P. L. Tu

Effect of intermetallic compounds on the thermal fatigue of surface mount solder joints

The effect of Cu-Sn intermetallic compounds (IMC) on the fatigue failure of solder joints during thermal cycling has been studied. The samples consist of components [leadless ceramic chip carrier (LCCC)] soldered onto FR-4 printed circuit board (PCB), and are prepared by conventional reflow soldering using a 63Sn-37Pb solder paste. The specimens are subjected to thermal cycling between -35/spl deg/C and 125/spl deg/C with a frequency of two cycles per hour until failure. The results indicate that the fatigue lifetime of the solder joints depends on the thickness of IMCs layer between Cu-pad and bulk solder, and the relation of the lifetime to the thickness can be described as a monotonically decreasing curve. The lifetime is very sensitive to the thickness of the IMC when the thickness is less than 1.4 /spl mu/m. During thermal cycling the thickness of the IMC layer increases and then the interface between IMC and solder becomes gradually flatter. The results of X-ray diffraction and scanning electron microscope (SEM) analysis show that cracks propagate along the interface between the IMC layer and the solder joint. The Cu/sub 3/Sn (/spl epsiv/-phase) is also found to form between the Cu-pad and /spl eta/-phase during thermal cycling. On the basis of the above results, the thick and flattened IMC layer is shown to responsible for the fatigue failure of solder joint during thermal cycling. The results of this paper can be used to optimize the reflow soldering process for the fabrication of robust solder joints.

GROWTH KINETICS OF INTERMETALLIC COMPOUNDS IN CHIP SCALE PACKAGE SOLDER JOINT

Department of Electronic Engineering, and *Department of Physics and Materials Science,City University of Hong Kong, Hong Kong, People’s Republic of China(Received May 18, 2000)(Accepted in revised form August 18, 2000)Keywords: Solder joint; Intermetallic; Aging; Kinetics; FatigueIntroductionAs the trend in requirements of electronic packaging is toward higher I/O, greater performance, higherdensity, and lighter weight, the use of area array packaging technology is expected to increase. The typeof packaging, such as ball grid array (BGA), chip scale package (CSP), and Flip Chip, provides theultimate in high I/O-density and count with superior electrical performance, and very small size. Themechanical properties of solder joint are recognized as one of the critical factors that determine theirreliability and lifetime [1–5]. During soldering, the formation of intermetallic compounds (IMCs) atsolder/substrate interface is inevitable and ensures a good metallurgical bond. However, during storageand field service, the growth of IMCs will influence the strength of solder joints and result inmechanical failure of the joints [6–10]. Our recent research result shows that the fatigue lifetime ofsolder joint decreases linearly with the increasing square root of IMCs layer thickness [11]. Since theintermetallic is responsible for the solder joint failures, attention has to be paid to the effect of IMCsgrowth on the lifetime and reliability of microelectronic assemblies.This paper discusses the Ni-Sn/Cu-Sn IMCs layer growth kinetics in the joint soldered on platedAu/Ni FR-4 printed circuit board (PCB), which is one of the most commonly used substrate for finepitch area array packaging. The kinetic model describing and predicting the IMC thickness can be usedto estimate the solder joint reliability and lifetime, according the relationship of the IMC thickness withvibration fatigue failure and cyclic bend fatigue.Experimental ProceduresIn this experiment, the chip scale package CSP46-T.75 with Sn/Pb-eutectic solder ball was placed andsoldered on FR-4 printed circuit boards using a high-speed flexible mounter (CASIO YCM-5500V) anda 5-zone N

Reliability studies /spl mu/BGA solder joints-effect of Ni-Sn intermetallic compound

