With the implementation of RoHS directives,pure tin plating is replacing lead in the tin-alloy used in the printed circuit boards and other worldwide electronics. Although use of tin provides a safer environment for electronic manufacturers and
helps meet the regulatory requirements,it is known to form tin whiskers,odd shape eruptions and dendrites that cause printed circuit boards or devices to fail. Research has been ongoing for decades in an attempt to understand and control the whisker formation with limited successes to date. Because of the RoHS requirements,efforts to find an appropriate solution to the tin whisker problem have increased significantly in the past couple of years. This paper reviews various options and presents a practical solution utilizing Parylene coating technology for suppressing the formation of metallic whiskers,OSEs and dendrites.
Parylenes are solvent and catalyst-free thin organic coatings that offer solutions to many existing protective,packaging and reliability challenges of electronic industry in part because of their excellent electrical,mechanical and barrier properties and chemical inertness. In addition,Parylenes truly conform to the parts due to their molecular level deposition characteristics. It has been observed that Parylene conformal coatings are more suitable for suppressing metallic whiskers than other conformal coatings or other proposed solutions.
This paper also highlights some key attributes of a recently commercialized Parylene HT that help mitigate the risk of metallic whiskers even further,particularly for electronics or PCBs used at higher temperature and high frequencies. Parylene HT offers long-term thermal stability at temperature exposures in excess of 350oC.

Our industry is slowly coming to the realization that many fluxes containing low molecular weight carboxylic acids cannot be adequately tested for surface insulation resistance and electrochemical migration at high temperatures. This paper describes the use of thermal gravimetric analysis (TGA) to look at the sublimation of succinic acid and glutaric acid from a paste flux formulation.

The widespread use of SAC-based (SnAgCu) lead-free solder paste drives the industry toward a smaller process window. This is due to higher reflow temperatures as well as the limited thermal resistance of the current generation of electronic components. Furthermore,differences in the wetting performance of Pb-free surfaces not only require reduced variation in all process steps,but also in the soldering materials.
SPC data has shown that the solder paste printing process is the primary source of soldering defects in SMT assembly. Consequently,verification of the specified printing properties of solder paste is of paramount importance in the pursuit of
higher quality goals and higher overall yields. Process variations such as temperature fluctuations in the printing area,changing printing speeds,and varying stencil life have been recognized as important factors in the characterization of solder paste.
In recent years,quality management standards such as ISO/TS 16949 have been driving the development of a reproducible rheometric methodology to characterize and quantify the aforementioned solder paste properties.
In the course of this development,we have learned that the thixotropic history of a solder paste has a major impact on the repeatability and reproducibility of its rheometric characterization. This paper describes the differences in this respect between the various measuring principles such as the spindle,spiral pump and plate/plate method. It also introduces the development and implementation of a sample preparation method to reduce the impact of the thixotropy of a solder paste in conjunction with enhanced plate/plate rheometry.

Wafer bumping using thin electroformed nickel stencils with ultra fine powder solder paste continues to gain popularity as a cost effective alternative to ball drop and electroplated technologies. This paper describes the results of an extensive study involving 120,000 bumps conducted using eutectic and lead free water soluble wafer bumping solder pastes to achieve high bump height to pitch ratios down to 150 micron pitch. The printing process,stencil designs and bump measurement methods are described in full detail. Additionally,results from the most recent work on a 200mm test wafer with pitches down to 100 microns defining the effect of various squeegee materials on bump height for lead free solder pastes is discussed. Lead free wafer bumping solder pastes with powder types 5,6,7 & 8 are also compared and contrasted for their bump height capability to bump pitches below 200 microns. In addition to the material deposition tools and techniques,the reflow and cleaning processes are described. Finally,preliminary results on UBM pad size effects on bump height are presented.

A family of SACX alloys has been developed with significant improvement in drop test performance on NiAu surface finish. Dopants such as Mn,Bi,Ti,Ce,and to a less extent Y for SAC105 have been observed to show very positive effect when used alone or in combination,with Mn exhibiting the most profound effect. SAC+Mn outperformed not only SAC alloys,but also Sn63,thus completely altered the shaky position of SAC systems caused by fragility of solder joints. The melting and intermetallic formation properties are not affected by the dopants. Mn tends to migrate toward MC and accumulate near IMC layer in the form of MnSn2 particles. Thermal aging results in further improvement of drop test performance.A family of SACX alloys has been developed with significant improvement in drop test performance on NiAu surface finish. Dopants such as Mn,Bi,Ti,Ce,and to a less extent Y for SAC105 have been observed to show very positive effect when used alone or in combination,with Mn exhibiting the most profound effect. SAC+Mn outperformed not only SAC alloys,but also Sn63,thus completely altered the shaky position of SAC systems caused by fragility of solder
joints. The melting and intermetallic formation properties are not affected by the dopants. Mn tends to migrate toward IMC and accumulate near IMC layer in the form of MnSn2 particles. Thermal aging results in further improvement of drop test performance.

