Rick Thompson said:
>With all the current discussion of solderability issues, I'm hoping some of
>you may be able to apply your expertise and give me some insight into a
>problem we seem to consistently have. When soldering components with
>palladium plated leads, we do not seem to get any wetting on the leads. It
>kind of looks like the 'footprint in mud' affect. On pcb's where we have
>this condition, all other components have soldered fine with good fillets
>and wetting. Checks of the oven profile indicate a good profile for the
>paste we're using (Kester R598, water soluble). Currently we've only found
>this on components from TI, so I don't know if it's only their components or
>the plating in general.
IMHO, TI is still the lone major semiconductor source using this leadframe
finish.
>Question: Are there any recommendations for profile, paste, process, etc.
>that differ from 'standards' when working with these types of components?
This thread dates back to October 2, and I have seen the replies from Dave
Hillman, Doug Romm, and several others.
In cleaning through the piles on my desk today, I ran across an article
from the Nov 97 Circuits Assembly magazine (p. 32) by Timothy McQuiggan and
Elizabeth Benedetto. The authors work for Compaq Computer Corp. The
conclusions of this year old article are along the lines that the soldering
of leads with palladium finish will require process changes to ensure good
results. You may wish to find the article and read it.
The article suggests:
- increasing the overall board temp reflow profile to accomodate relatively
long dissolution time for Pd
- manual touch-up of palladium joints
- changing to N2 atmosphere in reflow
- changing solder paste
I have also seen visual results on these parts which are classical
indications of poor wetting. My understanding is that TI suggests that
re-addressing the visual inspection criteria to permit the Pd lead finishes
may be done without compromising reliability.
While this may be true, I am still in the camp which prefers not to use
this lead finish if given a choice. With many assemblers and OEM's
switching to flux and paste chemistry that compromises activity for the
sake of cleaning elimination, the suppliers of the devices must meet an
obligation to show why this lead finish should be adopted, and provide
technical and cost motivation to the users. TI seems to be attempting this,
but there are skeptics in the industry.
regards,
Jerry Cupples
Interphase Corporation
Dallas, TX USA
http://www.iphase.com
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