Kathy,
The only way squeegee pressure can affect paste thickness when using
metal blades and SS stencil is if you are not getting a clean wipe, greater
then 0 snap-off, or too much squeegee speed. If you do not have a clean
wipe you will end up with thicker prints as the paste left on the stencil
will stay on the pad. You really cannot get the "scooping" affect with a
metal blade that you get with a rubber squeegee. Too much squeegee speed
with leave what I refer to as a "Ski sloop" deposit on the pads that run
vertical to the blade itself, the top of the deposit will not be level.
The pads that run horizontal to the blade will have less paste because the
blade passed over the short side of the aperture too quickly to fill it
completely. With a 0 snap-off, clean wipe, and the proper squeegee speed
you will get a set amount of paste per your stencil thickness. The actual
volume could be a little less depending on your paste release (if any paste
stays in the apertures), which is out of your control and undesirable.
One thing I found affects you paste thickness (increasing thickness)
is the stencil release distance and speed. If you pull the stencil away
from the board slowly, you get a deposit closer to your stencil thickness
then if you pull the stencil away from the board quickly. These settings
are controllable in most of the high end printers. This is a misleading
measurement because even though the print measures thicker, there is
actually not more paste. When you release slowly it gives the paste a
chance to release without pulling apart or stretching. Because you force
the paste into the aperture it is somewhat compacted in the aperture. When
you release quickly it will un-compress (stretches) the paste and therefore
making the thickness read high. We measure our paste thickness on a Alpha
Hi-check machine and have the data to show this phenomenon.
As far as squeegee speed is concerned you most likely will not have
one set speed for all assemblies. On a non-fine pitch assembly you can run
a faster speed with good results (50-75mm/sec). I have run 75mm/sec on a
board that I had to print one every 10 seconds or less (non-fine pitch).
With a 25-31 mil pitch you may be able to run 50mm, I am running 50mm on a
board with 31 mil pitch with great success. When you go to 20 mil pitch or
less you may want to cut back to 25mm. These numbers are starting points,
if you run 50mm and do not get the desired result, but you do get good
results at 25mm, start experimenting (good time for a quick DOE). Find the
speed that gives you the best throughput while maintaining good quality.
Good luck,
Brad Kendall
Hella Electronics Corp.
[log in to unmask] on 02/12/98 06:21:31 PM
Please respond to [log in to unmask]; Please respond to [log in to unmask]
To: [log in to unmask]
cc: (bcc: Brad Kendall/Hella North America Inc.)
Subject: [TN] Solder Paste Printing Process
When you are using a metal squeegee does more pressure still yeild less
solder paste? If so, could anyone tell me the phenomenon of why this
is?
And, would you say it is better to have one set speed for squeegee
blade travel, and only vary the squeegee pressure?
And if you use one set speed should you have one set speed for each
array of pitches, i.e. .025" and less 25mm/sec, .026" to .036" 35mm/sec
(or whatever), or just have one set speed for all lead pitches?
Thanks in advance for your thoughts on these questions.
Kathy Palumbo
################################################################
TechNet E-Mail Forum provided as a free service by IPC using LISTSERV 1.8c
################################################################
To subscribe/unsubscribe, send a message to [log in to unmask] with following
text in the body:
To subscribe: SUBSCRIBE TechNet <your full name>
To unsubscribe: SIGNOFF TechNet
################################################################
Please visit IPC web site (http://jefry.ipc.org/forum.htm) for additional
information. For the technical support contact Dmitriy Sklyar at
[log in to unmask] or 847-509-9700 ext.311
################################################################
################################################################
TechNet E-Mail Forum provided as a free service by IPC using LISTSERV 1.8c
################################################################
To subscribe/unsubscribe, send a message to [log in to unmask] with following text in the body:
To subscribe: SUBSCRIBE TechNet <your full name>
To unsubscribe: SIGNOFF TechNet
################################################################
Please visit IPC web site (http://jefry.ipc.org/forum.htm) for additional information.
For the technical support contact Dmitriy Sklyar at [log in to unmask] or 847-509-9700 ext.311
################################################################
|