This paper studies the bending and vibration effects on the fatigue lifetime of ball grid array (BGA) solder joints. The correlation between the fatigue lifetime of the assembly and the heating factor (Q/sub n/), defined as the integral of the measured temperature over the dwell time above liquidus (183/spl deg/C) in the reflow profile is discussed. Our result shows that the fatigue lifetime of /spl mu/BGA solder-joints firstly increases and then decreases with increasing heating factor. The optimal heating factor Q/sub n/ is found to be 300-680/spl deg/C. In this range, the assembly possesses the greatest fatigue lifetime under various mechanical periodic stress, vibration and bending tests. The cyclic bending cracks always initiate at the point of the acute angle where the solder joint joins the PCB pad, and then propagate in the site between the Ni-Sn intermetallic compound (IMC) layer and the bulk solder. Under the vibration cycling, it is found that the fatigue crack initiates at valleys in the rough surface of the interface of the Ni-Sn IMC with the bulk solder. Then it propagates mostly near the Ni-Sn IMC layer and occasionally in the IMC layer or along the IMC/nickel interface. Evidently, the Ni-Sn IMC contributes mainly to the fatigue failure of the /spl mu/BGA solder joints. The SEM and EDX inspection show that only Ni/sub 3/Sn/sub 4/ IMC forms between the tin-based solder and the nickel substrate. Moreover, no brittle AuSn/sub 4/ is formed since all the Au coated on the pad surface is dissolved into the solder joint during reflowing. The formation of the Ni/sub 3/Sn/sub 4/ IMC during soldering ensures a good metallurgical bond between the solder and the substrate. However, a thick Ni-Sn IMC influences the joint strength, which results in mechanical failure. Based on the observed relationship of the fatigue lifetime with Ni-Sn IMC thickness and Q/sub n/, the reflow profile should be controlled with caution in order to optimize the soldering performance.

Effect of intermetallic compounds on the shear fatigue of Cu/63Sn-37Pb solder joints

The effect of Cu-Sn intermetallic compounds (IMC) on the fatigue failure of solder joints has been studied by means of shear cycling. The samples consist of leadless ceramic chip carriers (LCCC) soldered onto FR-4 printed circuit boards (PCB), and are prepared by conventional reflow soldering using a 63Sn-37Pb solder paste and then aged at 150/spl deg/C for 1, 4, 9, 16, 25, 36, and 49 days. The specimens are subjected to low cycle fatigue shear tests controlled by the displacement. The results indicate that the fatigue lifetime of the solder joints depends on the thickness of the LMC layer between the Cu-pad and bulk solder, and the quantitative relationship between the lifetime and thickness can be described as a monotonically decreasing curve. The greatest decrease is over the thickness range up to 2.8 /spl mu/m, when the IMC/bulk solder interface becomes flat, corresponding to a lifetime decrease to 62% of the as assembled value. For further increase in IMC thickness the lifetime decreases more slowly. Evidently, the effect of the Cu-Sn intermetallic compounds on the joint fatigue lifetime is not only concerned with the IMC thickness hut also the interface morphology. A thick and flat LMC layer has a deleterious effect. The results of X-ray diffraction and metallographic analysis show that cracks initiate underneath the component metallization, and propagate along the IMC/solder interface, then toward the fillet. The Cu/sub 3/Sn (/spl epsiv/-phase) is formed between the Cu-pad and p-phase, and grows more quickly than the /spl eta/-phase during storage and long term operation or aging tests. However, the Cu/sub 3/Sn makes only a small direct contribution toward fatigue failure.

Effect of intermetallic compounds on vibration fatigue of /spl mu/BGA solder joint

