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December 2004

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Subject:
From:
Tom Gervascio <[log in to unmask]>
Reply To:
TechNet E-Mail Forum <[log in to unmask]>, Tom Gervascio <[log in to unmask]>
Date:
Wed, 1 Dec 2004 16:00:46 -0500
Content-Type:
text/plain
Parts/Attachments:
text/plain (209 lines)
We had experience with assemblies with 64 QFNs on a 14" x 9" high
density mixed technology board (it violates about every design guideline
known to man). Haven't experienced reflow related issues. Didn't have to
make drastic changes for Indium SMQ92J paste (63/37 Sn/Pb alloy).  We
ran it on a 8 zone Vitronics 820 oven and Electrovert Omniflo 7 oven.
The a QFN had a peak temp of 218 C with a time above reflow of 58.4Sec.
The coldest spot was a large BGA in the center with a peak temp of 213
and time above reflow of 70 sec and the hottest was a SO18 at the board
edges with a peak temp of 223 C and time above reflow of 74 sec. If
interested, contact me and I can provide the KIC profiles (usually I
hate someone doing this to me).

We did extend the PCB pads by 10 mils in the toe area to help with
maximizing paste volume and as an aid for visual inspection (we have
them look for a solder bump at the toe as a quick inspection guide).

Most of the major problems we had was with inspectability (less than 5
mm between adjacent packages) - cannot Ersascope and X ray has limited
value in detection of opens or non wetting. Other major problem was
rework- had to reball the packages since no space to stencil on board
and package is so small it is a pain to stencil paste onto part and
handle during rework.

Hope this helps

Tom Gervascio
Senior Process Engineer
Sparton Electronics
(352) 540-4040

>>> Guy Ramsey <[log in to unmask]> 12/1/2004 1:37:36 PM >>>
The QFN package can be hard to heat. Did you profile the part in
question?
You may find that the max temp under the component (where the solder
melts)
was 10 to 15 degrees cooler than the leads of a near by QFP, and the
dwell
above reflow substantially shorter than you think. This package and
those
"Land Grid Arrays" drive me nuts.


-----Original Message-----
From: TechNet [mailto:[log in to unmask]]On Behalf Of Kevin Jaworenko
Sent: Wednesday, December 01, 2004 12:34 PM
To: [log in to unmask]
Subject: Re: [TN] Help, Help, I have a QFN Package I don't know what
to
do !


In our facility we try to limit the max temperature during reflow to
220C.
Some of the components we use have max temperature ratings of 230C, so
we
prefer to have little process variation within our facility. One
profile
spec for all products. That may indeed change if/when we go lead-free,
but
for now it works for us.

-----Original Message-----
From: Joyce Koo [mailto:[log in to unmask]]
Sent: December 1, 2004 11:11 AM
To: [log in to unmask]
Subject: Re: [TN] Help, Help, I have a QFN Package I don't know what
to
do !


what is the reason to prohibit you go to 235C?
                             jk

-----Original Message-----
From: TechNet [mailto:[log in to unmask]]On Behalf Of Kevin Jaworenko
Sent: Wednesday, December 01, 2004 11:29 AM
To: [log in to unmask]
Subject: Re: [TN] Help, Help, I have a QFN Package I don't know what
to
do !


Thanks for the good questions back.

To answer the questions:

What is the lead finish of the QFN package? It is 85SN/15PB.

What is the component dimensions? They are 5.0mm by 5.0mm by 0.9mm

What paste am I using? We are using Kester easy profile 256.

How many layers in the PCB? This is a 4 layer PCB.

What is the PCB thickness? The PCB is 1.6mm thick.

Is this the only component that is not reflowing well? Yes



Thanks in advance.


-----Original Message-----
From: Morse, Carrie [mailto:[log in to unmask]]
Sent: December 1, 2004 9:32 AM
To: [log in to unmask]
Subject: Re: [TN] Help, Help, I have a QFN Package I don't know what
to
do!


Good Questions.
Also...What type of solder paste are you using?
What does your recipe/profile look like?  Ramp, Soak, Reflow times and
temps?
What is the thermal mass of the board?  Ex: # Layers, Thickness, Type
of
Layers?
Is this the only component that did not reflow well?

-Carrie


-----Original Message-----
From: TechNet [mailto:[log in to unmask]] On Behalf Of David D. Hillman
Sent: Wednesday, December 01, 2004 10:06 AM
To: [log in to unmask]
Subject: Re: [TN] Help, Help, I have a QFN Package I don't know what
to
do!

Hi Kevin! What is the lead finish of the QFN package? Having that
information will helpful to the TechNetee's in assisting you. Also,
what
are the package dimensions?

Dave Hillman
Rockwell Collins
[log in to unmask]




G'day Everyone.

        Well as the subject states we are currently running a
prototype
assembly with a new package style to me, a QFN (Quad Flat No-lead). I
had to look that one up myself.

        The problem is when we were trying to reflow the part, the
solder joint looks very poor, with little to no solder seen on the
edge
of the component lead and the pad on the PCB, or a bubble of solder
seen
on end of the component between the edge of the component lead and the
pad of the PCB.

        I'm currently setting up to X-ray the components to see if the
foot actually has a good solder joint underneath the component, where
the lead has been curled around the component, but has anyone else had
any dealings with this particular component type before and would be
willing to share?

        I looked up the component manufacture's website and they show
that this component has a required peak temperature of 235 (+5/-0)
degrees C.
That seems like a lot of heat for a non lead-free component.

        Feel free to email me offline if anyone likes.

Thanks in advance.

Kevin Jaworenko
Manufacturing Engineering Technologist
Vansco Electronics LP
(204) 453 - 3339 ext. 622
(204) 453 - 3663 FAX
[log in to unmask]
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