Could you define "gunshot".
Thanks,
Dave
Stadem, Richard wrote:
>Voids in solder paste are normal, I do not disagree with Werner on that
>point. However, what is "normal"? I see literally hundreds of X-rays of
>solder joints per week, and based on experience I know that producing
>void-free solder joints is entirely possible, but let me qualify that
>with that now-famous line: "It depends.....".
>There are a number of ways voids can form. You can have them in your BGA
>solder spheres when the components come in the door, they can be created
>from outgassing around the solder joint during reflow (especially in
>soldermask-defined boards), they can come from chemical contaminants on
>the board prior to print and reflow, they can be induced by the solder
>paste, they can be created by contaminants caused by improper handling
>of the board and/or components, they can be caused by improper reflow
>profiles, and many other ways.
>But most of them are typically caused by too much moisture in the solder
>paste. If you get rid of that problem, your chances of producing solder
>joints void-free, or nearly void-free, improve dramatically.
>Are voids bad? IMHO they do not cause a problem if they are smaller than
>the 25% limit, but again, it depends. I have worked at companies where
>the voiding caused a component to break off during a gunshot. The
>fracture went through the void and caused the component to lift off of
>the board on one end. Now get this, every capacitor on the assembly
>fractured on one end and lifted up during the gunshot, but only on one
>end. This was the end where the split terminal induced a void, but the
>solder wetting and IMF were otherwise good.
>There have been several published papers that state the void in a BGA
>solder joint can prevent total failure if a crack is present, as the
>void alleviates the pressure once the crack propogates into it.
>But two of the papers ended up stating that it is less likely for a
>solder joint to develop a crack if the grain structure is tightly
>defined (not coarse), and no voiding is present in the first place, and
>the pads are not soldermask-defined.
>If voids can be avoided by the use of a solder paste that was formulated
>to prevent it, why not take advantage of that?
>If you don't have any voids in your solder joints, then you do not have
>to depend on an operator to make judgement calls to determine if it is
>over 25%. You do not need to worry about whether your X-ray system is
>good enough to detect them or not.
>My feeling is that if all of the solder joints on a given assembly are
>extensively riddled with voids, there is something wrong somewhere in
>the process.
>Regarding which paste to use, I switched to a particular paste for a
>previous company. It almost totally eliminated all voiding seen in both
>SMT and BGA solder joints. Furthermore, it has been an
>evaluation/qualification champion in three different companies that I
>have worked with. There are other solder paste formulations out there
>that were created to prevent voiding as well, but this one seems to be
>the best overall performer in several different categories. There are
>new formulations coming out from different vendors quite often. If you
>are seeing a lot of voiding (or spattering, or wetting issues, or solder
>strength issues, etc.) then I strongly urge you to undergo a paste
>evaluation so you can go to sleep at night knowing that you have the
>best solder paste on the market being used on your assemblies. A new
>evaluation should be performed every so often to take advantage of the
>improvements being made. I do not want to recommend any paste over the
>other, as I think all solder process engineers should perform a paste
>evaluation themselves to see what works best for their particular
>process and product. The things you learn when you perform an extensive
>and detailed paste evaluation are invaluable. It is well worth the time,
>effort, and cost.
>
>
>-----Original Message-----
>From: TechNet [mailto:[log in to unmask]] On Behalf Of Steve Gregory
>Sent: Monday, July 18, 2005 7:12 PM
>To: [log in to unmask]
>Subject: Re: [TN] BGA voids
>
>Hi Richard!
>
>I know one goal is to have zero voids. But is seeing them really a
>problem?
>
>This all came about when I've been looking at upgrading our current
>x-ray system. To be honest, I haven't seen voids here using our current
>equipment.
>
>I've seen them now with some images from a sample that I've gotten with
>a system that has more capability than our current system has. I asked
>about the voids to the applications engineer, and was told that the
>voids that he observed were normal, and not to worry about them, with
>the caveat that if they should become larger, or more concentrated in
>any one area. He told me that he has about 25-years experience working
>with x-ray inspection...so I'm not that quick to go out and shoot him.
>He didn't say not to worry about voids, but the voids that he observed
>from the sample I sent him were not a concern...he said they were
>normal.
>
>It seems that Werner agrees...
>
>>From the differing opinions, it seems the jury is still out about what
>is still acceptable, and what really is rejectable when it comes to
>voids.
