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June 1998

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From:
"Rupert, Martha L." <[log in to unmask]>
Reply To:
TechNet E-Mail Forum.
Date:
Thu, 25 Jun 1998 10:34:31 -0400
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Hello Jon,

I would like to address your question concerning intrusive reflow.  To make
sure we're talking about the same thing, intrusive reflow is "the soldering
of leaded components in plated through holes using a surface mount reflow
process".

I will limit my comments as it pertains to connectors because that is what I
am familiar with.  There are many different styles of connectors used in the
industry and not all of them should be "true surface mount" products
(example, large I/O's, high current carrying, etc).  This being the case,
these same connectors may remain as "through hole" designs and still be
processed through surface mount reflow.

The first requirement is that the connector housing material be "high
temperature", that is, compatible with the temperature extremes of the
reflow process.  This information should be readily available from your
connector supplier and will most likely vary from product to product.  You
will encounter polymers such as PPA, SPS, PCT, LCP and HTN which are
available from many materials suppliers under various trade names.  If
you're using a convection reflow process, then pretty much any "high
temperature" polymer will be acceptable.  Years ago when IR and focused IR
units were prevalent, color was a concern because some of these connectors
are very large (compared to other devices) and are usually black and,
therefore, they make very good heat sinks.  This is not the case with
convection reflow.

The second requirement is for an "acceptable" plated through hole to
component lead cross section ratio.  "Acceptable" will depend on your
requirements for a final solder joint (i.e. IPC-A-610A). Typically, good
candidates for the intrusive reflow process will have square or circular
cross sections (i.e. 0.5-0.65mm square/diameter) vs a "blade" (i.e. 0.5 x
0.25mm).  (These types of leads will be placed in a 1mm diameter plated
through hole.  It will be very difficult to deposit enough solder paste for
the blade style contact to achieve complete hole fill on a final solder
joint.)  Remember, you will be using a stencil thickness that has been
chosen because of other fine pitch products on the board.  Because of that,
a solder deposit on only the surface of the board will not be near enough
paste (your deposit needs to be approximately twice the final solder volume
requirement) and you will need to deposit paste into the plated through hole
as well.  The majority of the volume of paste deposited will be in the
plated through hole. (Stepped stencils are an option, but you will lose
valuable board real estate surrounding your connector in order to account
for the transition areas.)

The number of rows in the connector is an issue as well.  One and two row
products are more acceptable vs three, four and five rows.  Again, the
solder deposit is the limiting factor.  The portion of the deposit on the
surface of the board may exceed the contact centerline spacing on the
connector.  (The paste does not have to be centered over the plated through
hole, therefore, two row product will still work.)

The aspect ratio of the plated through hole to board thickness will play a
role in determining the solder paste deposit.  If you have a small diameter
hole (i.e. 0.8mm) in a thick substrate (i.e. 2.36mm) you will not be able to
get enough paste into the plated through hole.  The solder pastes in use
today have such high viscosities (fine pitch requirement) that one pass of
the squeegee will not have enough "oomph" to fill the plated through hole.

Once the leaded component is placed on the board, the portion (or cross
section) of the lead extending into the plated through hole will displace
the solder paste. This paste will not "fall off" the lead but will "wick
back up" the lead and become part of the final solder joint.

I apologize for the lengthy response, but if you wish to discuss this
further, please contact me offline.  I do have some papers I would be happy
to share with you on work I've done with some connectors.

Regards,

Martha Rupert
AMP Incorporated
Phone:  717-592-4458
Fax:    717-592-3180
Email:  [log in to unmask]


> -----Original Message-----
> From: Benedicto Cruz [SMTP:[log in to unmask]]
> Sent: Thursday, June 25, 1998 7:19 AM
> To:   [log in to unmask]
> Subject:      [TN] see items stated
>
> Hi!
>
> I would like to ask for an effective stencil aperture design for
> fine-pitch ICs. We are encountering some solder bridging on these
> ICs. Also, there are solder balls present beside these components.
> Do you have any stencil design guidelines that you could share with
> us to improve our Screen Print Process?
>
> Also, Are you familiar with intrusive reflow? Can anyone share some
> informaiton about this?
>
> We also have a product that uses a MELF SMD resistor. The problem
> that we are encountering is that this component rolls during solder
> paste curing. So when it comes out, the component is already
> misaligned. Do you have any suggestions to control this without using
> any fixtures?
>
> Thanks.
>
> Regards.
> Jon Cruz
> Electronic Assemblies Inc.
> Manila, Philippines
> e-mail: [log in to unmask]
> Tel #: 823-7317/7593885
> Fax #: 7538629
>
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