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Subject:
From:
John Burke <[log in to unmask]>
Reply To:
TechNet E-Mail Forum <[log in to unmask]>, John Burke <[log in to unmask]>
Date:
Wed, 30 Jul 2008 15:46:50 -0700
Content-Type:
text/plain
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text/plain (235 lines)
How thick is that stencil? And am I correct in assuming that the squeegee
travel direction for that print is from the top to the bottom of the
photograph?

Also what type of stencil you using?, laser cut, etched, laser cut
chemically polished etc.

John

 
 
John Burke
 
(408) 515 4992
-----Original Message-----
From: TechNet [mailto:[log in to unmask]] On Behalf Of Leif Erik Laerum
Sent: Wednesday, July 30, 2008 1:56 PM
To: [log in to unmask]
Subject: Re: [TN] Solder Paste with craters.

Thank you everyone for contributing. Especially you, Richard.

First of all these are NOT via in cap boards and we are using FR4. Been 
there done that....

Most of the items brought up by Richard we do have established processes 
for. These are according to the recommendations of the solder vendor, 
but if others feel differently, please chime in.
We log the solder  used for each batch and  the Date of Manufacture for 
this. We  typically do not accept a batch of solder that is older than 3 
mts. This is so that we do not end up with too much out of date solder 
at the end.

- Solder is brought out of the fridge minimum 12 hours before use.
- We set a time limit that solder must can be stored out of the fridge a 
maximum of 3 mts. Practically this ends up around 1 month max.
- We do reuse solder that has been on the screen for up to two weeks. 
Then we throw it out.
- We never put solder back in the fridge.
- We are evaluating new suppliers of solder at the moment. We have not 
audited the currents supplier, but that is a good idea. We always get 
the solder couriered locally and in a cool container so I do not      
have any evidence that would lead me to put this on the solder vendor, 
but.....
- As our process works boards sit no longer than 1 hour with solder. 
Usually much shorter than that.
- We are using Type 5 solder actually.
- All misprints go though the wash before it is reprinted.
- We use a DEK 248 that is not as automatic as I would like and some of 
the issues we see are due to this repeatability problem, but from the 
data I have gathered, this is not the cause of the solder fines and craters.
- A Solder AOI would be nice. We do not have one of these (yet???)
- There could be an issue with too much solder. We are going to reduce 
apertures some. There is some evidence of excessive solder.
- I am going to experiment with slowing down the print separation speed. 
Good point.

Thx.

Leif Erik Laerum
Quality Assurance Manager
Texas Memory Systems
[log in to unmask]
Tel: (713) 266-3200 x468
www.texmemsys.com



Inge wrote:
> Hi Leif, you've just listened to His Master's Voice.
> You should take off your cap, when you speak to Richard Stadem.
> Impressive! I begin to feel that there are two exceptional stars at TN.
> Steve...we already knew
> Richard...a supernova
>
> Inge
>
> Gah...my example was not very clever...a  supernova is bright just for 
> a short time...hmmm...a red giant then? hmmm...or a white 
> dwarf...hm...none of them very striking....hmm....shining like 
> Betelgeuse...hmm.....maybe Master Whittaker can give a hand?
>
>
> ----- Original Message ----- From: "Stadem, Richard D." 
> <[log in to unmask]>
> To: <[log in to unmask]>
> Sent: Wednesday, July 30, 2008 6:53 AM
> Subject: Re: [TN] Solder Paste with craters.
>
>
> Hi, Leif
> Send your pictures to [log in to unmask]
>
> Can you tell me what solder paste it is you are using? How was it
> qualified for use?
>
> Here are just some of the questions you need to ask yourself in order to
> determine why you have an issue with solder fines:
> Do you have a good documented solder paste handling procedure? How is
> the paste handled, from vendor or distributor to your factory? When was
> the last time you stopped in at the distributor to verify their stock is
> being rotated and is kept refrigerated immediately upon receipt from the
> factory? How often do they turn the packages upside down to prevent flux
> separation? Are they a certified distributor who will pass on to you a
> lot recall notice from your solder paste vendor if there is a known bad
> lot?
> How long is the paste allowed to sit out on the stencil and how many
> times can a line of paste on the stencil be sheared (printed back and
> forth) before it is removed and replenished with fresh paste? Are the
> operators allowed to scrape up the unused paste on the stencil and
> re-deposit it into a jar for re-use later? Is the jar or tube of solder
> paste, once removed from refrigeration, allowed to set for two to four
> hours (depending on paste vendor and paste type) to reach room
> temperature prior to printing? Once removed from the refrigerator, is
> unused solder paste allowed to be put back in the refrigerator? Are you
> using Type 4 paste or Type 3? How good is the printer setup, ie, the
> repeatability of the registration of the stencil to the PWB? Are you
> performing some type of aperture reduction on all pads in general and at
> least a 50% reduction on large belly pads to prevent solder fines from
> being printed onto the board? If a board is misprinted, does the
> operator know better than to simply wipe off the board (embedding the
> paste into every space between the edges of the pads and the soldermask,
> into every small via, into every through hole, etc.) but is there a
> documented procedure detailing how the misprinted board is to be cleaned
> to prevent this? How is the solder paste packaged, jar or tube? (Tubes
> prevent a much larger volume of paste from being exposed to air and
> humidity, and also help prevent re-use of solder paste that has been out
> for awhile). Do you perform a good solder paste print inspection using a
> 3d AOI or some other type of automated inspection, and do you use the
> data from this inspection process to detect (real-time) paste defect
> trends and react to them with corrective actions?
>
> Sorry to ask so many questions, but all of these can contribute to
> fines, and there are many more factors that can cause them.
>
>
>
> -----Original Message-----
> From: TechNet [mailto:[log in to unmask]] On Behalf Of Leif Erik Laerum
> Sent: Wednesday, July 30, 2008 8:29 AM
> To: [log in to unmask]
> Subject: [TN] Solder Paste with craters.
>
> Technetters,
>
> We are having an issue with solder fines on our lead free boards. We are
> using SAC305 WS and no nitrogen. I noticed that our boards have craters
> in the paste on the pads after being printed. Not all pads are deposited
> this way, but maybe 25%. These craters have an air bubble in them. The
> bubbles usually burst before the boards goes into the P&P, but the
> crater stays. The screening process is exactly the same for leaded and
> unleaded paste. The leaded paste does not behave this way. Could these
> craters be a symptom of the cause of the solder fines? Anyone have any
> experience with this?
>
> How do I go about posting pictures to stevezeva.homestead.com?.
>
>
> -- 
> Leif Erik Laerum
> Quality Assurance Manager
> Texas Memory Systems
> [log in to unmask]
> Tel: (713) 266-3200 x468
> www.texmemsys.com
>
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