I can only comment on the thermal aspects of your process as that is my area
of expertise (my company manufacturers the Universal Instruments Reflow
ovens as well as the Nordson Select Coat Cure ovens).

If I were you I would take a closer look at the profile you are using. In
particular I would examine the rise rate during preheat, cross board
uniformity during relflow and also the top to bottom board temperature
differential.

I recommend that try running profiles on the board using at least 4
thermocouples. Be sure to place the thermocouples on various different
points on the board like so:


XXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXX
X                  T1                    X
X                  T7                    X
X                                        X
X                                        X
X                                        X
X         T2                 T3          X
X                                        X
X                                        X
X                  T8                    X
X                 (T10)                  X
X T4               T5                 T6 X
X                                        X
X                                        X
X                                        X
X                                        X
X                                        X
X                  T9                    X
X                                        X
XXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXX

Run the test board (using thermocouples T1 - T6, or 1, 4, 5 and 6 if your
profiler only supports 4 thermocouples) through the oven 4 times, rotating
the board 90 degrees after each run so that each edge of the board is used
as the leading edge at least once. Compare temperature differentials
recorded. This test is helpful for determining the cross board uniformity of
your oven.

Next test your top to bottom surface temperature differential (using
thermocouples T7 - T10). Thermocouple number 10 should be placed on the
bottom side of the board directly below thermocouple number 8. This test
will also provide information about your leading edge. Often times the
leading edge of an assembly sees excessive heat which may be causing your
problem.

If you would like to share the results of your profile testing I would be
happy to make recommendations to soften your profile which may eliminate
your problem. Of course you should also examine the solder paste for
moisture or other contaminates as well as the other portions of your process.

Sincerely,

Brian Stumm
ETS, LLC
509-483-0900 (voice)
509-483-0331 (fax)
[log in to unmask] (e-mail)


>Dear all,
>        I would like to introduce myself, I am the process Engineer working for
>the Electronics assembly company in Thailand.
>        I have the problem on one product which has the SMD and COB(Chip On
>Board). I found the dot solder splash on the bonding lead (use for COB)
>which effect to COB operation.
>        The cause are
>        1. the stencil wiping at screen printing is not effective. The past
method
>was manual wiping with the sheet of lint free, I solved by use the lint
>free paper in a roll form, wipe one direction only and roll up the paper
>every wiping. The solder splash from this operation
>reduce 95%
>        2. Too long reflow time but I can not adjust because with the PCB
design I
>use it require the long reflow time, if not the solder quality will be
>effected. I have evaluated the preheat in the range of solder paste's
>vendor recommendation but did not better.
>        Currently, we attach the Kapton tape on the COB area before go to SMD
>process. I can eliminate the solder splash on the bonding lead but the
>problem is the high cost of Kapton tape.
>        Moreover, I have evaluated the water soluble solder mask #110 of Alpha
>metal to instead the Kapton tape, I applied by using the silk screen
>technic, it worked it could protect the solder splash as good as the Kapton
>tape but the problem is short stencil life it dry within 4 hour that I have
>often cleaned stencil(15 minute/Time) then it is not suit for mass
>production.
>        Does anyone can help me to solve this problem.
>        1. Is there the other type of water soluble solder mask which has the
>longer stencil life?
>        2. Is there the other method able to support my current solder mask?
>        3. Is there the other mat'l to instead the Kapton tape?
>        4. Is there the other way to eliminate the solder splash otherwise the
>tape applying?
>
>        The return information will be aprreciation.
>
>With best regards,
>Noppadol S.
>
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