole technology. When surface mount came about in the early 1980s and these boards were used, it was the first time that the entire board was subjected to the surface mount reflow processes of the time, which could have been anything from IR ovens, vapor phase systems, Convection ovens, etc.. It was also discovered that the solder expanded under molten conditions and the when the second side of the product was prepared for surface mount component assembly, solder balls were discovered all over the surfaces of the solder mask, especially over any of the conductor traces. It was at this time that the board fabrication process was changed to remove the tin/lead etch resist materials and leave the board simply as a copper board. The soldermask was then applied over the bare copper, thereby eliminating the problems with solder balls migrating through the soldermask and interfering with the past deposition process on the second side of which was being assembled. After the soldermask was applied the board was then processed through a HOT Air Solder Leveling process coating all the exposed copper with solder, which became known as SMOBC, Soldermask over bard copper.

Leo Lambert
Vice President, Technical Director
EPTAC Corporation
At EPTAC Corporation, Mr. Lambert oversees content of course offerings, IPC Certification programs and provides customers with expert consultation in electronics manufacturing, including RoHS/WEEE and lead free issues. Leo is also the IPC General Chairman for the Assembly/Joining Process Committee.

If solder mask is applied over HASl it will fail because the solder will re-melt under the mask during reflow or wave and destroy the solder mask. The normal method is to solder mask all parts of the board that does not require solder and then do the HASL process. This is refered to as solder mask over bare copper (SMOBC). I have seen boards that were HASl'ed then solder masked but for the most part this does not work. It woud be hard to belive that any board house today would try to solder mask over HASL. I am sure the guys from the board house will have a more detailed answered but this is what we see on the wave side.

Greg Hueste
Senior Applications Engineer
Speedline Technologies
Greg joined Electrovert in February 1984. Based out of the Electrovert applications laboratory in Camdenton Missouri, Greg has been in the process applications support role since 2000. His primary responsibilities include providing process and machine applications support for the wave soldering lines as well as process, machine and operations training. He also provides applications support for the reflow and cleaner lines. Greg is a PBET certified trainer and holds two patents on wave solder nozzle design.

They likely used a silkscreen and did it maually.

Peter Greenland
Global Sales Director
Photonis USA
Mr. Greenland has 20 years electronics industry experience ranging from circuit board design to product support & repair services. Peter has spent the last 4 years at Pycon Inc. supporting customers that require high mix, low volume manufacturing solutions.
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