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April 2008

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From:
"Thayer, Wayne" <[log in to unmask]>
Reply To:
TechNet E-Mail Forum <[log in to unmask]>, Thayer, Wayne
Date:
Fri, 11 Apr 2008 09:27:09 -0400
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Hi Bev!

I have seen problems in this area, where I did not have enough data to
get satisfactory statistical evidence and all other indicators--X-RAY,
adjacent parts, visual--indicated things were OK.  I believe my problem
was running these parts with tin plated leads and the associated
oxidation through an RMA flux based SnPb line.  I am concerned that the
flux/temp combination of this line is not aggressive enough to strip the
oxide from these parts.

Now I am receiving parts from other vendors to be used in a higher level
assembly I'm responsible for--so I am asking for "evidence of wetting"
in order to pass IPC criteria.  So we are arguing about what this
evidence might be.

Wayne 

-----Original Message-----
From: TechNet [mailto:[log in to unmask]] On Behalf Of Bev Christian
Sent: Friday, April 11, 2008 9:19 AM
To: [log in to unmask]
Subject: Re: [TN] FN Parts--Evidence of Wetting

Wayne,
It doesn't prove it, but if the x-ray images show solder everywhere
across the pads or everywhere minus a few void areas then that is a good
sign.  If all the solder was jammed up to one side of the pads or off
the pads then the alarm bells should be going off.

Also, you could do solderability testing of components before they are
used. However, there are problems with this too. Unless the problem is
endemic, you will never statistically find it testing 5 to 10 parts.
And for a lot of these suckers are so small that I am essentially using
my wetting balance as a fancy dip and look tester, rather then a piece
of equipment that gives me numerical data.
Bev
RIM

-----Original Message-----
From: TechNet [mailto:[log in to unmask]] On Behalf Of Thayer, Wayne
Sent: Friday, April 11, 2008 9:12 AM
To: [log in to unmask]
Subject: Re: [TN] FN Parts--Evidence of Wetting

Hi Dave!

Saw you at IPC/Apex and was going to say hi, but you looked way too
busy.

So, from your response below, the FN parts are treated very differently
from all other SMT parts:  Whereas most parts require a visible,
measurable fillet to confirm wetting, acceptable evidence of wetting for
the FN parts a combination of the following:

-an adjacent part with fillets looks like it flowed real good and meets
IPC specs.
-the solder on any visible portion of the pads looks like it got plenty
hot and flowed on the pad -you have documented destructive SPC data on
"sister" boards and have demonstrated/documented good Cpk on all
contributing factors

OK, maybe that's acceptable to IPC, but it makes me darn uncomfortable
if I'm dealing with a short run of units I got built at a contract
manufacturer.  I guess I'm supposed ask for thorough documentation of
all of these contributing factors so I can confirm I have good joints. 

If this is really the case, it seems different enough from the rest of
the IPC specs that perhaps some additional clarification is called for
to explain the note "evidence of wetting".  Not only that, but if IPC
really thought that statistics is the right way to get the evidence,
they are smart enough to enter a numerical number for the statistical
criteria to meet.  Maybe different thresholds for the three classes.

The idea that the kind of evidence you have described is acceptable for
Class III (medical/military/aero, etc) gives me the creeps because many
of these apps have short production runs (likely not enough statistical
data to make me comfortable).  Furthermore, while the original FN
packages always had instructions from the manufacturer to place a
circuit board pad for and solder attach to the die paddle, many
manufacturers are neglecting this today--in fact I have found one QFN
where the data sheet says specifically to NOT attach to this.  So the
total stress is just on the pads where the only requirement is "evidence
of wetting".

Thanks,

Wayne Thayer

-----Original Message-----
From: TechNet [mailto:[log in to unmask]] On Behalf Of David D. Hillman
Sent: Friday, April 11, 2008 8:15 AM
To: [log in to unmask]
Subject: Re: [TN] FN Parts--Evidence of Wetting

Hi Wayne! Ah, if it were only that easy! A QFN component essentially is
a "blind solder joint" so you must rely on your process to produce an
acceptable solder joint and confirm that process consistency through
testing. We are faced with a variety of component styles now, such a
bottom only terminated devices, where there is no visible solder joint
available for inspection. Using a combination of process validation,
Xray inspection techniques and product integrity testing, you should be
able to confirm the solder joint integrity. A very common industry myth
is that we "inspect" quality into our electronic assemblies - the
reality is that the inspection step is only a confirmation that your
design and assembly processes are controlled/consistent. The JSTD-001
and IPC-610 specifications attempt to provide as much assistance as
possible. Good Luck.

Dave Hillman
Rockwell Collins
[log in to unmask]



Wayne Thayer <[log in to unmask]> Sent by: TechNet
<[log in to unmask]>
04/10/2008 09:35 PM
Please respond to
TechNet E-Mail Forum <[log in to unmask]>; Please respond to Wayne Thayer
<[log in to unmask]>


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Subject
[TN] FN Parts--Evidence of Wetting






Relevant IPC specs claim that no toe fillet is required for DFN/QFN
parts and there is no minimum solder joint height.  The only requirement
for the joint other than alignment/pad size is "evidence of wetting".
How do I get this 

evidence without a toe fillet?  I suppose if I have enough joint height
I can see under the part like a BGA, but I've never seen a FN part
mounted that high.

Would be nice if the spec included some pix/examples of this "evidence".

Any recommendations would be very much appreciated.

Thanks,

Wayne Thayer

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