BGA component removal and replacement are required during product development and repair operations. In general,a single rework on specific location is permitted without causing excessive damage to solder joints,board materials,and adjacent components. However,situations exist where a multiple rework is required on the same location. Multiple reworks could induce a thick intermetallic layer between the BGA solder balls and Cu pads on the boards,leading to reliability concern [1,2]. This paper examines the possible effects of rework cycles on bonding strength of solder balls,Cu pads and laminates. An effort was made to establish a sound scientific relationship among rework processes (cycle times),intermetallic thickness,and mechanical strength of BGA balls,as well as any possible intermetallic embrittlement. It is believed that these
results can help shed some insight on the maximum permitted limits of BGA rework cycles.

As companies transition over to lead-free assembly a certain amount of hand-soldering and rework will be performed. An article from Tech Search International last year did state that in Asia where lead-free soldering is much more common,handsoldering was more of a production problem than lead-free SMT or wave soldering or other soldering processes. In fact most user problem calls and requests for training these days are related to lead-free hand-soldering and rework. In many cases the assemblers are using materials from various solder suppliers with similar issues occurring in all cases. Often the problems are more than just material issues.
Switching to lead-free without proper preparation is not recommended. Although this seems easily understood some assemblers have attempted to transition without adequate training; resulting in line stoppages occurring only a few hours into lead-free hand-soldering. Operator complaints,loss of reliability and poor joint quality were experienced. This could be a production engineer’s nightmare but it need not be this way if the basic concepts of hand-soldering are revisited,some experience gained prior to the transition and adequate training of operators is performed before and after the switchover.
This paper is a compilation of questions often asked by assemblers in reference to hand-soldering with lead-free. These questions are in fact some of the issues addressed during lead-free hand-soldering on-site audits or training at assemblers using lead-free solders at their facilities. It offers practical answers as to enable the reliable implementation of lead-free.

The use of tacky fluxes is common throughout the industry in applications such as ball attach,BGA repair and hand soldering. These applications employ different heating profiles,meaning that the fluxes are required to endure a wide range of time and temperature conditions,while not compromising long-term reliability.
Tacky fluxes are generally made from the same types of materials that comprise standard solder paste products designed for typical SMT applications. Therefore,the processability and reliability of tacky fluxes that are subjected to a standard SMT reflow profile are well understood. However,when the same tacky flux is subjected to a shorter heating cycle,such as a hand soldering application,it is not necessarily known if the flux residues will have the same reliability as expected when subjected to the typical SMT reflow profile. This paper examines the long-term reliability of no-clean tacky fluxes when subjected to a variety of
processing conditions.

This paper describes a new lead-free SMT rework process which avoids component overheating,reduces board warpage and cratering,and prevents adjacent components from thermal damage. In the replacement of a BGA,on an organic substrate printed circuit board assembly,a solder alloy with the melting range lower than Sn-Ag-Cu solder is used. The lead-free solder pastes analyzed in this work include two Indium containing alloys,melting range 181-187°C and 138°C respectively,and a Bismuth containing alloy,melting range 195°-209°C. When the solder alloy melts,dissolution of the Sn-Ag-Cu balls occurs. The resultant composition and microstructure of the solder joints provide good mechanical and electrical properties with high fatigue life,while avoiding high temperature stresses on component bodies.
In this work CBGA937,PBGA196,and CSP46 components with SAC405 balls were assembled using SAC405 solder. Then rework was performed with three compositions of low melt solders using different profiles. The microstructures of mixed solder joints were analyzed using optical and scanning electron microscopy methods. Two solder pastes and rework profiles were chosen for thermal cycling. A limited ATC study was done in a temperature range of 0 -100°C. It was observed that fatigue life was component-dependant and when fully mixed,solder joints had better or equal reliability to that of the pure SAC405 assemblies.

In manufacturing flexible and rigid-flex boards,the metallization step using electroless copper poses a major challenge,namely the leaching of polyimide base material into the plating bath. Leaching is exacerbated by the combination of high alkalinity and extended dwell time in the bath during the plating cycle. The result of excessive leaching is the degradation of the quality of the plated deposit leading to compromised adhesion. This is presently overcome by increasing the renewal frequency of the electroless copper plating solution,increasing the demands on bath control as well as the overall cost.
Our process introduced in this paper forms a thin conductive Palladium film on the dielectric,eliminating the need for electroless copper deposition step. The conductive film readily accepts copper electro-plating and yields an adherent
deposit.
The process requires a short immersion time in an alkaline solution,alleviating any damage to the resin and minimizing the accumulation of leached byproducts. The palladium is specific to the dielectric resin and does not
adsorb to the copper surface eliminating the need for copper etching before electroplating. The treatment time is short leading to improved productivity. The process is stable and easy to control,resulting in enhanced adhesion and
increased inner layer connection reliability.