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September 2011

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Subject:
From:
Robert Kondner <[log in to unmask]>
Reply To:
Date:
Tue, 20 Sep 2011 14:12:35 -0400
Content-Type:
text/plain
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text/plain (297 lines)
Given that it is a wetting problem to the PCB can't we narrow down the
issues here?

  1.  There was something wrong with the paste. I think this is  unlikely.

  2. There was something wrong with the PCB.  Possible. Plating and
contamination are possibilities.

  3. There was a bad reflow profile.  Not likely since large components
seemed to reflow OK. 

 I mean the boards are scrap, right? Lifting a lead and pulling the solder
off the pad is a complete horror story, right?

 The only real question is who pays for everything?

 Please, enlighten me to the other possibilities! :-) 

Bob K.



  

-----Original Message-----
From: TechNet [mailto:[log in to unmask]] On Behalf Of Reuven Rokah
Sent: Tuesday, September 20, 2011 2:02 PM
To: [log in to unmask]
Subject: Re: [TN] large parts falling off finished surface mount board
assemblies

Hi Carl,
It seems wetting issue.  You should do the following:
1. Cross section of bare and assembled boards to measure the Ni and Au / IMC
layers thicknesses .
2. Check bare PCB with the same solder paste used in reflow but without
components, to check wetting.
Reuven

On Tue, Sep 20, 2011 at 7:52 PM, Carl VanWormer <[log in to unmask]> wrote:

