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December 2006

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Subject:
From:
"Stadem, Richard D." <[log in to unmask]>
Reply To:
TechNet E-Mail Forum <[log in to unmask]>, Stadem, Richard D.
Date:
Wed, 20 Dec 2006 13:39:07 -0600
Content-Type:
text/plain
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text/plain (102 lines)
A good wave profiler can be made of Durostone CAS 760, with TCs in the
far corners of the fixture (flush to the bottom of the fixture, exposed
to the wave) to measure lag between corner 1 and corner 2, for the
purpose of establishing coplanarity of the wave to the board. There
should not be a significant time lag between the spike seen by TC1 and
TC2 when they hit the pot.
Then put TC3 in the center of the fixture (mounted the same way, the
same distance from the leading edge as TC1 and TC2) to measure dwell
time in the pot all the way across. TC1 and 2 can also be used for this,
but TC3 ensures the time in the solder is the same across the bottom of
the fixture. Realize that with larger pwbs the sag can cause differences
in the total immersion time at different locations.
Three lug-terminal TCs (www.omega.com) should be clamped to the bottom
to measure the pre-heat temps from the bottom preheater, and three on
the topside for measuring the topside temps.

This fixture is not intended to measure the temperatures that a pwb
would see, it is only intended to be used as a process control fixture
for the sake of verifying the machine is producing a repeatable profile
for dwell time, co-planarity, solder peak temp, etc.. It should be run
and checked each day that the machine is used for production, and
compared with a benchmark profile. Over time, as the preheater and pot
temps drop (normal) the machine settings will need to be adjusted to get
the profile back in the required range. Measurements should be made
based on a trigger, ie, phase-adjusted to the point in time that the
profile hits 50 deg. C. to accomodate small changes in climate. The
reflow temp just prior to the spike when the TC hits the pot, the peak
solder temp, the ramp rate, the time lag between TC1,2,3 (coplanarity)
and the time lag between the average of TC1,2,3 and the average of
TC5,6,7 (belt speed repeatability) should be monitored and tracked with
MINITAB or other process control software and the CpK established and
monitored.
This is what the minimum process control is for a wave solder machine.
But you can do better than that.

To determine an optimum profile for a particular pwb, you should use a
pwb (scrap solder coupon). But don't use a pwb for a daily process
control check fixture, as a pwb will not hold up very long for this
purpose and the readings will change drastically over time, even from
one run to the next. Having to remove busted thermocouples invalidates
the medium used to measure the temps, as every time you need to replace
one, it reads differently than the last. That is why a good fixture with
TCs embedded in the material so that only the business end (the weld) is
exposed, or with lug-terminal TCs that remain very robust over time will
work well. 
Have fun.

-----Original Message-----
From: TechNet [mailto:[log in to unmask]] On Behalf Of Dominic Boudreau
Sent: Wednesday, December 20, 2006 12:46 PM
To: [log in to unmask]
Subject: [TN] Wave soldering profiling

Hi Technet!

My question is simple: Where should I put the thermocouples to do an
accurate wave soldering profile?

We have different points of view in this matter, some say that I should

put the probes on top of PTH and some inside PTH.

Anyone have any suggestions?

Is there any literature which should help?

Thank you

 

Dominic Boudreau

Digico Fabrication

 

 


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