Tusen takk, Inge!
Your analysis makes perfect sense. It also looks like that the craters
do not cause the fines really. I am glad to hear you have seen the
issues and at this point I will ignore them and focus on the
stencil/solder type/reflow interaction.
Leif Erik Laerum
Quality Assurance Manager
Texas Memory Systems
[log in to unmask]
Tel: (713) 266-3200 x468
www.texmemsys.com
Inge wrote:
> Leif Erik,
>
> seems as the focus is now on the stencil. Good, because this may be
> the red thread. Imagine the squeege has just passed and the extremly
> small laser cut holes are filled with your paste. Next, the stencil is
> ready to separate from the board. What happens? I guess the rather
> rough metal walls are little unwilling to let go the paste, while the
> paste has already established a grip on the ENIG pad through surface
> wetting and adhesion forces. The later one wins over the adhesion from
> the stencil walls (otherwise, there had been no prints). However, the
> adhesion from the walls will expand or elongate the paste body for
> fractions of a second. But the true volume is constant still, isn't
> it. Then a compensation must be done, namely, the middle of the paste
> body must sink. You get a small , what shall I call it, a inward bend
> instead of a flat paste surface. When the the stencil wall adhesion is
> overcome, the paste glides backward and leaves the stencil, but not as
> a perfect cube or cylinder, but more like a volcano crater. What
> happens next is all dependent on the rheology of the paste. We found
> that pastes with little lower viscosity never gave any bubbles, while
> 'thicker' or 'drier' pastes had a tendency to build a 'depression' in
> the middle. This was only seen for very small apertures. In our case,
> we could ignore the phenomenon, didn't have any negative impact. We
> learned this all from our thickfilm printing guy. Wished I had saved
> all hundreds of technical report from the 60s and 70s (it was the
> time we had people who knew about rheology) but they are gone. Later
> generations thought this old stuff was useless, a lot not even stored
> for the IT age... Trike Man may still have something hidden in his
> drawers,,,
>
> Inge
>
>
>
> ----- Original Message ----- From: "Inge" <[log in to unmask]>
> To: <[log in to unmask]>
> Sent: Wednesday, July 30, 2008 2:21 PM
> Subject: Re: [TN] Solder Paste with craters.
>
>
>> 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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