As the electronics industry has transitioned to RoHS compliant products there was (and continues to be) much research on reliability issues. Much of the early research was focused on solder joint reliability under temperature cycling and vibration. Only in the past several years has the question of corrosion and PCB surface finishes been studied extensively due to several companies experiencing failures with immersion silver PCB finishes. One potential failure mechanism has not received much attention and that is the potential of sulfur-driven corrosion failures of palladium plated leadframes (1) used in IC packaging. This leadframe plating makes the component very susceptible to failures due to copper sulfide creep corrosion. This is a result of 1) the palladium being a very noble and accelerating the copper corrosion and 2) the copper sulfide corrosion films can creep quickly across the palladium (or gold) surface. This failure mechanism is certainly not unknown but has not received much attention. This is surprising since ICs packaged using the leadframe plating are more sensitive to sulfur-driven corrosion than the more publicized failures of immersion silver PCBs. The palladium plated leadframe method has been utilized by Texas Instruments for many years but with the search for a RoHS compliant leadframe plating which is guaranteed not grow tin whiskers, many companies have joined TI in utilizing the leadframe plating . Samsung, Toshiba, ST Micro, Infineon and others are using this leadframe plating for some products (2).
However the underappreciated danger of this type of leadframe is corrosion in environments with airborne sulfur. Under these conditions the Pd plated leadframe component will fail quickly and will be the weak link in terms of corrosion resistance. A vulnerable area for creep corrosion to originate is the tie bar cut-off area in the gull wing part. In this area there is exposed copper adjacent to the palladium and the creep kinetics across the palladium surface are quite high. Even conformal coating on the part can not guarantee corrosion resistance since most coatings will tend to run off the knee of the gull wing lead thereby exposing the vulnerable area. Considering the sensitivity of these parts to creep corrosion and the increasing use of the devices in RoHS designs, I predict that the electronic industry will see more instances of sulfur-driven component failures with Pd plated leadframe components. Stand by….
References
1. Assessment of Ni/PdAu-PD and Ni/Pd/Au--Ag Pre-plated Leadframe Packages Subject to Electrochemical Migration and Mixed Flowing Gas Tests, P. Zhao and M. Pecht, CALCE
2. Reality of Pb-Free Reliability, C. Hillman, DfR Solutions, SMTA Lead-Free Academy, Toronto, May 2010
Tuesday, December 11, 2012
Sunday, November 18, 2012
Tin Whisker Risk Assessment Software Review
My previous blog described risk mitigation for tin whiskers
using conformal coating. Now I will
continue on the subject of tin whiskers by looking at some software that
attempts to quantify the reliability risk of tin whisker failures. I have found two software packages that
provide some assessment of potential risk due to tin whisker growth: the CALCE
tin whisker calculator (1) and the Pinsky tin whisker assessment calculator (2)
from Raytheon Analysis Laboratory. The
CALCE model actually gives a quantitative reliability assessment while the RAL
spreadsheet only gives a 1 to 10 relative risk assessment.
As an exercise I applied the CALCE tin whisker calculator to
an electronic module. This assembly uses
several larger QFPs but is not considered a leading edge high density
module. The CALCE calculation was
performed after entering the appropriate input parameters. These parameters include how many pins of each
device, the lead spacing are various lead dimensions. Using the calculation software the predicted
reliability after 5 years was only predicted to be 12%! That is, 88% of the modules of this design
are predicted to have failed due to tin whiskers. The actual assembly analyzed was already 5
years old and no tin whiskers were observed on the module. This seems to suggest that the prediction
calculator is overly pessimistic in giving a life estimation. The one positive aspect of being overly
conservation is that if the predicted reliability is good then you are standing
of firm ground. The downside is that I cannot really use the predicted lifetime
to make risk decisions.
The second assessment algorithm is the Raytheon Analysis
Laboratory algorithm by David Pinsky.
This calculator uses an Excel spreadsheet and is not as detailed as to
the input variables. For example it does
not asked for exact spacings between pins.
Interestingly it does require inputs for variables such as tin plating
thickness, tin annealing history (if any) and forced or convective
airflow. This calculator is more suited
to single component analysis rather than system analysis. Therefore only one component was analyzed: a
204 pin QFP. In the CALCE calculator the
probability of any two pins shorting was 1.05% which means that the probability
of any pair of pins within the 204 pins failing is high. The Pinsky calculator does not give a
quantitative assessment but rather a risk number between 1 and 10 where 10 is a
very high risk. For the 204 QFP, the
risk number is 8.41 which is considered a high risk. This could be considered in accord with the
pessimistic CALCE prediction.
The specific module that I analyzed has been in the field
for over five years and no known tin whisker failures have been observed. Furthermore examination of modules that were
several years old have been examined with no evidence of tin whisker
growth. Therefore both the CALCE and the
RAL calculators should be viewed very conservative. While there could be some value as a relative
risk comparison between modules, these models cannot be used to accurately
predict field reliability and cannot be input into reliability prediction
programs. Considering this, it appears
that more research needs to be done in order to develop these models
further. The CALCE calculator is more
precise but the tin whisker growth rates must be modified and it needs to
include some of the input variables from the Pinsky method. As of the date of
this blog posting I am not aware of a planned major revision to the CALCE
calculator. Tin whiskers have been
studied since the 1940s (4) but it does appear that more papers need to be
written and more degrees awarded. Then we can move on to study zinc whiskers!
References
Saturday, September 15, 2012
Do Conformal Coatings Prevent Tin Whisker Failures?
