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TECHNET  July 2008

TECHNET July 2008

Subject:

Re: Solder Paste with craters.

From:

John Burke <[log in to unmask]>

Reply-To:

TechNet E-Mail Forum <[log in to unmask]>, John Burke <[log in to unmask]>

Date:

Thu, 31 Jul 2008 14:14:36 -0700

Content-Type:

text/plain

Parts/Attachments:

Parts/Attachments

text/plain (326 lines)

If you want REALLY old papers on Rheology Mike Fenner will have/remember
them - sorry Mike!!...........NOT

 
 
John Burke
 
(408) 515 4992
-----Original Message-----
From: TechNet [mailto:[log in to unmask]] On Behalf Of Inge
Sent: Thursday, July 31, 2008 1:52 PM
To: [log in to unmask]
Subject: Re: [TN] Solder Paste with craters.

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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