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.jpg
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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