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Poor wetting to palladium

Scott B

#5072

Poor wetting to palladium | 29 January, 2001

I have recently come across a device which has a palladium over nickel finish which when soldered shows evidence of poor wetting to the lead ( i.e. a wetting angle greater than 90�) about three quarters of the way up the side of lead. This is only visible at 10-20 times magnification (due to these parts being 20 thou pitch) which is confirmed when slight pressure is applied to the heel of the component lead and the lead pops out of the solder leaving an imprint of the lead.

The component in question is specified as having a minimum of 3 microinches of palladium over 40-60 microinches of nickel plate on a copper base metal. 3 microinches sound very little to me. Is this adequate to prevent oxidisation of the nickel plate underneath.

Has anyone else seen this problem.

We have checked our profile as having a peak of 221�C. It has been suggested to us that we should incresase peak to at least 230�C but we are worried about the effect on the BGA's at this higher reflow temp.

These parts pass the normal Mil-Spec solderability testing but I am not convinced by this as it is not representative of the surface mount assembly process.

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#5077

Poor wetting to palladium | 29 January, 2001

Several points about your query are:

* TI�s solderability protection specification is a minimum of 3 u" of palladium over 40-60 u" of nickel plate over a <5u" palladium nickel strike over a <5u" nickel strike on a copper base metal.

* Many people require 3 to 8 u" of gold over nickel as part of their ENIG solderability protection. Palladium is much less dense than gold (12.0/19.3). So, your minimum of 3 u" of palladium is probably adequate to prevent oxidization of the nickel plate underneath. [It might be reasonable for your to pursue that idea from a different angle by questioning if the nickel was corroded before the over-plate was applied, if the plating is actually GT 3 u".]

* Palladium prevents of the nickel under-plate from oxidizing. During soldering, the palladium dissolves into the solder and then the solder joins to the nickel. The dissolution rate of palladium in solder is dependent on temperature.

Temperature �C | Pd dissolution rate into solder u"/sec|| 215 | 0.7||250 | 2.8

This indicates the temperature / time necessary to dissolve the palladium into the solder before a solder connection can be made to the nickel. [So, you might consider increasing temperature to a level that you feel your components will bear together along with decreasing your conveyor speed]

* In "Evaluation of Water Soluble and No-Clean Solder Pastes with Palladium Plated and Solder Plated SMT Devices", D Romm and N McLellan [1/93 SMT magazine] found that the solder paste formulation has a strong impact on contact angle of the solder joint. Palladium plated devices exhibited a greater sensitivity to changes in solder paste, however the performance trend of the pastes was similar for both lead finishes. At least one no-clean paste and at least one water soluble paste that they tested performed well for both palladium plated and solder plated devices.

* TI talks to palladium at http://www.ti.com/sc/docs/products/logic/package/palladm/index.htm

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