Liqiang Cao

Effect of curing condition of adhesion strength and ACA flip-chip contact resistance

There has been steadily increasing interest in using electrically conductive adhesives as interconnecting materials in electronics manufacturing. Simple processing, low processing temperature and fine pitch capability are the major advantages of conducting adhesive technology. A new and innovative connection technology geared towards achieving increased functionality at a lower total system cost is anisotropic conductive adhesive (ACA) interconnection. ACAs provide electrical as well as mechanical interconnections for fine pitch applications when used for flip chip assembly. In the other hand, this new technology still has reliability limitation. One major concern is the poor adhesion strength of adhesive interface, another is the contact resistance shift. The mechanism of adhesion interface and contact resistance shift during elevated temperature and humidity aging had been discussed in previous study. The ultimate goal of this work is to develop conductive adhesive with stable contact resistance and desirable interface adhesion strength. The purpose of this study is to investigate optimum curing degree and curing temperature to achieve the best performance of ACA. The curing reaction of a conductive adhesive was studied with a differential scanning calorimeter (DSC) under isothermal condition and temperature scanning. Adhesion strength was measured by 90 degrees peel test and contact resistance was also studied as a function of curing degree and curing temperature. Microstructural investigations of the fracture surfaces were studied by scanning electron microscopy (SEM). It was shown that the electrical and mechanical performance is dependent on curing conditions. By comparing the dynamic DSC results of the conductive adhesive with different bonding condition, it can be seen that it is not fully correct that high temperature cured sample have higher adhesion strength than low temperature cured sample, peel strength increase proportion with the increase of bonding temperature up to 120/spl deg/C and then decrease when the bonding temperature was above 120/spl deg/C. Contact resistance was found to be strongly dependent on the curing degree, but also revealed no related with curing temperature. In this case, the optimum temperature for bonding with ACA was concluded to be at 120/spl deg/C.

Isothermal low cycle fatigue behavior of Sn-8Zn-3Bi on single shear solder joint

Tin-zinc solder with the low melting temperature below 200 /spl deg/C is an attractive alternative to eutectic tin-lead. The addition of bismuth improves wettability of Sn-Zn alloy. Low cycle isothermal mechanical fatigue testing of solder joints of Sn-8Zn-3Bi (wt%) and Sn-37Pb were tested in a solder joint of single lap shear sample with a multi-functional Instron 4458-Microtester. Solder joints around 0.5mm high and 1.6mm in diameter was used. The testing was executed in a displacement -controlled mode, at three different amplitudes namely, 40, 50 and 60/spl mu/m. The test frequency was kept constant at 0.2Hz for all tests. The failure of the solder joint was defined as a 50% load reduction. Finite element (FE) modeling were simulated to analyze the strain and stress distribution in solder joint and compared with the experimental results. The cross-section showed that there were mixed phase of small cellular-shape and coarse needle-shape in the microstructure of Sn-8Zn-3Bi. SEM-EDS analysis indicated that the Au-Zn intermetallic compound was observed near the interface. The total average lifetime of Sn-8Zn-3Bi solder joint was 17% increase comparing with Sn-37Pb solder joint from low cycle fatigue testing. The ductile-fracture manner and brittle-fracture manner were both involved in fracture surface, and more features of brittle-fracture manner appeared in small displacement loading and more features of ductile-fracture manner in large displacement loading. From FE simulation, the maximum stress was found at two corners, coinciding with the experimental observation of crack initiation and propagation.

Low cycle fatigue testing and simulation of Sn‐8Zn‐3Bi and Sn‐37Pb solder joints

Purpose – Sn‐Zn based lead free solders with a melting temperature around 199°C are an attractive alternative to the conventional Sn‐Pb solder and the addition of bismuth improves its wetability. Whilst lead‐free soldering with Sn‐8Zn‐3Bi has already been used in the electronics assembly industry, it is necessary to study its low cycle fatigue properties since such data have not been reported up to now.Design/methodology/approach – In this study, displacement‐controlled low cycle fatigue testing of Sn‐8Zn‐3Bi and Sn‐37Pb solder joints was done on lap shear samples. The test amplitude was varied whilst the frequency was kept constant at 0.2 Hz and failure was defined as a 50 per cent load reduction. Finite element (FE) modelling was used for analysis and the results were compared to the experimental data.Findings – The microstructure of the Sn‐8Zn‐3Bi solder showed a mixed phase of small cellular‐shaped and coarser needle‐shaped areas. Au‐Zn intermetallic compounds were observed near the interface from the...

Residual Stress in Flip Chip Joining Using Anisotropic Conductive Adhesive

Flip chip packaging with anisotropically conductive adhesive (ACA) joint is increasingly used in the electronic industry because of its environmental benefits, fine pitch capability and simpler process. The mechanical deformation has a great impact on electricity ability of the particles in flip chip assembly. The stress analysis of ACA joining in flip chip assembly is one of key steps towards a better understanding of the performance of ACA joints under various process and service conditions. A new finite element simulation for large deformation and residual stress in ACA joining has been made in this work, and there has been no known prior reported on it. A new method was suggested to simulate the force equilibrium and residual stress in conductive ball and resin matrix in flip chip joining using anisotropic conductive adhesive. The link elements with varied Youngs module were introduced in present FE simulation. The results from this simulation in ACA bonding process indicated that after the pressure removed, the deformation ratio RD decreased from 80% to 75.7%, and the deformation recovery ratio DeltaRD = 4.3% only. A force equilibrium between resin and ball was built, and the ball can not revert its original state after the external load on the chip had been completely removed. The residual equivalent stress Seqv,ball in ball center was remained at a high value, 318 MPa, and the average residual stress Seqv,resin in resin matrix was only 25 MPa. Comparing with the residual stress in ball, the residual stress in resin matrix would be not important, due to the simulation result Seqv,resin << Seqv,ball The assumption Seqv,resin = 0 usually used, i.e. the residual stress in resin matrix can be omitted, may be a useful approximation in reliability analysis of ACA joining. The relaxation of residual stress in conductive ball may play a more important role for reliability of ACA joining. It is necessary to investigate the viscoelastic and plastic behavior of ball material, and to build its constitutional equation more reasonably in the future research. The simulation method suggested in this work can serve as a basis for stress analysis and for lifetime analysis of flip chip packaging with ACA joint in the future work