*****CORRECTION********
Interesting feedback.
The stencil is indeed 6 mil thick.
The resistor pack pads are 22 x 10 mil and have 20 mil pitch. The
smallest BGA pads are 13 mil and have 30 mil pitch.
Is it the case that type 5 has a higher flux percentage than the type 3.
If so that could explain some of our findings.
Leif Erik Laerum
Quality Assurance Manager
Texas Memory Systems
[log in to unmask]
Tel: (713) 266-3200 x468
www.texmemsys.com
Guy Ramsey wrote:
> 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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