Dear sir,
This is idea shearing, any mistake please accept my apology.
For the solder on gold finger, There are three major root causes, raw PCB(
if your PCB has the pre-solder), raw solder paste, screen printing and
reflowing process. Inspection under 10X magnification after screening can
not determine the solder ball because the solder particle is only 45-25
microns. O.K. let to see item by item.
1. Raw solder paste. The solder paste is tested the solder ball test or
not, the solder ball test can determine the solder explosion during
reflowing. See IPC-TM-650 standard method 2.4.43 or ANSI/J-STD-005.
2. Bare PCB. If your PCB has the pre-solder from PCB maker please make
sure no any solder splash on the gold finger pad.
3. Solder paste screen printing. The residue solder particle under stencil
is the major. Stencil wiping every print can help but the wiping method is
one important to consider. The manual wiping by hand can not eliminate the
solder particle under stencil, if your M/C has auto-wiper please use it. If
your M/C has not the auto-wiper then you can use a roll of lint free paper
wipe the stencil, please be reminded the wiping should be a plane wiping
not spot wiping.
4. Reflow soldering. First start verify the reflow profile conform to the
vendor's recommended reflow profile. Next, try to bring the max temp down
and raise the pre heat time up.
How to prove the cause of problem.
1. Prepare the product to be 2 groups
1.1 Attach the Kapton tape onto the gold finger pad before screen
printing and process the unit to normal process.
1.2 Screen the solder paste and attach the Kapton tape onto the gold
finger pad before process to Pick and place.
2. After reflowing, inspect the solder dot at the gold finger pad but do
remove the Kapton tape.
2.1 If the solder dot is found under the Kapton tape then the cause
come from screen printing, next count the defect percentage.
2.2 If the solder ball is found on the Kapton tape unit then the cause
come from the reflowing. Next count the defect percentage.
Do pareto graph to represent what is the 1st, 2nd problem to fix.
Good luck.
Noppadol S.
At 01:41 16/7/98 +0800, you wrote:
>Hi,
>
>I would like to know how do the solder get onto the gold finger, if one
>can't find it after printing, under 10X magnification.
>
>Please do advise me.
>
>
>Poh
>
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