If water residue from the cleaning process causes real shorts, there is a
processing problem that must be solved prior to looking at coating. I
suspect that water is under the microBGA and not properly dried. Coating
will only trap the remaining moisture, if it even covers the water residue.
I do not recommend water washing SMD, especially fine pitch. I have never
worked with BGAs. It is difficult to dry under the parts and you cannot
tell whether flux residue is trapped under fine pitch without destroying
the panel, until its too late. This failure can exist even when the Omega
Meter says the panel is clean.
I suggest that you have your supplier either bake dry the panels to ensure
all water is removed from under the SMD or use a no clean process and see
if you even need coating.
If its not trapped water, read on.
If water residue interferes with your RF, how long will it last in any
environment.
I suggest you use either a robotic spray or vapor deposition for your
coating. Dip will not provide even and repeatable coverage thickness
relative to the above conditions. Tunnel spray has higher operating
expenses.
The coating material chosen depends upon your end environment and circuit
design. All coatings have some capacitance and moisture permeability.
Depending upon the final environment, system capacitance, sytem
inductance...., different coatings will be recommended. I know of one
material that worked fine at one environment, but at high humidity,
absorbed enough moisture to change dielectric constants and changed the
response characteristic of a sensor (at another company). I expect some
coatings to do this to RF circuits.
Also, you must tune your circuits after coating to compensate for the above
reason.
ARIC PARR
Sr. Manufacturing Engineer
Eaton Corp
1400 S. Livernois
P. O. Box 5020
Rochester Hills, Mi 48308-5020
[log in to unmask]
248 608 7780
Fax: 248 656 2242
-------------
Original Text
From: C=US/A=INTERNET/DDA=ID/TechNet(a)IPC.ORG, on 7/29/98 2:29 AM:
To: Aric Parr@01635@Lectron_RH,
EatonWHQ@CorpMail@WHQCleveOH[C=US/A=INTERNET/DDA=ID/TechNet(a)IPC.ORG]
Dear Net member,
We have a hybrid PCB with microBGA and some resistors/capacitors. It
is a through hole component for data transmitting. When the customer
insert the component to a PCB and finish the wave soldering. It is
necessary to clean the board. Due to its application in high
frequency, the residue of water after cleaning will cause short of the
circuit.
To solve the problem, we like to try the conformal coating the PCB. Do
anyone can recommend the conformal material and the process?
Eden
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