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March 2011

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Subject:
From:
Graham Collins <[log in to unmask]>
Reply To:
TechNet E-Mail Forum <[log in to unmask]>, [log in to unmask]
Date:
Thu, 17 Mar 2011 09:52:49 -0300
Content-Type:
text/plain
Parts/Attachments:
text/plain (289 lines)
That is some funny s**t right there!  Made my day.  Who is it?

regards,
 - Graham Collins

-----Original Message-----
From: TechNet [mailto:[log in to unmask]] On Behalf Of Steve Gregory
Sent: Thursday, March 17, 2011 8:06 AM
To: [log in to unmask]
Subject: Re: [TN] Query on failure of assembled PCB

Here 'tis:

http://stevezeva.homestead.com/Black_Pad_Blues.wmv

Steve

-----Original Message-----
From: TechNet [mailto:[log in to unmask]] On Behalf Of Wenger, George M.
Sent: Thursday, March 17, 2011 6:39 AM
To: [log in to unmask]
Subject: Re: [TN] Query on failure of assembled PCB

Rex,

Well put!.  The real problem with Peter's button is that it does exist
but
it doesn't work very well.  It doesn't make ENIG disappear but it does
add
another strike against it every time it is pushed.  I know several
people
including myself who push that button as often as possible and slowly
but
surely it has an effect.

I'm not sure who wrote the attached "Black Pad Blues" tune but I like
harmonic music and except for the name of the song, which I hate and I
which
they would redo the song as "ENIG Blues", but I keep a copy on my laptop
desktop and I play it for every designer who asks me what I think about
ENIG.  Actually I'm surprised the tune isn't on Steve's web page.  If it
were I know it would be played often.

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 Rex Waygood
Sent: Thursday, March 17, 2011 4:45 AM
To: [log in to unmask]
Subject: Re: [TN] Query on failure of assembled PCB

I went to a lecture about 20 years ago by Peter Starkey who said that if
there were a button that could make ENIG disappear as a surface finish
from the face of the earth he would push it. I have often been second in
line for that button.

Some ENIG defects are worse with second reflow.
Thin Ni can cause solderability problems. The problem often reveals
itself when the second reflow is attempted. Thin Ni can result in joints
that look OK but are not mechanically sound as complete wetting hasn't
taken place. 
Then you can have porous gold or partially passivated Ni prior to gold
deposition. Again this produces joints that look OK but the wetting to
the Ni has occurred in patches on the pads. 
Both of these defects do not just affect BGAs. Shear testing on other
chip components can reveal the weakness of the joints. When the cause is
at its worst solderability is obviously very bad. Unfortunately there is
an in between where the joints don't look too bad, but they are
mechanically unreliable. I have seen components fall off the PCB when
the board edge is tapped on a bench from these defects!
There is also an ENIG problem which can be just associated with BGAs. I
know this as we are awaiting the arrival of a remake! If the designer of
the pcb has put tented or half tented vias amongst the BGA pads then
microetch can get trapped in the vias which then prevents the
electrochemistry working correctly at Ni plate. This gives poor Ni at
the BGA pads causing poor solderability and unreliable joints. 
Any of these defects mean the boards are not usable but could result in
shipped product with a compromised reliability if the defect is at a low
level..

In the recent batch problem we had we thought the yield would allow us
to recover the situation for the customer if we had the boards Hot Air
Levelled.
The boards looked absolutely normal but the yield was very low.
Interestingly using state of the art HASL machines produced a finish
even in the fine pitch BGA area that we would have been very comfortable
doing the SMT on. The company doing the HASL were unable to optimise
their hot air level as the pcbs were in routed panels and the machines
are designed to work on complete master panels. These boards were HASL'd
lead free.

Where is the evidence that HASL is not suitable for fine pitch work?
If we receive a HASL'd board it is unlikely to not solder. I would
prefer to receive a pcb that has been through a HASL process prior to
BBT and prior to our reflow process.

I am sure there are other ENIG defects that may cause your problem but
we have experience the three I have listed that might cause your
problem. We have experienced other ENIG problems but they won't cause
intermittent joints on BGAs! :-)

Regards
Rex

Rex Waygood
Technical Manager
 
Hansatech EMS provides value manufacturing through engineering and
quality
 
Hansatech EMS Limited
Benson Road
Nuffield Industrial Estate
Poole
Dorset
BH17 0RY
 
+44 (0)1202 338200
 


-----Original Message-----
From: TechNet [mailto:[log in to unmask]] On Behalf Of Uppina Nagaraj
Sent: 17 March 2011 02:28
To: [log in to unmask]
Subject: [TN] Query on failure of assembled PCB

Dear All,

1. We are manufacturers of PCBs. One of our customer has reported
   failure in assembled boards. The main observation is that after
   assembly, the assembled units were functioning normally
   and had passed all the stipulated electrical tests. These parts
   were kept at the assembly house for around over 3-4 months. Later
   when the assembled units were scheduled to be integrated into
   the system, the assembled units were tested again, but now they
   see failure.

    These are the inputs given to us..........
    a) Failures are intermittent in nature.
    b) Failures are concentrated in and around the area where
       BGA is mounted.
    c) Some failures are recovered by applying mechanical stress on
       the PCB.
    d) Some are recovered by subjecting to reflow.
    e) There are no component failures.

2. Our customer's assembly process flow is...
    a) 100Deg C, 16 hours baking
    b) Assembly done within 24 hours after baking
    c) Pb-free reflow conducted with a peak temperature of 262Deg C
    d) Reflow is done 2 times (once for each side of components)
    e) Visual and physical inspection done
    f) Cycling done at -20~+70Deg C for 10 cycles. First 9 cycles
       are 30minutes per cycle and the last cycle is 1 hour cycle
   g) Electrical testing

3. We had retrieved a blank PCB from the same batch and conducted
   the following trial......

   a) A standard step of baking prior to assembly (100Deg C,
      16 hours baking) was skipped due to time constraint.
   b) 2x reflow done at customer's place using standard profile
      used for assembly of this part.

     Electrical Testing (BBT) on bare PCB passed .

   c) 10 cycles of temp. cycling done at customer's place using the
      standard cycling profile used for assembled unit.

      Electrical Testing (BBT) on bare PCB passed .

   d) Repeated 2x reflow (2nd time) at customer's place using
      standard profile used for assembly of this part.

      Electrical Testing (BBT) on bare PCB passed .

     Based on the above trial, we feel that the failure seen is
     not due to PCB failure.


4. This particular PCB is a 8 layer PCB, built using ISOLA FR370HR
   material.
-----------------------------------------------------------------


Please share your inputs and thoughts on the failure phenomenon and how
to ascertain if the failure is due to PCB or assembly.

Thanks in advance!

Best Regards,
U.Nagaraj



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