Challenges & Next-Generation Pogo Solutions For
Contacting Pb-Free Devices In High-Volume Production Test
Semiconductor
manufacturers are in full swing, converting a majority of their IC packages
away from lead (Pb) bearing solder to contact finishes that are more
ecologically friendly, and compliant with the Restriction of Hazardous
Substances (RoHS) legislation.
Traditionally,
tin-lead (SnPb) solder-plated leads and solder balls have been contacted with
hard gold electroplated interconnects.
The gold-plated contactor interconnects have always had a tendency to
accumulate solder from the IC leads, and have had issues with the formation of
intermetallic layers and oxides that degrade the electrical performance of the
connection. With proper application of
tip geometries that provide a wiping surface, as well as adequate normal forces
and cleaning techniques, the gold interconnects have proven to be reliable and
economical for production testing of SnPb-plated device leads.
Leadframe-Based
Package Contacting Challenges
Many leadframe-based packages (QFP, SO, and QFN, etc.) have
now been redesigned to have either matte tin (Sn), tin-bismuth (SnBi) or
palladium-nickel (PdNi) surface finishes.
Each of these new surface finishes have presented contactor manufacturers
with unique challenges in providing the reliable temporary interconnects needed
for production testing.
It
has been particularly challenging to make a reliable test connection to matte
Sn and SnBi. Quite often, the matte Sn
device leads have a relatively thick oxide layer on the surface that is hard,
rough, and difficult to pierce. Contact
geometries that easily displaced SnPb finishes, exposing fresh surfaces, no
longer adequately work against a harder oxidized matte Sn finish. Laboratory studies and field testing have
shown that a normal force of 30 grams or more is required to achieve good test
yields, compared with lower forces acceptably used when probing SnPb parts.
One
drawback of the higher force required is the tendency for the interface
asperities to cold-weld, erode and fracture, resulting in adhesive wear (Figure
1). The increased stress on the surface
of the plated contact layers also can induce fractures that expose base
material on the contacts and result in fretting corrosion that over time
increases contact resistance (Rc) and, just as important, increases
Rc variability. The solution
for overcoming this type of wear and corrosion is to engage the IC lead with a
metallurgy that has a lower affinity for Sn and SnBi. The PrimeGuard-II™ contact finish available
on ECT probes has a metallurgy that resists adhering to Sn and SnBi surfaces
during IC test conditions, and provides a stable Rc.
Leadframe IC’s with PdNi-plated leads present a very
different interconnect challenge. Oxide
layers and intermetallic compounds at the interface are greatly reduced,
however the very high surface hardness of these leads (185 HKnoop
and above) results in extreme abrasive wear on contacts plated with gold, such
that the base material is quickly exposed, and fretting occurs. (Figure 2)
The
best way to overcome the extreme hardness and abrasive wear of the contacts is
to encapsulate the surface of the probes with a material that is substantially
harder than the NiPd. ECT’s PrimeGuard-I™
proprietary interconnect surface finish has been production proven to hold up
against PdNi device leads to hundreds of thousands of insertions and have low
and stable Rc. The
PrimeGuard-I finish can be selectively applied to different contact types on the
DUT side of the interconnect, while maintaining an optimized gold-plated
contact surface against similarly plated gold loadboard pads.
Ball Grid Array Contacting Solutions
Prior
to the switch to Pb-free interconnects for Ball Grid Array (BGA) packages,
contactor manufacturers had been providing a variety of solutions to contact
the relatively soft, low melt temperature (183° C) alloys of eutectic SnPb
solder. Contacting eutectic SnPb solder
balls has always been a challenge, as the traditional interconnect designer’s
toolkit of normal force and wiping action has tended to transfer large
quantities of SnPb to the contact surface, especially at elevated test
temperatures. When solder gets
transferred, intermetallic layers form with gold contacts and oxide films form
when the contact is exposed to moist ambient air. The other constraint faced by the
interconnect designer is that the contact technology may physically distort the
solder balls to the point that the packages could be out of specification or create
down-stream soldering reliability issues.
Contact manufacturers have devised many unique, proven shapes that
minimize solder transfer, while making reliable contact with SnPb BGA’s.
The Pb-free solder ball alloy adopted for BGA’s is
predominantly a tin-silver-copper (SnAgCu or SAC) alloy that is mostly Sn
(>95%). The melting temperature of
SAC alloys is higher (217° C), and the
balls are harder. The harder ball allows
for a more aggressive normal force and wipe for a probe contacting the ball, thus
improving the Rc consistency in test. It is interesting to note that the SAC balls
exposed to ambient environments can appear very dull, and electron microscope
inspection reveals that the dull finish is a result of a nearly pure Sn
dendrite growth on the ball surface. The
dendrites are very rough, but can be easily displaced with proper contact
design. (Figure 4)
The
ultimate contact solution for BGA’s is a very hard finish that is chemically
inert to the alloys in SnPb and SAC balls.