> If this really is a PCB problem, is there anything the assembly house 
> can do to mitigate the problem?  Is there an easy (nondestructive) way 
> to tell if a board will have this problem?
> Thanks,
> Carl
>
>
> -----Original Message-----
> From: Wenger, George M. [mailto:[log in to unmask]]
> Sent: Tuesday, September 20, 2011 6:58 AM
> To: TechNet E-Mail Forum; Carl VanWormer
> Subject: RE: [TN] large parts falling off finished surface mount board 
> assemblies
>
> Carl,
>
> The pictures indicate you have a brittle fracture at the ENi/Ni-Sn 
> IMC/ Solder interface.
>
> Regards,
> George
> George M. Wenger
> Senior Principal Reliability / FMA Engineer Andrew Corporation - 
> Wireless Network Solutions
> 40 Technology Drive, Warren, NJ 07059
> (908) 546-4531 Office (732) 309-8964 Mobile
> E-mail: [log in to unmask]
>
> -----Original Message-----
> From: TechNet [mailto:[log in to unmask]] On Behalf Of Carl VanWormer
> Sent: Tuesday, September 20, 2011 9:51 AM
> To: [log in to unmask]
> Subject: Re: [TN] large parts falling off finished surface mount board 
> assemblies
>
> Steve has posted pictures of my PCB failures on his site.  Upon 
> further examination, prompted by several suggestions from the group, I 
> have found that the solder seems to stay with the components, leaving 
> just the dull, fine granular surface that seems to be the actual pad 
> texture without any Gold showing on the top surface.  Does this mean 
> that the Gold combined with the solder (as expected), and then the 
> remaining solder/Gold/Nickel/inter-metallics (Technet words I picked 
> up) boundary failed?
>
> We have seen 2 bad assemblies (from this batch of 30) that failed in 
> this manner, and 5 good assemblies (still in our stock) that take 
> significant force without failures.  We are in the process of checking 
> any more assemblies we can locate.
>
>  - What made these bad boards different from the others in the same 
> production run?
>  - Can these bad boards be "fixed" by re-processing through the IR oven?
>
>  - How can I keep this from ever happening again?
>
> Any more thoughts, hints, and/or opinions are appreciated.
>
> http://stevezeva.homestead.com/csi1-pads_and_solder_on_leads.jpg shows 
> a crystal that popped off with medium fingernail pressure.  Almost all 
> of the solder stayed on the leads, with the surface of the PCB pads 
> left flat with no major accumulation of solder.
>
> http://stevezeva.homestead.com/csi2-pads_and_solder_on_leads.jpg shows 
> an inductor that popped off with medium fingernail pressure.  Like the 
> crystal, the solder stayed with the inductor pad.  It could be peeled 
> of the inductor pad with the knife blade.  When the knife blade was 
> scraped across the PCB pad, there was no significant quantity of 
> solder, and the hard granular surface made a much "harder" scraping sound.
>
> http://stevezeva.homestead.com/csi3-pads_after_component_popped_off.jp
> g shows the pads where a SOT-23 transistor was popped off, leaving 
> similar no-solder PCB pads with all the solder left with the good 
> looking fillets on the SOT-23 leads.
>
> http://stevezeva.homestead.com/csi4-pad_before_soldering_thumb.jpg 
> shows a bare board pad, before the soldering process.
>
> http://stevezeva.homestead.com/csi5-pad_after_component_popped_off.jpg
> shows the same position, after the pad has been processed, then the
> SOT-23 had been popped off of that pad.  There was no significant 
> accumulation of solder left on the PCB pad.
>
>
> Thanks,
> Carl
>
>
> -----Original Message-----
> From: Steve Gregory [mailto:[log in to unmask]]
> Sent: Monday, September 19, 2011 3:32 PM
>
> Hi Carl,
>
> Send me some pictures and I'll post them...
>
> Steve
>
> -----Original Message-----
> From: TechNet [mailto:[log in to unmask]] On Behalf Of Carl VanWormer
>
> To test this possibility, I wedged the X-Acto blade under the plastic 
> package of a SOT-23 transistor.  A pin lifted up.  I held that pin 
> down with a fingernail, re-inserted the blade, and the other two pins 
> popped off the board.  All 3 pins had solder blobs the shape of the 
> pads, and all 6 surfaces were uniformly granular.
>
> Does anybody want to see some pictures?  If so, I can try to get some 
> to Steve.
>
> Thanks,
> Carl
>
>
> -----Original Message-----
> From: TechNet [mailto:[log in to unmask]] On Behalf Of Goodyear, Patrick
>
> I'm with Phil, sounds to me like the thermal mass on the component was 
> too large for the 'BIG' components to fully heat and they never 
> actually got a melt or good reflow, due to too short a dwell time, the 
> only thing that was holding the component on was the binder in the 
> flux in the paste which had slightly solidified with a minor melt.  
> But that is just a guess.
>
> Pat
> Pacific Gas and Electric
>
> -----Original Message-----
> From: TechNet [mailto:[log in to unmask]] On Behalf Of Phil Bavaro
>
> I might have missed something but are you sure the solder actually 
> melted?  It almost sounds as if the bottom side heaters in the reflow 
> were off and you only got topside heat applied.  You should not have 
> been able to see any yellowish metal beneath the solder connections.
>
>
>
> -----Original Message-----
> From: Carl VanWormer <[log in to unmask]>
>
> A surface mount PC board assembly (made by a local assembly house and 
> passed by our outgoing test) was returned from the end user, along 
> with an electrolytic capacitor that they had "knocked off" the board.  
> After looking at the assembly and component under a microscope, I 
> noticed that the "gold plated" PC board (Lead free) had a fine 
> granular solder appearance (much smaller grain than solder paste 
> balls) on the pads newly exposed, with the whole "melted solder" blobs 
> (in the shape of the
> pads) attached to the capacitor leads (with a similar granular 
> appearance).  After minor scraping of various parts, I had come to no 
> brilliant conclusions, so I pressed my finger nail against an inductor 
> that was of similar size to the capacitor.  It popped off the board 
> with the same symptoms as the capacitor.  After recovering from my 
> surprise, I realized that I have no clue about the causes or cures for 
> my component adhesion problem.  Can any of the more experienced 
> members of this fine group direct me to some key words that might lead 
> me to a quick education (and solution)?
>
>
>
>
>
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-- 

Best Regards,

*Reuven Rokah*

Mobile: 972-52-60-120-18
Tele-fax: 97239360688
<http://www.rokah-technologies.com/>[log in to unmask]
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