Tin whiskers are needle-like crystals of tin growing from pure tin or high-tin alloy surfaces,which may grow long enough to cause electrical shorts. Conformal coatings provide some protection against these shorts,partly by suppressing or slowing whisker growth,but primarily by deflecting or buckling a whisker growing from an opposing surface. The longer an unsupported cantilevered whisker,the more likely it is to bend or deflect. This paper addresses the capability of a whisker to penetrate an adjacent coating,and does not consider the effect of a coating on the propensity for whisker growth. In the present work,an analysis was conducted to determine the critical spacing between coated conductors below which a whisker is likely to penetrate the conformal coating on the adjacent conductor and cause a short. The analysis is based on the critical
buckling force of an angled whisker using two different boundary conditions: fixed at one end where the whisker slides along the coating surface,or fixed at one end and hinged at the other,where friction prevents the whisker from sliding. By using the critical compressive strength of the coating (derived from Durometer measurement) and the area of the whisker tip,the critical force needed to penetrate the coating is determined and compared to the tin whisker buckling force. The computed compressive force that is required for the whisker to penetrate the coating can be considerably less than if the coating is
assumed to behave elastically. By solving the buckling relationship for whisker length at various whisker angles,the minimum coated-conductor spacing is determined as a function of whisker angle. Then,by comparing the computed
spacing-angle relationship with published data on the distribution of whisker angles,the minimum expected (mean) coating gap can be determined,in addition to the absolute minimum gap. Based on this analysis,whisker re-penetration is unlikely for components with lead pitch of 1.27-mm and above,though risk is higher for fine-pitch components with some “soft” coatings.

This paper will present the testing performed to date by Lockheed Martin Missiles and Fire Control on Tin Whisker test coupons. Coupons are representative lead materials and are half coated with 3 different conformal coatings in 3 different thicknesses. Testing has been performed over two years at 50/50 temperature/humidity exposure. This paper will also describe the capability of automated conformal coating equipment to provide complete coating coverage of various packages.

Soldering with lead-free solders has become an international standard. A study conducted by the Dresden Technical University and rehm Anlagenbau examines which influences various parameters have on soldering results. The study
substantiated that,amongst other factors,the soldering atmosphere (nitrogen or air) and oven parameters have a significant influence on tombstoning. Excessively high soldering speeds cause more tombstones than lower speeds. Wetting results for the examined surface finishes (nickel-gold and immersion tin) depend upon the composition of the solder paste. Nitrogen in the soldering atmosphere significantly improves solder spreading.
The study also included ultrasonic examinations of various IC packages after reflow soldering. The tendency of the different packages to delaminate after convection soldering was compared with vapor phase and vacuum soldering.

This paper provides a high-level overview of many of the key lessons learned thus far in working with PCB fabricators in qualifying laminate/fabrication processes that will yield fundamental printed circuit board (PCB) attributes that will meet IBM server reliability requirements,with the additional requirement of being subjected to the higher temperature processes required for mixed-solder assembly and for totally lead-free soldering. Special focus is placed upon
significant laminate integrity issues,the solutions of which will probably require far greater integration among different levels of the supply chain.

Corrosion of solder pots and solder pot components in wave soldering equipment has been reduced with the introduction of corrosion resistant coatings and improved lead free solder alloys. The latest trends in protecting wave solder machine components from liquid metal corrosion by lead free solder alloys will be presented in order to provide guidelines for evaluating existing equipment as well as for purchasing new systems.
New lead free alloys are available that are less corrosive than earlier Sn/Ag and Sn/Cu alloys and can be utilized in older,unprotected equipment for a longer period of time. Cast iron,titanium alloys,and stainless steels coated with titanium nitride and Melonite QPQ are discussed and evaluated. Coatings and materials evaluations are based upon chemical,structural,and visual analysis along with field experience. Additionally,solder pot contamination sources,and tips on how to avoid contamination,are presented.

This paper will present seven different materials used for the production of both wave solder and reflow solder pallets. The goal of this study will be for the purpose of depicting machining capabilities for depth,wall thickness,and
accuracy of machining for each of the seven materials presented. Additionally,a lead free wave pallet (used for a high running Printed Circuit Board (PCB) product) will be built out of each of the materials being tested. Each pallet will be
tested for the purpose of showing long-term wear effects over multiple heat cycle run times; monitoring pallet flatness for heat warping of the pallet; wall thickness changes; and,overall changes in surface condition. For the purposes of
this study,each of the materials used is listed in terms of basic material properties.

