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TECHNET  July 2008

TECHNET July 2008

Subject:

Re: Solder Paste with craters.

From:

Paul Edwards <[log in to unmask]>

Reply-To:

TechNet E-Mail Forum <[log in to unmask]>, Paul Edwards <[log in to unmask]>

Date:

Thu, 31 Jul 2008 17:36:37 -0700

Content-Type:

text/plain

Parts/Attachments:

Parts/Attachments

text/plain (472 lines)

Richard,

We have been using Type 5 for 2 years with no problems...
Also used type 6 and 6+ without the issues Leif has been having...
Difference is shorter shelf life, more abrupt change when the solder paste reaches end of life BUT you can get more solder down on finer features...

The solder paste cratering I have seen was from SAC type 4 W/S paste that was exposed to moisture and glycol... What occurred was the during paste printing an air bubble was deposited in the paste or on the paste...After a few minutes the bubble would rise to the surface or if on the surface, would just burst leaving a small rounded crater in the top of the paste...

We went back to alcohol for stencil cleaning and opened up a new solder paste jar and the problem went away...

Paul Edwards
Surface Art Engineering


-----Original Message-----
From: TechNet [mailto:[log in to unmask]] On Behalf Of Stadem, Richard D.
Sent: Thursday, July 31, 2008 2:23 PM
To: [log in to unmask]
Subject: Re: [TN] Solder Paste with craters.

Hi, Leif
Thanks for your response earlier. It looks like you have a good handle
on the paste handling, etc.
I was going to ask why a type 5 paste was being used. I have a tough
time with a type 4, and generally speaking the industry uses a type 3.
With a type 5, you have an order of magnitude more fines, and they are
much smaller. Because this effectively raises the true metal content
above 94%, there is actually a little less flux, which I think is a good
thing.
But a type 5 is much tougher to control from a printing standpoint. The
fines, being smaller, separate more easily from the agglomeration of the
printed mass, and thus there are more issues with stray fines, and with
small solder balls spitting away in large flat areas, such as under cap
terminations and belly pads, etc.
So the first question is whether you need a type 5 in order to print
through some very small apertures, such as on flip chips, etc.
If not, try going to a type 4 or 3, and this may help with the stray
fines.

-----Original Message-----
From: TechNet [mailto:[log in to unmask]] On Behalf Of Leif Erik Laerum
Sent: Thursday, July 31, 2008 2:46 PM
To: [log in to unmask]
Subject: Re: [TN] Solder Paste with craters.

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 20 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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