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 >>>>> >>>>> --------------------------------------------------- >>>>> Technet Mail List provided as a service by IPC using LISTSERV 15.0 To >>>>> unsubscribe, send a message to [log in to unmask] with following text in >>>>> the BODY (NOT the subject field): SIGNOFF Technet To temporarily halt >>>>> or >>>>> (re-start) delivery of Technet send e-mail to [log in to unmask]: SET >>>>> Technet NOMAIL or (MAIL) To receive ONE mailing per day of all the >>>>> posts: send e-mail to [log in to unmask]: SET Technet Digest Search the >>>>> archives of previous posts at: http://listserv.ipc.org/archives Please >>>>> visit IPC web site http://www.ipc.org/contentpage.asp?Pageid=4.3.16 >>>>> for >>>>> 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