Designs of new electronic products show a significant drive for smaller and more complex PWBs. This continuing trend of miniaturization affects components in the same way as for the connecting pads on a printed circuit board. Driving force on the one hand is the telecommunication industry,in particular mobile phones and hand held devices and on the other hand by developments in networking and server applications. The effect of this is a transition in assembly technologies from through hole technology in the past,to surface mount,up to latest technologies with BGAs,CSPs,MCM or Flip Chip. With continuing changes in assembly technologies,the demands of the final finish of a PWB increases,where HASL as final surface was first choice for through hole assembly and for the beginning of SMT. With an increase of the interconnect density combined with an increase of fine pitch,HASL reached its technical limits. Features of HASL are variations in tin/lead thickness distribution – from 1µm up to 50µm and high thermal stress to the PCB while processing / dipping into a molten solder alloy. This kind of processing is leading to warpage. Features like these implemented the move to alternative coatings; HASL could not fulfil all technical requirements for assembly anymore. Due to technical demands,new surface finishes were required. First choice for multiple reflow was electroless nickel immersion gold - ENIG. This finish has world wide a high acceptance on the market,with benefits unique to ENIG. These days other finishes are moving into the market with the same advantage of planar surfaces for multiple reflow cycling,as there are electroless palladium,immersion tin and immersion silver. All these HASL alternatives combine low thermal operating temperatures during processing and planar surfaces for assembly.

The technology for embedding components in PWBs will change the process how PWB are fabricated over the next
few years. At the present time,polymer thick film is the most frequently used material to embed “passives” in
PWBs. However,these do not meet the stringent tolerances that will be needed in the future and will be replaced by
new materials based on ceramic,thin film or even with silicon. The coefficient of thermal expansion (CTE) of such
new technology embedded components is,in most cases,lower compared to the CTE of standard glass-reinforced
materials used today. In addition,the distance between the component lead and the PWB connection will be shorter
and will thus absorb less stress. For the effective use of embedded components,the dielectric thickness between the
conductive layers will be reduced. High dielectric strength,CAF resistance and low migration are required,while
resin recession cannot be accepted on plated inner via holes. This paper explains the impact of CTE imbalances
caused by the different materials used in a PWB. It also provides guidance on how PWB manufacturing process can
be modified to accommodate the new requirements from the PWB fabricators,assemblers and OEMs. These
requirements are for high dimensional stability of the laminate and prepreg to improve manufacturing yields during
bare PWB fabrication,during assembly and during the life cycle of the electronic device in the field. The paper also
explains the risks of metal migration and conductive anodic filaments (CAF) in PWBs and the need for lower
dielectric constant (Dk). The new materials are processable in standard PWB fabrication lines and can accommodate
the needs engendered by halogen-free resin systems.

This paper describes a new technology for speeding the initiation and uniformity of electroplating deposits that does
not depend on modifying the chemistry or physical environment of the plating bath. Rather,it involves the treatment
of conductive surfaces outside of the plating environment and is therefore not dependant on any particular type of
bath,the pH of the bath,or the chemical agents contained in the bath. Process improvements include a more rapid
initiation of plating; an increase in "throwing power" into low current density areas; and improved metal-to-metal
bonding such that very thin deposits exhibit unusual resistance to corrosive testing environments.

This paper provides a comparison of the air-to-air thermal cyclic reliability and associated failure modes of second level interconnects in lead-free,1.27 mm pitch,256 I/O BGA devices with eutectic tin -lead assemblies. Both electroless nickel/immersion gold (ENIG) and copper OSP test board surface finishes were included in this study. The assemblies were subjected to two different 0/100oC accelerated thermal test conditions. Solder joint failures were determined with in-situ event detection and verified by resistance measurements,x-ray,cross-sectioning and dye penetration. Elemental analysis was performed on selected failed solder joints. The results of the experiment indicate that the average solder fatigue life of the lead-free alloys investigated was higher than that of the tin-lead solder. However,the fundamental crack propagation behavior of the lead-free alloys was not the same as the tin-lead samples. Failure analysis revealed that intermetallic formation,fatigue crack characteristics,and solder fatigue propagation mechanisms associated with the lead-free alloys were not common to the tin-lead samples. In addition,lead-free solder joints evaluated in rigorous failure analysis show unique features such as stress voids,crack path redirection around intermetallic formations,vertical cracking and spalling of portions of the solder joint,which are not common in traditional tin-lead BGA solder joints.

A comparative study of package-to-board interconnections of a 1.27mm pitch BGA package using two Pb-free alloys and Sn-Pb solder in extended –40 to 125°C thermal cycling is described. The microstructural evolution,intermetallic compound growth,and progressive damage in the solder joints were documented using visual,x-ray,SEM,and EDS analysis. The analyses indicate that intermetallic compound growth in the Pb-free solder joints can contribute to void retention in Pb-free solders. Sn/Ag intermetallic plates often formed across the grain boundaries and redirected or retarded crack propagation in many instances. In addition,Sn-whiskers were found inside voids and intermetallic formations extending outward from Sn/Ag and Sn/Ag/Cu solder joints as the number of temperature cycles increased. Crack propagation included the formation of vertical and horizontal cracks in random solder joints. The preferred propagation path of the vertical cracks is shown to be through the large angle grain boundaries in the solder. Spalling or separation of portions of the solder joints can also occur. The observations provide insight into the effects of long-term thermal mechanical stresses and the attendant failure mechanisms in Pb-free solder interconnections.

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The texture of a tin deposit has been described as a key factor influencing whisker growth. Based on X-ray diffraction (XRD) and whisker growth data,this paper intends to show one possible mechanism that contributes to the formation of whiskers. By correlating observed XRD patterns with whisker growth performance in tin electrodeposits,a model has been created to approximate the risk of whisker growth. With some limitations,this model was found to be valid in predicting the extent of whisker growth in tin deposits.

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There are two main driving forces that are causing the electronics industry to take a look at alternatives to hot air solder leveling (HASL) as a surface finish for printed wiring boards (PWB). First,finer pitch array packages like micro-BGA and chip scale packages are being used in increasing volumes and for a broader variety of devices. With reducing pitches,and thus smaller ball diameters,it becomes necessary to have a predictably flatter surface in order to achieve a consistently more reliable solder joint. The inherent nature of the HASL process to leave inconsistent and uneven topographical profiles of the PWB pad surface,has been well observed and documented. Alternative finishes such as organic solderability preservative (OSP),immersion tin,immersion silver,electroless nickel/immersion gold (ENIG),etc.,have been proven to provide the flatness desired – several orders of magnitude flatter than HASL. The second reason to consider HASL alternatives is the move toward lead-free electronics,particularly in Europe and Japan. With legislation already approved in Europe and a strong focus on environmental marketing in Japan,the demand on PWB fabricators to produce “unleaded” product will no doubt see a marked increase over the next several years. In the United States,organizations such as IPC,the National Electronics Manufacturing Initiative (NEMI),and others have been heavily involved in research activities related to lead-free materials over the last several years. Although the US has not produced any legislative mandates (as of this writing) on the removal of lead from electronics,research activities are ongoing.

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