Having been in the solder mask business for the past about 20 years, I can
give you a general overview of the evolution of SMOBC and PISM (with
approximate time lines) and how these technologies have become dominate in
the PWB isdustry,
In the early days of circuit boards, most circuitry was Sn/Pb. Solder tends
to be a very solderable surface, whereas copper becomes oxidized easily and
won't solder. There were some anti oxidant treatments back then, but they
were not all that reliable and had poor shelf lives. On these Sn/Pb boards
solder masks were applied (usually one or two part epoxies) by standard
screen printing with a pattern. Because of "bleed" large clearances around
the pads had to be used; however, this was not too much of a problem back
when 50 mil lines/spaces were considered "high tech" and plated through holes
were 35 mils or greater. Unfortunately, during wave soldering the Sn/Pb
circuits under the solder mask would melt and the mask tended to flake off.
>From my perspective, this precipitated lots of nasty phone calls about lousy
adhesion of solder mask, but there was litttle that I could do (once the
Sn/Pb melts the adhesion is gone). Even so, this was still mostly a cosmetic
problem because of the wide spaces between circuits.
By the early to mid 70's circuit lines/spaces became smaller and smaller and
the adjacent circuit traces with Sn/Pb circuitry would tend to fuse together
when the circuits melted during wave soldering and this became a serious
functional problem instead of just a cosmetic one.
The advent of the hot air solder leveling (HASL) process (1970's) was the
savior because now you could strip the Sn/Pb after etching, scrub and apply
solder mask over the now copper circuits, apply solder to the plated through
holes by HASL and have a nice sodlerable metal during wave soldering without
worrying about melting the circuits because now they are copper and will not
melt.
So by the late 70's early 80's, the issue of melting circuits, solder mask
flaking and shorts had been solved by SMOBC and HASL; however, the issue of
solder mask bleed with traditional screen solder mask was still there and
became more of a serious problem as plated through holes became smaller,
circuits became denser and "mask on pads" became a major issue.
In the USA the solution to this problem was the advent of dry film solder
masks. It is interesting to note that at about the same time the solution in
Europe was the advent of the Probimer curtain coated, solvent developed,
liquid photoimagable solder mask and in Japan it was a number of screen
applied, aqueous developed liquid photoimageable solder masks. All of these
processes applied the mask to the entire surface of the circuit board and the
pattern was difined by using standard UV exposure units and phototools
(either silver or diazo).
By the mid to late 80's Probimer came to the USA and took the market by storm
at many of the larger circuit board shops. This product was cheaper, much
tougher and more durable and could be applied at thinner coatings. The
thinner coating became a major advantage as surface mount technology became
the norm because the thicker dry film solder mask (usually 4.0 mils) caused
solder shadowing and solder skip problems.
By the late 80's, the aqueous PISM products from Japan made their debut in
the USA. These products were cheaper, tough and durable, environmentally
friendly (aqueous developed), had low VOC emissions, could be applied as
thinner coatings and did not require large capital equipment investments to
run.
As we moved into the 90's, the aqueous developed, liquid, photoimageable
solder masks became the dominant solder masks used in the USA and this market
is still growing. Many equipment companies developed specialized equipment to
apply and process these masks.
I hope this gives you an overview of the SMOBC and PISM history. If you want
to dig deeper and need more specific information, you may want to contact PC
FAB magazine. Over the years they have published many articles on solder
masks and would highlight solder masks in one of their issues every year.
Regards,
Larry Fisher
Dexter Electronic Materials
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