Good morning Darrell,

The following is an answer I have given a while ago to a circuit board
designer
asking almost the same question you are asking. And yes, the aperture size
(volume) is determined by the need for solderpaste to attach the component
to
the board (between component and pad and to produce the required / desired
fillet).


IPC-7525 is a set of Guide Lines, not a spec. So a company that starts from
scratch can use these to set up their library for aperture dimensions.
However, remember, these are Guide Lines. The reason they are not called a
Specification is that the required aperture dimensions can vary
significantly depending on many parameters, e.g. pad size, pad finish,
solder type (ball dimensions, amount of solids), squeegee type, squeegee
pressure, squeegee speed, squeegee angle, temperature, operator habits (or
lack thereof), and probably a few more.
Because of all these variables, we cannot "design" the stencil for the
customer. It is the cooperation between the Manufacturing Engineer (who
knows all the process details) and the stencil maker (who can provide
advise) that can lead to a stencil which will give the desired results.
All the above is especially difficult when dealing with contract assemblers.
From them we get a great variety of differing data knowing less about the
details that impact stencil design.
In the case of captive assemblers, the library is known, as are the process
details and even the board can be known. Under those conditions and with
cooperation between the players excellent results can be obtained. The best
results we know of resulted in placing 7,000,000 bricks of solder with zero
errors  --  that is real quality!! (remember that 50 dpmo is considered
worldclass!)

If you are working as a board design engineer, it is
desirable to set up a standard design procedure and maybe even to have your
own library based on IPC-7525. I believe this to be possible and even an
advantage for you in your market place.

Regards, Ahne

One more example of potential trouble:
Customer wanted a stencil for a board, which had high current runs. It
turned out that the solder mask openings were larger than normal and the fat
runs were connected to the surface mount pads (or sometimes the pads were
part
of the run) causing insufficient solder. Such a problem is next to
impossible
to predict by the stencil manufacturer.

Ahne.
A-Laser, Inc.
503-641-9428

-----Original Message-----
From:   TechNet [mailto:[log in to unmask]] On Behalf Of Darrel Therriault
Sent:   Thursday, January 03, 2002 17:07
To:     [log in to unmask]
Subject:        [TN] Solder Stencil

TechNet,

What considerations are used for selecting the thickness and aperature
of a solder stencil?  I would guess it has to do with solder volume and
maybe proximity/spacing, but are there other things considered as well?
Are there any "rules of thumb" or guidelines for making this selection
or standards?

All comments and preferences appreciated.

DT
--
Darrel Therriault
VP, Mfg. Operations
INCEP Technologies, Inc
(858)547-9925 223
[log in to unmask]

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