>
>Right now, the J-STD-001 and -610 call out that more than 25% of the
>area of the solder sphere is rejectable...it does not specify where they
>are located.
>
>Anymore thoughts? Anyone?
>
>-Steve Gregory-
>
>***************************************************************
>
>
>
>>Hi Steve,
>>
>>
>
>
>
>>Seeing voids is indeed normal.
>>What you are referring to is in IPC-7095, Section 7.5.1.6 Process
>>Control Criteria for Voids in Solder Balls.
>>This was a suggested process control example, and is NOT a requirement,
>>
>>
>
>
>
>>nor was it meant to be one--otherwise, I would have voted negative on
>>the
>>
>>
>document.
>
>
>>Putting it like your customer did is absurd.
>>
>>
>
>
>
>>Werner
>>
>>
>
>Voids can be caused by a number of things, but the most common cause is
>the moisture content of the solder paste, and its condition.
>That being said, a certain paste that has been known to be the mainstay
>for many years currently is the worst one today as far as voiding. A
>different paste manufacturer made an excellent paste that was formulated
>to prevent voiding.
>I recently performed a paste evaluation of standard 63/37, a leadfree
>water soluble, and a leadfree no clean paste. There were 27 different
>tests, with 5 vendors. On the standard 63/37, the paste formulated to
>prevent voiding had ZERO voids in 1330 BGA joints, multiplied by 15
>samples. The previous paste vendor had voids in every single BGA joint.
>All variables were strictly controlled, and I did not know which sample
>was being tested (they were numbered by someone else to prevent any
>bias).
>While I do not want to give out vendor names on the forum, I would be
>glad to share this info with anyone offline.
>The application engineer who told you not to worry about voids should be
>taken outside and shot. While some data exists that says voids are OK if
>they are small enough, you should certainly strive for an alloy that
>provides you with an excellent grain structure free of voiding, with
>evidence of good IMF at the pad/ball interface. Sorry, pal, but you
>cannot determine this with an X-ray machine. It can only be done with a
>good microsectional analysis.
>
>
>-----Original Message-----
>From: TechNet [mailto:[log in to unmask]] On Behalf Of Stephen Gregory
>Sent: Monday, July 18, 2005 1:07 PM
>To: [log in to unmask]
>Subject: [TN] BGA voids
>
>Good Day Technetters!
>
>I have a question about BGA voids. We are bidding on some work and it
>looks like they have taken statements that are in the IPC-7095, and
>placed them as notes on their drawing.
>
>One of the notes has to do with a limit on the percentage of balls on a
>BGA device that are allowed to have voids. For example; "Less than 5% of
>the balls can have voids, and the percentage of the void area must meet
>class
>3 requirements."
>
>I can understand the void area being critical, but the percentage of the
>number of balls that are allowed to exhibit them?
>
>The reason that I'm asking, is that I was told last week by a
>application engineer from a x-ray machine company (who shall remain
>nameless), that seeing voids is normal. Just as long as they aren't too
>big, or being concentrated at either the pad/ball interface, or the
>ball/device interface.
>He said you should worry if you don't see any voids on BGA's with
>eutectic balls.
>
>Just curious as to what you all think about this?
>
>Kind regards,
>
>-Steve Gregory-
>Senior Process Engineer
>LaBarge Incorporated
>Tulsa, Oklahoma
>(918) 459-2285
>(918) 459-2350 FAX
>
>---------------------------------------------------
>Technet Mail List provided as a service by IPC using LISTSERV 1.8e 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
>additional information, or contact Keach Sasamori at [log in to unmask] or
>847-615-7100 ext.2815
>-----------------------------------------------------
>
>---------------------------------------------------
>Technet Mail List provided as a service by IPC using LISTSERV 1.8e
>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 additional information, or contact Keach Sasamori at [log in to unmask] or 847-615-7100 ext.2815
>-----------------------------------------------------
>
>
>
>
--
Dave Seymour, CID+
Catapult Communications Inc.
800 Perimeter Park Dr, Suite A
Morrisville, NC 27560
Direct: (919)653-4249
Main: (919)653-4180
Fax: (919)653-4297
[log in to unmask]
---------------------------------------------------
Technet Mail List provided as a service by IPC using LISTSERV 1.8e
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 additional information, or contact Keach Sasamori at [log in to unmask] or 847-615-7100 ext.2815
-----------------------------------------------------
|
