Oliva
We have seen many failures over the last 10 years associated with No Clean
low solids assembly processes. The failures fall into four general
catigories, electrochemical migration failures, electrical leakage failures,
visible (cosmetic only) residues and insulative failures on contact areas.
It sounds like you are experiencing electrical leakage and electrochemical
migration problems. These problems typically stem from bare board and
component cleanliness (HASL flux is high in chloride and / or bromide
activators) from fluxes, etchants, rinse water quality. The other failure
mechanism that we have seen is the flux from hand solder or touch up not
completely heated (complexed means to drive all the carrier from the flux
after it has reacted it's activation temperature, so as not to absorb
moisture). I would recommend Ion Chromatography analysis on the failed
assemblies and compare this to current production samples. After the failure
analysis is completed and a corrective action plan is put into place I would
suggest a process qualification using a test board built by your fabrication
house and run through your primary (SMT and PTH) processes and your secondary
(hand solder, temporary maskents) processes. Then subject these test boards
to SIR testing protocol identified in J-STD 001.
I have included information from a recent article that we published on
failures.
Electrochemical Migration and Leakage Failures
This type of failure occurs when the following key variables are combined.
1st is a voltage differential (power to ground),
2nd is the transfer fluid (e.g. absorbed surface moisture - in micro-droplet
form) and
3rd is a corrosive (conductive residues for leakage failures)) residue that
will create the deplating of the anode and carry the metal salt into solution
and allow plating along the current path.
All three variables must be present in order for the electrochemical
migration failure to occur. With a power requirement of as little as 1.5
-2.0 volts to drive the dendrite formation, nearly all electronic circuits
are susceptible to this type of failure criteria (as long as the three
conditions exist). Generally, a failure occurs when the spacing between
power and ground is connected by a thin layer of moisture that combines the
corrosive residues and the voltage to create a metal dendrite that shorts the
circuit. This conductive metal path creates a short circuit on an assembly in
the field, and this assembly is then returned to the manufacturer where a
typical failure analysis is performed. This typical failure analysis will
often include a SEM/EDX analysis showing the following elements, carbon,
oxygen, tin, lead, and copper. This elemental investigation provides some
wonderful photos of the dendrite and shows that copper, tin and lead metals
were the metals that created the short, but it doesn't tell us what caused
the dendrite to grow.
We still need to understand the contamination types and levels, as well as
determining the sources and why the assembly surface was absorbing moisture.
Our focus should not be on which metal created the short (it has to be one of
the metals in the area of the failure), but rather, on what corrosive
residues caused the dendrites and where they came from. We have found that
tools such as Ion Chromatography and SIR testing give us a very detailed
understanding of the specific residue species. The No Clean flux residue is
not encapsulating enough to keep the board fabrication residues away from
absorbed moisture and the circuit voltage.
We have published a number of case studies on these issues and are availble
from our web site.
Terry Munson
CSL Inc.
P 765-457-8095
F 765-457-9033
<A HREF="www.Residues.com">www.Residues.com</A>
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