Det var nettop en virkelig specialist som talte og beviste tesen den gangen
, Gert Becker
Inge
----- Original Message -----
From: "Leif Erik Laerum" <[log in to unmask]>
To: "TechNet E-Mail Forum" <[log in to unmask]>; "Inge"
<[log in to unmask]>
Sent: Thursday, July 31, 2008 2:37 PM
Subject: Re: [TN] Solder Paste with craters.
> 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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