This paper studies the vibration fatigue failure of /spl mu/BGA solder-joints reflowed with different temperature profiles, and aging at 120 /spl deg/C for 1, 4, 9, 16, 25, 36 days. The effect of the Ni/sub 3/Sn/sub 4/ and Cu-Sn intermetallic compound (IMC) on the fatigue lifetime is also reported. During the vibration fatigue test, in order to identify the failure of /spl mu/BGA solder joint, electrical interruption was monitored continuously through the daisy-chain network. Our results show that the fatigue lifetime of the solder joint firstly increases and then decreases with increasing heating factor (Q/sub /spl eta//), which is defined as the integral of the measured temperature over the dwell time above liquidus (183/spl deg/C) in the reflow profile. The greatest lifetime occurs when Q/sub /spl eta// is near 500s/spl deg/C. Moreover, the lifetime of the solder joint decreases almost linearly with the increasing fourth root of the aging time. The SEM/EDX inspection shows that Ni/sub 3/Sn/sub 4/ IMC and Cu/sub 6/Sn/sub 5//Cu/sub 3/Sn IMCs are formed at the interface of the solder/nickel-plated PCB pad, and the no-aging solder/component-metallization, respectively. And during long term aging, Ni/sub 3/Sn/sub 2/ and NiSn were found at the Ni/Solder interface with X-ray diffraction, except Ni/sub 3/Sn/sub 4/. For nonaged solder joint, the fatigue crack generally initiates at the interface between the Ni/sub 3/Sn/sub 4/ IMC and the bulk solder. Then it propagates mostly near the Ni/solder, and occasionally in the LMC layer or along the Ni/solder interface. After aging, the fatigue track mostly initiates and propagates in the Cu/sub 6/Sn/sub 5/ -phase/bulk-solder interface or the Cu/sub 3/Sn/Cu/sub 6/Sn/sub 5/ interface on component-metallization. Evidently, the intermetallic compounds contribute mainly to the fatigue failure of /spl mu/BGA solder joints. The thicker the IMC layer, the shorter the fatigue lifetime of solder joint. The initial formation of the IMCs at the interface during soldering ensures a good metallurgical bond between the solder and the substrate. However, a thick IMC layer influences the toughness and strength of the solder joint, which results in mechanical failure.

Vibration fatigue of /spl mu/BGA solder joint

This paper studies the vibration fatigue failure of /spl mu/BGA solder-joints reflowed with different temperature profiles, and ageing at 120/spl deg/C for 1, 4, 9, 16, 25, 36 days. The effect of the thickness of the Ni/sub 3/Sn/sub 4/ and Cu-Sn intermetallic compound (IMC) on the fatigue lifetime is also reported. During the vibration fatigue test, in order to identify the failure of /spl mu/BGA solder joint, electrical interruption was monitored continuously through the daisy-chain network. Our results show that the fatigue lifetime of the solder joint firstly increases and then decreases with increasing heating factor (Q/sub n/), which is defined as the integral of the measured temperature over the dwell time above liquidus (183/spl deg/C) in the reflow profile. The greatest lifetime occurs when Q/sub n/ is near 500 s/spl deg/C. Moreover, the lifetime of the solder joint decreases linearly with the increasing fourth root of the ageing time. The SEM/EDX inspection shows that only Ni/sub 3/Sn/sub 4/ IMC and Cu/sub 6/Sn/sub 5//Cu/sub 3/Sn IMCs are formed between the solder and the nickel-plated PCB pad, and the solder/component-metallization interface respectively. For non-aged samples reflowed with different profiles, the fatigue crack generally initiates at valleys in the rough surface of the interface of the Ni/sub 3/Sn/sub 4/ IMC with the bulk solder. Then it propagates mostly near the Ni/solder, and occasionally in the IMC layer or along the Ni/solder interface. For aged samples, the fatigue crack mostly initiates and propagates in the Cu/sub 6/Sn/sub 5/-phase/bulksolder interface or the Cu/sub 3/Sn/Cu/sub 6/Sn/sub 5/ interface on component-metallization. Evidently, the intermetallic compounds contribute mainly to the fatigue failure of /spl mu/BGA solder joints. The thicker the IMC layer, the shorter the fatigue lifetime of solder joint. The initial formation of the IMCs at the interface during soldering ensures a good metallurgical bond between the solder and the substrate. However, a thick IMC layer influences the solder joint strength, which results in mechanical failure due to volume shrinkage during the transformation from solid phase to the intermetallic compound.

