FROM: BRUCE HOUGHTON 27/552/844/TOR
*** Resending note of 05/01/95 13:05
Phone (416) 448-5701 Tie 778-5701
Subject: Re: No-clean process impact
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Reply to Dale's note of May 1:
Dale,
Welcome to the real world of no-clean processes. The old tests are
not adequate to verify that the PWB will solder in the assembly shop.
You need to be able to test the PWB's solderability with the same
type of flux and process that the assembler will use. You used
a different flux and saw a fail. You may also have to look at the
solder pot in the future. In wave soldering, a PWB is only over
a wave for 3 to 5 seconds, not 10 as in the solder pot test. Its
not going to be easy!
Bruce Houghton
Celestica, Inc.
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>> Date: Mon, 01 May 95 08:20:01 MST
>> From: [log in to unmask]
The situation I have here is that a customer of mine has rejected a lot of
our
PCB's at incoming inspection for SMT pads that did not all wet during their
solderability test. They are thus, not accepting the product. They sent
samples
back to me for analysis; the samples did indeed show random non-wetted pads
after their test. The board is a double sided with SMT pads on the top side,
.130" overall thickness, solder mask used is Probimer 52 from Ciba Geigy.
I took one of the pcb returned that had not been tested for solderability and
measured random smt pads for solder thickness, the results were an average of
142f", this indicated to me that there was plenty of solder and the
possibility
of intermetallic growth a non issue. I then took the pcb's that the customer
had done the solderability test on and measured the smt pads that appeared not
to wet, the results were an average 337f".
I then took one of the raw pcb's to have our lab do the solderability test the
specification is as followed: solder pot temp: 474 +/- 8 degree F, Flux
mixture:650 ml flux (type R rosin), 433 ml isopropyl alcohol (99%), Once per
week, the older solder is bailed out and a minimum of the pot volume is filled
with new solder. The test is as followed:1.) Immerse the sample for 10 seconds
in the flux mixture. 2.) Skim the dross off the top of the solder within 10
sec
of running the test. 3.) Slide the sample onto the surface of the solder using
tongs. Allow it to remain on the surface for 10 sec. 4.) Slide the sample off
the surface of the solder. Place it on the workstation to cool. Immerse the
sample in isopropyl alcohol. Following this procedure on the raw pcb resulted
in all smt pads wetting.
The lab tech then took another raw pcb and folloed the same steps as above,
but
this time used R-Rosin flux non-activated. The results: We were able to
duplicate the same results as our customer samples. Our customer used a non-
activated flux, Rosin flux 25%, 75% isopropyl Alcohol.
My question is: Would this lack of wetting on random pads actually cause an
assy issue on the line with regards to solderability of components? Please let
me know what your opinion is on this.
Thanks,
Dale
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