As the electronics assembly industry adjusts to building boards with 0201 components,the next level of shrinkage for chip components is just around the corner with the emergence of 01005s. A key objective of this work is to demonstrate assembly success using common 0201 build parameters of Type 3 solder paste and 4-mil stencil thickness. Stencil artwork strategies are implemented across 27 different pad designs to determine how assembly yield is influenced by aperture area ratio,component termination overlap level in paste,and tolerance to stencil defined misregistration levels. A pad design of 9 mils
wide by 11 mils long and separated from the opposite pad by 6 mils is recommended when printed with 9 mil wide by 10 mil long apertures to achieve acceptable print quality and high assembly yield results. A direct comparison of tombstone ratios for 0201 and 01005 resistor components assembled together shows that 01005s are more stable across a variety of profile atmospheres compared to 0201s.

For some years now,an area ratio of 0.66 or greater has been the criterion for stencil apertures to achieve acceptable volume control in the printed solder paste “brick,” for small apertures Meeting this criterion,in the stencil printing process,has been a constant concern as the volume consistency of the solder paste brick is among the most critical metrics in determining high yields in the assembly process.
With the advent of 0201 and most recently 01005 passives components,meeting the area ratio target of 0.66 is a challenging task; especially with a 5 mil thick stencil and a Type 3 powder solder paste. These tiny passive technologies and minimalist IC packages like 0.4mm CSPs have created a need to re-evaluate the area ratio criterion of 0.66 and other related issues. In light of this need,we developed a series of designed experiments (DOEs) evaluating Type 3 and Type 4 solder pastes,varying aperture designs for 0201,01005 and 0.4 mm CSPs,and 3 and 4 mil thick laser cut and E-Fab stencils. In these experiments we determined optimum print parameters for the various combinations of pastes,components,aperture design,and stencil fabrication mentioned above. Our results also suggested new criteria for area ratios. We believe that with the concurrent implementation of lead-free assembly,this work could not be timelier.

During the development of a new medical imaging system,via quality was identified as a potentially source of infantile failures. Premature via failures were precipitated in a critical 14 layer board,specifically in the 0.8 mm BGA vias with an
aspect ratio of approximately 9:1. Failure analysis indicated two dominant failure mechanisms: dry film lock-in,and drill debris in the via barrel causing insufficient plating of the via sidewall. Dry-film lock-in was corrected by process control
improvements at the fab supplier. The fab material was identified as a major driver in the presence of drill debris in the via barrel.
To address these infant mortality issues and to demonstrate long term via reliability,IST testing was identified as an industry recognized tool to quickly assess fab reliability. Three parameters were identified as key variables in fine-pitch via quality and reliability: fab resin material,inclusion of non-functional pads (NFP),and type of drill machine at the fab supplier. A DOE was developed to understand the influence of these factors on overall via reliability. The following conclusions were observed. The lower CTE fab material outperformed the other fab materials in mean cycles to failure (CTF). In addition,an interesting result with NFPs was found. Contradictory to industry recommendations,the
presence of NFPs actually improved the mean CTF for the same material and process. The drill machine had little to no influence on CTF for the same material and design. In addition,at the lead-free preconditioning temperature,separation of the glass fiber bundles was observed in most of the materials tested. An understanding of this phenomenon and the other failure modes is critical to developing a robust lead-free fab.

This paper continues work by Sun Microsystems and the University of Maryland,CALCE[1] to predict plated through via life using laminate material properties,plated copper material properties,and the physical via geometry to model via life. The new method presented in this paper uses non-linear laminate material properties and a damage-fatigue model to predict the accumulated damage to a plated through via as it is thermally cycled through assembly and field life conditions.
Copper is a ductile metal so it is possible to construct a Log-Stress versus Log-Life plot that follows an Inverse Power Law (IPL).[2] The key to doing a Log-Stress versus Log-Life plot is developing the relationship of stress versus temperature of the laminate material. Use of a Log-Stress versus Log-Life plot allows increased testing efficiency since you can perform an accurate life analysis by testing at only the high and low temperature extremes. Once the Log-S versus Log-N plot is
constructed,it is possible to predict plated through via life over a wide range of temperatures. For this paper,we will use thermal cycle to failure test data obtained from Interconnect Stress Test (IST),[3] but the analytical methods developed apply equally to other thermal cycle methods like Highly Accelerated Thermal Shock (HATS)[4] and Air-to-Air Thermal Shock (AATS).
Last,a Finite Element Model simulation is conducted that uses material properties that are easy to obtain and is then validated against the large database from IST testing at multiple temperatures. Once the Finite Element Model validation is complete,the model is used to make assembly and field life predictions for two case studies involving thick,complex printed wiring boards.