Study of the self-alignment of no-flow underfill for micro-BGA assembly

Abstract As a concept to achieve high throughput low cost flip-chip assembly, a process development activity is underway, implementing next generation flip-chip processing based on large area underfill printing/dispensing, IC placement, and simultaneous solder interconnect reflow and underfill cure. The self-alignment of micro-BGA (ball grid array, BGA) package using flux and two types of no-flow underfill is discussed in this paper. A “rapid ramp” temperature profile is optimized for reflow of micro-BGA using no-flow underfill for self-aligning and soldering. The effect of bonding force on the self-alignment is also described. A SOFTEX real time X-ray inspection system was used to inspect samples to ensure the correct misalignment before reflow, and determine the residual displacement of solder joints after reflow. Cross-sections of the micro-BGA samples are taken using scanning electronic microscope. Our experimental results show that the self-alignment of micro-BGA using flux is very good even though the initial misalignment was greater than 50% from the pad center. When using no-flow underfill, the self-alignment is inferior to that of using flux. However, for a misalignment of no larger than 25% from the pad center, the package is also able to self-align with S1 no-flow underfill. However, when the misalignment is 37.5–50% from the pad center, there are 10–14% residual displacement after reflow. The reason is the underfill resistant force inhibiting the self-alignment of the package due to rapid increment of underfill viscosity during reflow. The self-alignment of micro-BGA package using no-flow underfill allows only 25% misalignment prior to the soldering. During assembling, although the bonding force does not influence on the self-alignment of no-flow underfill, a threshold bonding force is necessary to make all solder balls contact with PCB pads, for good soldering. The no-flow underfill is necessary to modify the fluxing/curing chemistry for overcoming the effect of tin metal salt produced during soldering on underfill curing, and for maintaining the low viscosity during soldering to help self-alignment.

Endoscopic inspection of solder joint integrity in chip scale packages

This paper reports the use of endoscopy for the nondestructive examination of solder joint integrity in chip scale packages (CSP) such as flip chip on flex. Borrowed from the medical instrument technology, the endoscope is used to examine visually the inside of an organ. This concept has now been developed and refined by ERSA and KURTZ, and led to an ERSASCOPE inspection system coupled with sophisticated but user-friendly software for data and image analysis. We have successfully used this endoscopic method to evaluate the solder joint integrity in CSP such as cold joints, standoff height measurements, and probing of inner rows of ball joint integrity. Since this new instrument has only been introduced very recently, we will also report the latest results for obtaining the optimal examination of solder joint integrity achievable. By analyzing many varieties of CSP samples, we have witnessed the tremendous power of endoscopy when applied to the nondestructive examination of CSP solder joints, and found this method of immense use to the quality assessment of miniaturized electronic packages. We also give a critical review of this inspection method when applied to CSP.

Study of self-alignment of μBGA packages

In this paper, a detailed study of the self-alignment of /spl mu/BGA packages is presented. Complete self-alignment can be achieved even for a misalignment of the package of larger than 50% off the test board pad centres. A small residual displacement of the package from perfect alignment after reflow is observed. The reason for this displacement is the action of gas flow viscous drag on the package during reflow. The use of eutectic SnPb solder paste slightly reduces self-aligning ability, due to the increase in the solder volume, which reduces the restoring force. Exposure of the solder paste to a 25/spl deg/C and 85% RH humidity environment also has a detrimental effect on the self-alignment of the /spl mu/BGA package, due to solvent evaporation and moisture absorption in the paste causing solderability degradation. The self-alignment of the package is also affected when there is slow spreading of molten solder on the pad surface. This is attributed to the reduction of restoring force due to the decrease in effective wetting surface area of the board pad.

Study of self-alignment of /spl mu/BGA packages

In this paper, a detailed study of the self-alignment of /spl mu/BGA packages is presented. Complete self-alignment can be achieved even for a misalignment of the package of larger than 50% off the test board pad centres. A small residual displacement of the package from perfect alignment after reflow is observed. The reason for this displacement is the action of gas flow viscous drag on the package during reflow. The use of eutectic SnPb solder paste slightly reduces self-aligning ability, due to the increase in the solder volume, which reduces the restoring force. Exposure of the solder paste to a 25/spl deg/C and 85% RH humidity environment also has a detrimental effect on the self-alignment of the /spl mu/BGA package, due to solvent evaporation and moisture absorption in the paste causing solderability degradation. The self-alignment of the package is also affected when there is slow spreading of molten solder on the pad surface. This is attributed to the reduction of restoring force due to the decrease in effective wetting surface area of the board pad.