Patrick
Two other things to consider:
1) ramp rate - your ramp rate may be high enough that the volatiles in
the flux are blowing solder off the paste deposits. The rule of thumb I
work to is < 2 deg. C / sec.
2) stencil design - if your apertures are too big you may be squeezing
solder out to the sides of leads. This can become free range
solderballs, and they could make their way over to the gold.
As with Bill, let me know what you find, please!
regards,
Graham Collins
Process Engineer,
Litton Systems Limited, Atlantic Division
(902) 873-2000 ext 215
> -----Original Message-----
> From: [log in to unmask] [SMTP:[log in to unmask]]
> Sent: Tuesday, November 04, 1997 5:52 PM
> To: [log in to unmask]
> Subject: Re: [TN] ASSY, solder residues on gold fingers
>
> Patrick:
>
> How fresh is the paste being used? Also, how wet has the weather
> been
> during the time of this phenomenon? In other words, has the
> paste had
> the chance to absorb moisture before screen printing?
>
> One mechanism for solder splatter MIGHT be that during the
> preheating
> operation, entrapped moisture in the paste could rapidly expand
> and
> "explode" from within the deposited paste, especially if a damp
> surface layer of paste is spatulated into the jar before placing
> it on
> the screeen printer. This rapid expansion of moisture COULD send
>
> solder balls flying to nearby areas on the board (e.g. gold
> fingers).
> They probably result in melted spots of ~0.005" - 0.010" in
> diameter
> and are probably located on the end of the finger nearest the
> deposited solder paste, right?
>
> An experiment you could run to prove or disprove the theory is:
>
> 1) Tape the fingers of a several boards with Kapton tape.
> 2) Paste and reflow one set of boards with paste that has been
>
> allowed to sit open for a period of time to collect moisture
> from
> the air.
> 3) Paste and reflow another set of taped boards with fresh
> paste
> from the supplier.
> 4) Carefully inspect the gold finger area under a microscope
> and
> see how many solder balls are present on the tape for each
> set.
>
> The N2 environment should have no direct affect on the phenomenon
> you
> are seeing. In either case, the gold fingers would be protected
> while
> the data is being collected, so the product quality would NOT be
> compromised.
>
> When or if you find the root cause of the splatter, I would
> appreciate
> it if you would let me know the results.
>
> Good luck!
>
> Bill Fabry
> Plantronics, Inc.
> [log in to unmask]
>
>
> ______________________________ Reply Separator
> _________________________________
> Subject: [TN] ASSY, solder residues on gold fingers
> Author: "TechNet Mail Forum." <[log in to unmask]> at INTERNET
> Date: 11/4/97 4:07 PM
>
>
> Hi, I would greatly appreciate any help or any recommendation in the
> following subject...
>
> We are having solder residues on gold fingers of PCBs, those are
> seeing
> immediately after the reflow oven (it's a recirculation convection
> reflow
> solder system). A visual inspection has been done after the printer
> machine at high magnification and nothing showed up. We were thinking
> that
> the problem was from the printer machine since a long time but we
> found
> that it is not the case today.
>
> I suspect that the problem is coming from the reflow oven but I dont
> see
> the problem mechanism. The reflow atmosphere is nitrogen and the ramp
> up
> temperature at the entrence of the oven is relatively high, arround
> 3.4
> C/sec.
>
> Does anyone of you have an idea concerning this problem, thanks in
> advance.
>
> -------------------------------------------------
> Patrick Ducas
> Process Quality Engineer
> Matrox Electronic Systems Ltd
> E-mail: [log in to unmask]
> Tél:(514)969-6000 ext.2971 Fax:(514)685-3415
> -------------------------------------------------
>
>
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