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July 2008

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Subject:
From:
Guy Ramsey <[log in to unmask]>
Reply To:
Date:
Thu, 31 Jul 2008 15:00:52 -0400
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Smallest pads under  mm BGA + 6 (60 mil??)laser cut stencil + type 5 paste =
me confused.

This would be, in my opinion, a recipe for hot slump and solder fines. A
design with components and features like you describe should be fine with
type 3 paste. Let's talk about those capacitor and resistor networks. What
is their pitch and land pattern dimension? Smaller than the BGA? A lot
smaller? Okay maybe type 4 paste and a 4 or 5 mil stencil. 

Guy 

-----Original Message-----
From: TechNet [mailto:[log in to unmask]] On Behalf Of Leif Erik Laerum
Sent: Thursday, July 31, 2008 11:34 AM
To: [log in to unmask]
Subject: Re: [TN] Solder Paste with craters.

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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