A number of alternative contact finishes have been proposed and
implemented, with some success in minimizing the material transfer from the
balls to the contacts. One solution for
all BGA’s, including SAC balls, is again the PrimeGuard-II finish available
from ECT. The PrimeGuard-II finish
contacting the BGA results in only a slightly higher overall probe Rc,
but with increased stability over thousands of DUT insertions. The PrimeGuard-II finish is also much harder
than gold, so contactors can be cleaned with more aggressive methods vs.
gold-plated probes. As with the
PrimeGuard-I solution for NiPd leadframe parts, the PrimeGuard-II finish is
selectively applied, retaining the loadboard-side gold finish with optimized
radius tip geometry for overall contact integrity.
Caution: One
Solution May Not be Right for All
Various
surface finishes and contact shapes have been developed by ECT and other
contactor manufacturers to address solder migration, surface wear, and other
contact corrosion issues commonly dealt with in production test. A solution for a particular application can
not be assumed to work in all cases.
Beyond the first tier of issues of interface metallurgy and geometry,
come other parameters that can have a significant influence on contact
reliability.
Semiconductor
package leadframe base material has a measurable influence on Rc. Leadframes with copper substrate material
tend to have a far greater susceptibility to Rc variation vs.
iron-based leadframes (Alloy42 or FeNi42).
The Alloy42 base material is much harder than copper, and socket
contacts with alternative surface finishes have less of a Rc variability
reduction vs. standard hard gold finishes against these leads. Careful analysis and understanding of the
device test parameters needs to be made – perhaps the pass/fail threshold is
broad enough to allow more traditional finishes to perform well.
Other
factors that come into play in making the proper probe surface finish choices
are: thickness of leadframe finishes, surface roughness, cleanliness of the
test
environment,
chamber temperatures, and acceptable DUT witness mark levels on packages.
In
production environments that use common hardware to test both Pb-free and
Pb-bearing packages, special care must be taken not to use the same contact
sets for both package types. Contactor
setups must be identified and labeled for which type of packages are to be
tested. Cross contamination of Pb and Sn
on probes can lead to very unpredictable test results and the potential for
contaminating Pb-free packages violate RoHS guidelines.
Interface Challenges
are Increasing
Compounding
all of the interconnect challenges with Pb-free devices is the fact that the
devices themselves often have more demanding performance requirements. Low power, resistance-sensitive applications
such as measuring MOSFET RDSon resistances of less than 100 milliohms in
production test require absolutely low and stable Rc. Even the most exotic contact finishes may not
be enough to meet the test requirements imposed by the device specifications,
so a true Kelvin solution may need to be deployed in the contactor, loadboard,
and test program.
Automotive
applications that force high current through contact interfaces often cause an
accelerated intermetallic formation from the galvanic effects of current flow,
exacerbated by elevated temperatures due to interfacial Joule heating or
extreme test chamber environments. To
minimize the effects of the thermal extremes, a very low Rc is
required, so low-resistance materials and finishes must be used. Similar high-current issues exist for power
hungry CPU’s and GPU’s. Alternative
contact finishes and innovative housing solutions allowing for parallel probe
use combine to overcome these high-power issues.
Overall Effect of
Pb-free Packaging on Contactor Technologies
The
level of research and development, scientific analysis, and industry
partnership has greatly increased over the last 5 years, and will continue to
climb. The change to Pb-free packages
has increased the number of interconnect options used in production test
contactors, and choosing the right solution for your application requires
careful analysis.
In
addition to understanding the package outline and handler specifications that
typically drive a contactor outline, the contactor user must have a better
understanding of the package construction, metallurgy, power requirements, chip
performance, test program requirements, and loadboard signal launch
limitations. All of these considerations
have an impact on the contactor reliability in a high volume, production test
environment.
ECT
is up to the challenge, and has invested in the technical resources needed to
understand and overcome the difficulties in making reliable contact for
production semiconductor test.
Innovative surface finish technologies, such as the PrimeGuard family of
coatings, as well as advanced probe designs, such as the Bantam® and
DuraPak™ families of contactors enable the test engineer to effectively develop
test systems for next-generation semiconductors. Technologies and processes developed for
solving contacting issues for Pb-free applications today are being utilized in
the development of future products for high frequency, ultra-fine pitch test
contactor applications of tomorrow.