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July 2008

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Subject:
From:
Leif Erik Laerum <[log in to unmask]>
Reply To:
TechNet E-Mail Forum <[log in to unmask]>, Leif Erik Laerum <[log in to unmask]>
Date:
Thu, 31 Jul 2008 10:34:02 -0500
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Paul et al,

The bubbles are more prevalent on the smallest apertures such as 1 mm 
BGA pads and the resistor/capacitor pack pads.
The temperature in the production area is between 68F and 75F. Humidity 
is relatively steady at 40%. (We are in Houston....)

If anyone has any suggestions how to test the screens for cleanliness 
that would be great. That is a part of our process we do not control. We 
have a stencil cleaner, but we can only visually see if they are really 
getting clean. We also bought some stencil cleaner for manually wiping 
down a screen, but it actually makes the screens "greasy". We stopped 
using it.

Ours stencils are the standard 60 mil laser cut. We do use a metal squeegee.





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



Paul Edwards wrote:
> Leif,
>
> I just have 2 questions...
>
> What size apertures do these bubbles occur on and is there a 
> particular aperture orientation in which they are prevalent?
>
> What is the humidity and temperature of the environment for the paste 
> prior to putting it on the stencil>>
>
> Have seen something similar in certain kinds of SAC W/S pastes that 
> have been exposed to too much moisture and certain stencil cleaning 
> compounds...The saponifier was making soap bubbles when rolling in the 
> paste...
>
> Paul
> Paul Edwards
> Surface Art Engineering
>
>  
>
>  
>
>
> -----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 <http://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 <http://www.texmemsys.com>
> >
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