Hi Tom,
I'll jump in with a little comment regarding the use of soldermask, and the
reliability of PCB's in general.
Soldermask has a limited scope to its application on Printed Wiring Boards.
As we already know, it is intended to limit the solder distribution on a pcb
during the reflow solder or wave solder process to just the land area where
the component being attached to the board for mechanical and electrical
connection. It really has no other purpose. Copper traces on a PCB will
survive for many many years without any protection other than the natural
oxides that form on it's surface when exposed to air and moisture. (anyone
have copper bottomed pans at home? or penny's in your pockets? or copper
wiring and pipes in your home?) Unless, of course, there is exposure to
etching compounds, acid rain, sulfuric acid, and other corrosive
chemicals..there will be no failure due to natural oxidation if sufficient
copper thickness is present on the board to allow an oxide barrier to form.
RF boards and High speed logic boards are unaffected by oxidized copper at
the surface of the board. The place where you have trouble with oxidation is
in connector contacts and in soldering.
The surface oxide layer forms as a barrier to corrosion(and soldering)...
and actually protects the copper from degrading further. In the 'old days'
we needed to remove the natural oxide layer just prior to soldering by
sanding the surface,... This was a lot of work, so some very smart people
invented 'flux'or 'Rosin', which is an acidic compound that activates with
the heat of the soldering iron, to clean the copper surface of natural
oxides while soldering. Eventually someone decided it would be convenient to
put this rosin into the solder so that as the solder melted, it released the
cleaning rosin from an imbedded 'core' down the center of the solder
'thread' ... This helped soldering a great deal...
Mass production of PCB's brought new innovations, for example, if the
surface of the board could be 'tinned' or solder coated all over, then the
soldering process required less labor, and less flux... leaving the surface
clean and prepared to solder components to and could be automated... instead
of hand 'tinning' every component land. Dipping the surface of the board
into molten solder and then wiping the surface to reduce the lumping of
excess solder became a standard in the early 70's if I remember correctly.
Most companies where using a machine called a 'hydrosqueege' which sprayed
hot oil on the surface of the board to remove excess solder after dipping
the board into a solder pot. The solder coating did not oxidize as quickly
as the bare copper surface and helped with shelf life of the product..etc.
I remember in the late 70's using soldermask for the first time and being
surprised to find the solder plating had reflowed under the soldermask.. due
to the pcb manufacturer putting the soldermask over the standard tin/lead
surface coating that everyone was using at the time... it looked like a real
bad case of wrinkles or a 'prune texture' in the surface of the
soldermask...after we put the boards through wavesolder. We could not
reject the boards because they built it just the way we specified it...
needless to say, there was a lot of rework and scrap until we fixed this...
This is when we started the use of the SMOBC callout on the drawing...
"Solder Mask over Bare Copper" insured adhesion of the soldermask to the PCB
material and it functioned well through wave solder.... Many people had
misgivings back then over doing this, they were afraid that the lack of
solder on all the traces would somehow compromise the copper and it would
rapidly degrade due to oxidation... bla bla bla...This of course is just not
the case. Even though solder mask provides protection to the copper beneath,
it not the function of soldermask to do so. Nor is it a legal method of
insulating electrical circuits even though it has insulating properties.
Thickness of soldermask varies and I guess the mixture can affect it's
properties as well...
Soldermask is used strictly a corral for the solder to keep it localized
around the component land area. Nothing more.
Rejection of PCB's is entirely up to the Purchaser of the PCB's and the
specifications that are quoted (before manufacture) in the FAB drawing are
the controlling authority...Non-compliance is grounds for rejection. If it
is not specified in the drawing... you have a problem. Quality of soldermask
compounds and their proper usage can be commented on by one more intimate
with the process of application and use, than I. However, it could be
rejected if the soldermask does not conform to the specification in the
drawing notes. If there is a contaminant present in the mask, you may be
able to reject them on the basis that they did not use approved materials...
etc. Most shops however will bend over backwards to help you solve the
problem, be it cosmetic or functionally a failure oriented problem. I have
specified soldermask in different colors and some QA dept's rejected for
cosmetic reasons...
It's really up to your quality standards and how well they are documented.
Hope that helps a little.
Bill Brooks
PCB Design Engineer
DATRON WORLD COMMUNICATIONS INC.
3030 Enterprise Court
Vista, CA 92083
Tel: (760)597-1500 Ext 3772 Fax: (760)597-1510
mailto:[log in to unmask]
IPC Designers Council, San Diego Chapter
http://www.ipc.org/SanDiego/
http://home.fda.net/bbrooks/pca/pca.htm
-----Original Message-----
From: Tom Parkinson - Quality System Manager
[mailto:[log in to unmask]]
Sent: Monday, February 19, 2001 8:07 AM
To: [log in to unmask]
Subject: [DC] Solder Mask
We have a number of printed wiring assemblies that are showing
indications of bad (?) solder mask. Tape test is fine on PCBs prior to
wave solder. And overall appearance prior to wave solder appears normal.
After the wave soldering process, boards are water washed and dried.
There is a powdery, whitish, streaky look to the boards. This does not
come off, and it apparently is not on the surface. It appears to be
inside the solder mask. Tape test still passes, no blistering or loss
of adhesion is apparent.
The customer wants Class 3 standards to apply, and cosmetically this
looks terrible (yes, 610-C says okay - but also says customer must
determine what is acceptable).
Checking with a third party board house, we are being told this may be
caused by the solder mask not being applied over clean PCB, or improper
mixing of the mask and catalyst.
Our concerns are: what is the long term consequences - will this break
down, allow growth of something, or what. The boards are used in power
distribution systems for 19 inch computer racks.
Any help would be appreciated.
Tom Parkinson
WinTronics, Inc.
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