Neda -

Touching up chip caps with a soldering iron is a very risky proposition. The
problem is the rapid temperature delta between the point where the tip
touches the cap and the rest of the component. You can easily cause
microcracking internal to the capacitor that would not initially be visible
and would probably pass all your testing. However, it is a latent failure
mechanism especially with high bulk capacitors such as those with a Z5U
dielectric.

As an analogy, think of the glass windshield on your car. If you get a small
crack in it, the crack will gradually grow even with relatively slow
temperature changes. Ceramic and glass have very similar thermal properties.

Hot air provides a much more even temperature delta and a slower ramp rate
to solder liquidous, and the process will be somewhat slower. Obviously, if
you are throwing away the cap as opposed to touching up, use whatever heat
source you want as long as you don't overheat or burn the board.

Ed Valentine
Electronics Manufacturing Solutions
8612 Mourning Dove Road, Raleigh, NC 27615
Phone: (919) 270-5145, Fax: (919) 847-9971
Email: [log in to unmask]
Website: http://www.ems-consulting.com

----- Original Message -----
From: "Neda Thrash" <[log in to unmask]>
To: <[log in to unmask]>
Sent: Wednesday, December 05, 2001 4:53 PM
Subject: [TN] SM Ceramic Caps - Touch-Up/Replacement


Quite often I see the need to add solder to the end caps of surface mounted
ceramic capacitors. Normally I would do this with hot air and solder paste
because of the part being more heat sensitive but some of the touch up areas
I go to only have soldering irons for rework. Assuming the operator is using
the correct tip size for the part and the temp set at 600 degrees F,  does
this pose a great threat to the component ( internal cracking from heat) ?

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