Tin whiskers are small filaments that can grow spontaneously from tin and tin alloy surfaces. The conditions necessary for tin whisker growth have been much debated. Papers have been written, conferences have been held and PhDs have been granted, yet the exact mechanisms and growth conditions are still not completely determined. The growth of tin whiskers is a stress relief mechanism with either internal plating stress or external mechanical stress driving the whisker growth mechanism. For those who would like some additional background information, read G. Gaylon’s A History of Tin Whisker Theory: 1946 to 2004.
Electronic suppliers have employed a variety of risk mitigation approaches. An OEM has little control over the global supply chain and therefore the question arises as to what tin whisker mitigation methods are available to the OEM. One risk mitigation idea is to use conformal coating as a tin whisker barrier. On first appearances this makes sense since tin whiskers are thin and fragile and therefore it would seem that a layer of coating would easily prevent whisker growth. However in 2006, Woodrow and Ledbury (1) demonstrated the surprising result that most conformal coatings could be penetrated by tin whiskers. These include acrylic, silicones and urethanes. Only the vacuum deposited parylene coatings showed good resistance to whiskers. Their research was performed on brass coupons plated with bright tin which can easily grow tin whiskers. In the July 2012 Journal of Electronic Materials, S. Han et al have reported (2) results of tin whisker experiments on real electronic modules. These modules used a variety of component package styles and a variety of conformal coatings. The results were the same as those by Woodrow. That is, all conformal coatings (except parylene) will allow penetration by a tin whisker. This study also reported the very practical finding that conformal coating doesn’t consistently cover irregular surfaces. For example, gull wing leads will not be covered continuously by conformal coatings. Therefore the edges and the knee of the lead can (and will) be exposed. For corrosion protection it is desirable that the component lead be entirely covered but in practice this cannot be achieved. Studies have shown than even multiple coating passes will not provide complete coverage.
One area which has not been studied is the probability of a tin whisker to penetrate a conformal coating from the outside inwards to the lead. After all, in order for a whisker to short between two leads, it must penetrate the coating extending outwards and then penetrate a second time growing inwards. This second penetration seems unlikely although this must be corroborated with additional studies which will be experimentally difficult.
In conclusion, can it can stated that conformal coatings will greatly reduce the risk of shorts due to tin whiskers but this cannot be considered a foolproof risk mitigation strategy. There are two reasons for this: 1) tin whiskers can grow through conformal coatings and 2) conformal coatings typically do not provide uniform coverage over uneven geometries such as the edges of electrical conductors. However having long tin whiskers grow through the coating or having a long whisker which grows from the small uncoated area of a leadframe is small. This low probability whisker must then contact an uncoated conductor which is also a low probability. If these small possibilities are multiplied by the small possibility of having tin whisker failures on an uncoated module, then the resulting reliability is very high indeed. The chance of a tin whisker failure under these circumstances is virtually zero.
References:
1. T. Woodrow and E. Ledbury, Evaluation of Conformal Coatings as a Tin Whisker Mitigation Strategy, SMTAI, September 2006.
2. S. Han et al, Evaluation of Effectiveness of Conformal Coatings as Tin Whisker Mitigation”, Journal of Electronic Materials, July 2012.
Electronic suppliers have employed a variety of risk mitigation approaches. An OEM has little control over the global supply chain and therefore the question arises as to what tin whisker mitigation methods are available to the OEM. One risk mitigation idea is to use conformal coating as a tin whisker barrier. On first appearances this makes sense since tin whiskers are thin and fragile and therefore it would seem that a layer of coating would easily prevent whisker growth. However in 2006, Woodrow and Ledbury (1) demonstrated the surprising result that most conformal coatings could be penetrated by tin whiskers. These include acrylic, silicones and urethanes. Only the vacuum deposited parylene coatings showed good resistance to whiskers. Their research was performed on brass coupons plated with bright tin which can easily grow tin whiskers. In the July 2012 Journal of Electronic Materials, S. Han et al have reported (2) results of tin whisker experiments on real electronic modules. These modules used a variety of component package styles and a variety of conformal coatings. The results were the same as those by Woodrow. That is, all conformal coatings (except parylene) will allow penetration by a tin whisker. This study also reported the very practical finding that conformal coating doesn’t consistently cover irregular surfaces. For example, gull wing leads will not be covered continuously by conformal coatings. Therefore the edges and the knee of the lead can (and will) be exposed. For corrosion protection it is desirable that the component lead be entirely covered but in practice this cannot be achieved. Studies have shown than even multiple coating passes will not provide complete coverage.
One area which has not been studied is the probability of a tin whisker to penetrate a conformal coating from the outside inwards to the lead. After all, in order for a whisker to short between two leads, it must penetrate the coating extending outwards and then penetrate a second time growing inwards. This second penetration seems unlikely although this must be corroborated with additional studies which will be experimentally difficult.
In conclusion, can it can stated that conformal coatings will greatly reduce the risk of shorts due to tin whiskers but this cannot be considered a foolproof risk mitigation strategy. There are two reasons for this: 1) tin whiskers can grow through conformal coatings and 2) conformal coatings typically do not provide uniform coverage over uneven geometries such as the edges of electrical conductors. However having long tin whiskers grow through the coating or having a long whisker which grows from the small uncoated area of a leadframe is small. This low probability whisker must then contact an uncoated conductor which is also a low probability. If these small possibilities are multiplied by the small possibility of having tin whisker failures on an uncoated module, then the resulting reliability is very high indeed. The chance of a tin whisker failure under these circumstances is virtually zero.
References:
1. T. Woodrow and E. Ledbury, Evaluation of Conformal Coatings as a Tin Whisker Mitigation Strategy, SMTAI, September 2006.
2. S. Han et al, Evaluation of Effectiveness of Conformal Coatings as Tin Whisker Mitigation”, Journal of Electronic Materials, July 2012.
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