Ioan's original question was; "with the advent of lead free processing, how
do you qualify you PC FAB House?".  Offered below is our response when ask
how we qualify vendors for lead free processes.

The first point to understand is that since the product design is unique and
the fabrications process (including the base materials) is unique, we are
not testing against an established standard.  This is not a conformance
test, it is a performance test.  We are quantifying the performance of a
product exposed to a lead-free assembly and rework environment.  What we are
attempting to do is establish the reliability of a product, to understand
how assembly and rework degrades the reliability and then compare the
results to similar products based upon historical data.

This is reliability testing so it assumes some sort of accelerated thermal
cycle testing.  This testing can be done by many methods; all perform
thermal excursions over a specified temperature range, whilst monitoring
changes in resistance of specific circuits.  Some methods require
specifically designed coupons; others could be preformed on actual product.
Please understand I am trying to be circumspect here as to not violate the
rules of this forum, and, at the same time, not to dilute a procedure to the
point that this information is of no use.

Qualification to Lead Free

1. A test vehicle needs to be defined.  The test vehicle must use the same
construction and design attributes as the product.  Particularly important
are overall thickness, hole size, grid size and material type.
2. The test vehicle must be measured for resistance (and in our case)
capacitance of the test circuits before and after preconditioning, and at
failure or end of test.
3. The sample size should be statistically valid; usually 12 to 18 test
vehicles per variable (depending on budget constraints).
4. A typical test would be 12 to 18 samples tested "as received",
preconditioned to tin lead assembly and rework requirement (4 to 6 cycle @
230C) and preconditioned to lead free temperatures (4 to 6 cycle @ 260C).
5. The sample are preconditioned and then tested by thermal cycling.  We
test near Tg, 150C for most applications, microvia are tested at 190C.
6. Sample are tested to failure (usually 500 to 1000 cycles are required),
this is generally a 10% increase in resistance.  Testing stops when a sample
reaches 10% increase in resistance.  We try to achieve at least a 50%
failure rate.
7. The first failed coupon of each test condition is subjected to failure
location (we use thermal-imaging cameras).  We find the one hole that is
contributing the most to the 10% increase in resistance.
8. Microsections are done of the first failed sample in each test group and
failure analysis performed.
9. Samples are evaluated for delamination knowing that delamination is not
usually visible by external examination.  All of our generic coupons can be
used to detect delamination electrically.  Failing that capability, the
samples from each group with the longest cycles to failure should be
subjected to failure location, microsectioning and, hopefully if
delamination is present it would be found.  Finding delamination by random
microsectioning is not very effective so increase the sample size as budget
permits.
10. All data is then analyzed.  The "as received" sample established the
"entitlement" of the PWBs.  The tin/lead-preconditioned samples represent
the typical degradation of product.  The LF samples represent the effects of
lead free.
11. Concurrently thermal analysis of the material should be performed.
Specifically Thermal Mechanical Analysis (TMA) and possibly Dynamic
Mechanical Analysis (DMA) are used to characterize materials in the PWB (not
the raw laminate).  The TMA samples include the copper ground planes but not
PTHs.  Early failures which are not directly attributed to fabrication
problems (ex. thin copper, poor electroless adhesion), may be explained with
materials analysis.
12. All data is reviewed, observations made and conclusions are drawn.  The
mean cycles to failure are recorded and used as the "touchstone" against
which all other tests are measured.

The failure mode analysis is particularly important because we have been
seeing a shift in failure modes with lead-free processes. Product that would
be acceptable in tin-lead assembly can fail with lead-free assembly and the
failure mode is not what is anticipated by tin/lead failure history.  In
other words you can be blind-sided by a process that was thought to be
robust (ie, material or electroless copper).

Lead free temperatures stress the plated through via, internal
interconnections and the knee of the hole much more than tin-lead
temperatures.  Pad rotation is pronounced and stress is redirected toward
the surfaces of the PWB.  Metal fatigue of the barrel is not always the
dominant failure mode with LF thermal excursions.

Since no standard is established, nor is it likely to be established soon,
due to the complexity and variability of printed wire boards, you need an
educated eye to establish if the vendor meets or exceeds your needs, for
your specific product, in your anticipated end use environment, with you
field failure history, your warrantees, your customer needs etc.  You can
review established requirements in your industry and compare the means
cycles to failure against a baseline of similar product.

Once the fabricator has been through the qualification process a certain
expectation (performance baseline) can be established.  The demonstrated
cycles to failure become the standard by which subsequent tests are
measured.  Usually a system of acceptable testing for future product is
established.  Other tests (E.g. alternative suppliers, materials type,
different constructions, the use of non-function pads, three point contact
vs. flush interconnect, Etc.) can proceed comparing results with the
original studies.

Presently, there are no standards established because of the inherent
variability of circuit boards and fabricators.

If you wish to know more I would be happy to discuss in more detail the
process involved in qualifying lead free products.

Best Regards,


Paul Reid


Program Coordinator
PWB Interconnect Solutions Inc.

Tel:  613-596-4244 Ext. 229
Fax: 613-596-2200

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