C.W. Tang

Correlation between Ni 3 Sn 4 intermetallics and Ni 3 P due to solder reaction-assisted crystallization of electroless Ni–P metallization in advanced packages

Ni 3 Sn 4 intermetallic was formed by the depletion of Ni from electroless Ni‐P, and a Ni 3 P layer was formed simultaneously due to solder reaction-assisted crystallization during solder reflow. Both Ni 3 Sn 4 and Ni 3 P grew rapidly due to the solder reaction-assisted crystallization and their growth was diffusion controlled during the first 15 min of annealing at 220 °C. After that, the growth rate of Ni 3 Sn 4 was greatly reduced and the crystallization of electroless Ni‐P to Ni3P was no longer induced. Based on kinetic data and scanning electron microscope morphology observations, underlying mechanisms causing this specific phenomenon are proposed. This finding is indeed very crucial since we may control the growth of Ni‐Sn intermetallics by monitoring the solder reaction-assisted crystallization of electroless Ni‐P.

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.

Metallurgical reaction and mechanical strength of electroless Ni-P solder joints for advanced packaging applications

We have studied the metallurgical reaction and mechanical strength of the electroless Ni-P solder joints as a function of reflow time at 220 °C. It is found that both Ni3Sn4 intermetallics and Ni3P are formed due to the solder reaction-assisted crystallization. However, after the first 15 min of reflow, an unusual depression of Ni3Sn4 growth has been observed. A detailed description of the diffusion mechanism has been presented to explain the prohibition of the Ni3Sn4 growth. It is found that the growth of Ni3Sn4 and Ni3P may have a mutual effect on each other during the solder reaction since there is a direct correlation between the depression of the Ni3Sn4 growth and the ending of Ni3P growth. The characteristic of the mechanical strength of electroless Ni-P solder joints has been demonstrated. A correlation between the mechanical strength and the interfacial metallurgical reaction has been discussed. Also, it is found that different reflow times will result in different fracture interfaces of the sheared electroless Ni-P solder joints. The detailed explanation of the fracture surface morphology has been explored.

Effect of pinhole Au/Ni/Cu substrate on self-alignment of advanced packages

Abstract The self-alignment of advanced packages (μBGA) on both non-pinhole and pinhole Au/Ni/Cu pads has been discussed. It is found that a slight reduction of self-alignment of the packages using pinhole pads occurs. Rutherford backscattering spectrometer (RBS) results suggest that this reduction should not be attributed to the oxide formation of the surface or interface layer in the Au/Ni/Cu pads. The solder wetting experiments show that slow spreading of molten solder on pinhole pads may result in a reduction of effective board pad surface area that can be wetted. This will reduce the restoring force of the solder joints, and thus causing a less better self-alignment of the packages using pinhole pads. Oxidation of nickel at the exposed area and Au/Ni interface is observed to occur by direct exposure of substrate pads through pinholes during aging. The solder wetting of the aged pads has been described. For flux reflow soldering, the aging of the pads seems to have no serious effect on the self-alignment of the package. However, it is found from the peel-off test that a few solder joints of the samples after reflow have weak adhesion strength at the solder and aged pinhole pad interface. The mechanism for this weak adhesion strength has been proposed.

Nondestructive methodology for standoff height measurement of flip chip on flex (FCOF) by SAM

Flip chip technology is the emerging interconnect technology for the next generation of high performance electronics. One of the important criteria for reliability is the width of the gap between the die and the substrate, i.e., the standoff height. A nondestructive technique using scanning acoustic microscopy (SAM) for the standoff height measurement of flip chip assemblies is demonstrated. The method, by means of the implementation of a pulse separation technique, time difference of the representative signals of the die bottom and water interface and water and substrate surface interface from the A-scan image can he found. Then, the corresponding standoff height can be calculated. When compared to the traditional destructive measurement method (SEM analysis on sectioned sample), this nondestructive technique yields reliable results.

Scanning acoustic microscopy investigation of engineered flip-chip delamination

The rapid uptake of flip-chip technology within the electronics industry is placing the reliability of such assemblies under increasing scrutiny. A key feature of the assembly process is application of underfill to reinforce the die attachment to the PCB. This has been identified in numerous studies as one of the major ways in which device reliability can be improved, by mitigating coefficient of thermal expansion mismatch between chip and board. However, in order for the underfill to be effective in coupling the die to the PCB, its adhesion to the passivation layer of the die and the solder mask layer on the PCB must be maximised. There is a growing body of literature that indicates that poor adhesion at either interface (delamination) due to contamination can result in premature assembly failure through stress fracture of the solder joints. In order to investigate further the effect of delamination on the reliability of flip-chip assemblies, surface chemistry has been used to control the adhesion of the underfill to the die passivation. This paper reports how modification of the die surface by application of a low surface energy coating, which prevents strong underfill adhesion, has enabled selective device delamination at the chip-to-underfill interface. Using scanning acoustic microscopy (SAM), the effectiveness of this treatment in creating controlled delamination before and after thermal cycling has been monitored. The ability to engineer delamination can enable experimental studies of the mechanics of flip chip assembly failure, which complement current finite element modelling work.

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.

Standoff height measurement of flip chip assemblies by scanning acoustic microscopy

Flip chip technology is the emerging interconnect technology for the next generation of high performance electronics. One of the important criteria for reliability is the size of the gap between the die and the substrate, i.e. standoff height. A nondestructive technique using Scanning Acoustic Microscopy (SAM) for standoff height measurement of flip chip assemblies is demonstrated. The method, by means of the implementation of the pulse separation technique, time difference of the representative signals of the die bottom and water interface and water and substrate surface interface from the A-scan image can be found. Then, the corresponding standoff height can be calculated. When compared with the traditional destructive measurement method (SEM analysis on sectioned samples), the results suggest that the present method yields reliable results.