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| Date: | Fri, 10 Jan 1997 10:27:14 -0600 |
| Content-Type: | text/plain |
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Thad,
After reading your reply to Pratap I have a few other questions for
you or anyone else on the forum. I have considered Intrusion Reflow
on several PWAs that are 98% SMT with the thru-holes being connectors.
Since we are using a 6' table top Zevatech Reflow oven its hard to get
the appropriate reflow desired versus using a 15-20 foot Heller (its
coming).
I've set-up several evaluation test parameters (ESS, Xsectioning,
thru-hole pull test - as compared to Wave, etc.) but would like some
input on cost issues. I don't mind making a matrix for cost but would
prefer some platforms to begin from.
1. What cost differences (more or less) has anyone determined between
using Solder Paste and Solder Pot? My view, your cost will
defintely increase by using paste rather than standard SnPb Wave.
Assume that you can not turn your Wave into scrap metal due to some
thru-hole designs.
2. What time studies, if any, have been done to compare the two
processes? A typical 22mil, 25mil or 32mil Connector thru-hole
will require at least 3-4 passes per side to achieve an optimum
solder joint (as described by Prasad).
A recessed solder joint should still be acceptable based on the
PWAs environment and use.
3. How is anyone maintaining the past removal as the thru hole
component or connector is Manually or Automatically inserted?
There is definetly a paste volume loss in this operation.
Please advise.
John Gulley
Inet Inc.
Plano, TX
______________________________ Reply Separator _________________________________
Subject: Re: ASSY: IR Reflow of Thru-Hole Components
Author: Pratap Singh <[log in to unmask]> at Internet
Date: 1/9/97 7:56 PM
Thad McMillan wrote:
>
> Some questions for the Technet,
>
> One of our PCA Assembly subcontractors uses IR Reflow to reflow
> through hole parts instead of the traditional Wave. They typically
> use this technique for high density double side SMT/Through hole
> assemblies where there is no room for the keepouts necessary to mask
> the SMT parts during the wave process.
>
> The technique involves a double Paste screening operation to squeegee
> paste into the through holes. The board is run through IR to reflow
> both the SMT and the Thru Hole parts. The solder joints look
> completely different than a typical wave reflowed thru-hole part. In
> fact I think they would be considered rejectable per IPC-A-610B. The
> solder joints look as follows (forgive the art, I hope this tranlates
> well):
>
>
>
> | |
> ___________ | | __________
> | | | |
> |* | | *|
> |** | | **|
> |****| |****|
> |****| |****|
> |****| |****|
> |****| |****|
> |****| |****|
> |****| |****|
> |****| |****|
> |****| |****|
> |****| |****|
> |****| |****|
> |****| |****|
> |****| |****|
> |** | | **|
> |* | | *|
> ________| | | |___________
> | |
> | |
> |______|
>
> Where "*" is solder.
> ---------------------------------------
>
> I have several questions for the forum and would greatly appreciate
> feedback:
>
>
> 1. Is there an industry term for this type of reflow. I've heard
> it referred to as "intrusive reflow". Since I love catchy names, I'm
> soliciting one.
*Intrusive reflow is the current term used for PTH joints made through
SMT process of paste screen and reflow. I call it Solder Reflow In Hole
(SRIH).
> 2. Is the technique in widespread practice in the industry or is it
new?
*It is a relatively new technique, however IBM developed a similar
process 1986-1989 time frame.
>
> 3. Any comments on the reliability of such a solder joint.
*There is no reason (metallurgically) for it to be any different from
that of Wave solder so far as there is proper wetting of the interfaces
at the pin and PTH copper by the solder.
>
> 4. Do standards (IPC or otherwise) exist that describe a "good" joint
and a "bad" joint. I'm interested in IPC Class II product.
*Any joint that works in a given environment is "GOOD" and that which
does not work is "BAD" (no pun intended). PTH joint is a robust joint in
almost all cases except when poor quality copper is plated in the holes.
An indication of poor PTH plating is 'blow hole' and 'solder balls' and
very poor fillets or no fillets at all (for wave solder process).
>
> 5. Are there any tricks of the trade in designing boards for this
> type of assembly. For example do I need to use modified (i.e.
> smaller) thermals on my through holes to insure good reflow.
*Heatsinking effect of 'thermals' should remain same or a little less
pronounced because SMT reflow is at higher temperature.
>
> 6. Are there special processing techniques in assembly that will
produce good joints. Example - screening paste on both sides or
using special pastes.
*Proper volume of paste in the hole is critical part of the process.
>
> 7. How can these joints be non-destructively evaluated since they
can't be seen other than microsection. Do periodic destructive
tests need to be done per an AQL.
>*The establishment of optimized process parameters is important.
xsectioning in early process development phase is critical to set the
screening and reflow parameters.
>
> 8. I'm aware that any through hole components that use this
> technique must be made of high temperature material to withstand IR, but
are there any other restrictions regarding what type of
components this assembly technique can be used for.
>*Check the compatibility of the PIH component temperature profile with
reflow temperature profile. Call your component supplier.
> I would appreciate any feedback or comments you might have on this.
>The answers are marked with *.
> Thanks,
>
> [log in to unmask]
>
>
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--
Pratap Singh
Tel./Fax: 512-255-6820
e-mail: [log in to unmask]
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