SnPb Surface Research Articles

Failure analysis of contact probe pins for SnPb and Sn applications

A study has been conducted to investigate the failure mechanism of pogo pin-type probe contacts. Probe pins are used for electrical test of microelectronic components in manufacturing. A false rejection of parts due to high probe contact resistance results in a penalty in cost and yield. The probe pin contact bears distinctive characteristics of failure compared to the conventional contact systems such as mechanical switches and interconnects. Moreover, the transition to Pb-free component leads demands understanding of different probe failure mechanisms between a SnPb and Sn surface. The objective of this study is to understand this unique failure mechanism and the effect of lead coating metals on probe pin life. This has not been clearly elucidated to date in spite of its significant impact on yield and cost of electronic package manufacturing. A simulated probe tester with 3-axes actuation capability was devised to mimic the actual test process. The force required to penetrate the surface oxide layer and develop electrical contact was measured. Contact resistance history revealed that probe pins mating to Sn surfaces failed earlier than pins used on SnPb surfaces. Through periodic inspection of probe pins using microprobe/EDS as a function of probe actuations, the general root cause of probe pin failure was found to be probe pin tip wear out associated with the Sn oxide growth on its surface. The matte Sn surface wears the probe pin more than SnPb due to the rough and abrasive nature of the matte Sn surface.

Improvement of adhesion and electrical properties of reworkable thermoplastic conductive adhesives

The adhesion and electrical properties of a PAE-2-based isotropically conductive adhesive (ICA) material were investigated. PAE-2 (thermoplastic polymer)-based ICAs, as well as a commercial epoxy-based ICA, showed dramatic reduction in adhesion to a NiAu surface after the 85°C/85% RH aging test, while the adhesion to a SnPb surface did not change much. To improve the adhesion strength of PAE-2 to a Ni:Au surface after the humidity aging test, coupling agents and epoxy blends were used. The formulation with a higher amount of epoxy showed a higher adhesion strength. The combined use of epoxy blending and coupling agents was effective in increasing the adhesion strength of PAE-2 before aging and also in maintaining adhesion at a 50% level after humidity aging. However, its adhesion strength to NiAu was reduced to less than 50% after 85°C/85% RH aging. PAE-2-based ICAs showed excellent reworkability.

Thermal and mechanical stability of soldering QFP with Sn-Bi-Ag lead-free alloy

Thermal stability of the circuit boards with a quad flat package (QFP) soldered with Sn-58wt%Bi-(0, 0.5 and 1.0) wt% Ag and their microstructural features were evaluated. The addition of 1.0 wt% Ag causes the formation of large primary Ag/sub 3/Sn precipitates in the solder while no primary Ag/sub 3/Sn is found in Sn-57Bi-0.5Ag. Thermo-Calc calculation indicates that the lowest limit content for the formation of primary Ag/sub 3/Sn is about 0.8 wt%. Heat-exposure below 100/spl deg/C has no serious degradation on the joint structure for all solders. Heat-exposure at 125/spl deg/C caused serious degradation in joint strength for all alloys. The contamination of Pb from Sn-Pb surface plating on the components reduces the interface tolerance by forming ternary Sn-Pb-Bi phase melting at low temperature. Thermal fatigue between -20 and 80/spl deg/C does not have any significant influence on joint structure.

Reliability of electrically conductive adhesive joints for surface mount applications: a summary of the state of the art

Electrically conductive adhesives are of large potential interest for the bonding of surface mount devices, especially in those cases where the use of soldering processes is restricted or impossible. Among the claimed benefits are mild processing conditions, process simplicity, flexibility, high resolution potential, and lead omission. Most of the presently available knowledge on the utility and reliability of conductive adhesives has been gathered with silver filled isotropic conductive epoxy adhesives (ICAs). In addition, the use of gold or nickel filled anisotropic conductive film or paste adhesives has been investigated quite intensively. Even conductive adhesive joints made with unfilled nonconductive adhesives are being considered for flip chip applications. To date, quite some work has been published demonstrating the usefulness and limitations of isotropic adhesives to bond a variety of IC (e.g., QFP) discrete (e.g., SOT) and passive components (R and C) under different climate conditions using various circuit and component-metallizations. Most of the isotropic conductive adhesives require noble metallizations (e.g., An or AgPd) to survive harsh environmental conditions as for instance 85/spl deg/C/85%RH and temperature cycling from -40 to +125/spl deg/C. Most of the adhesives give bad results on SnPb surfaces, a few special types show better results in 85/spl deg/C/85%RH. Deterioration of the electrical properties is due to an increase in the contact resistance. The bulk resistance of the adhesive, although considerably higher than that of solder, usually remains quite constant. On passivated Cu substrates reasonable good results are obtained. With Au finished surfaces large components may give problems in drop tests and temperature cycling due to insufficient bond area, unfavorable bond geometries or failures due to thermal expansion differences. The use of a coating or glob top on the leads may overcome these problems. With Au coated plastic spheres filled anisotropic conductive adhesives [ACA(F)] results opposite to the ICAs were obtained. For QFP80 (Au) joints with ACAF on FR-4 (Au) considerable increase in R-value was found after Temperature Humidity (TH) testing at 85/spl deg/C/85%RH and Temperature Cycling (TC,-20/100/spl deg/C), while with QFP80 (SnPb) R-values did not increase. This was found to be due to the formation of microsolder bridges. Microsolder joints can also be made with commercially available SnBi filled ACA. Some insight has been developed into the possible failure causes with ICAs. Surface oxidation of SnPb and interfacial cracks and delamination have for instance been observed, although the extent of their contributions under various conditions still needs to be resolved. The danger of Ag migration seems to be low as long as no liquid water is present. In summary, present results indicate that reliable connections with conductive adhesives even under harsh climate conditions are possible with the right choice of adhesive and metallizations of components and boards.