Do you use a thick stencil?
Have you tried to slow down the stencil lift speed?

Inge


----- Original Message ----- 
From: "Leif Erik Laerum" <[log in to unmask]>
To: <[log in to unmask]>
Sent: Wednesday, July 30, 2008 1:56 PM
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 This is so that we do not end up with too much out of date solder at 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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