Peak Reflow Temperature Research Articles

Solder Paste로 접합된 비아볼의 Ball-off에 관한 연구

Package reliability test was conducted to investigate the effect of solder paste composition at BGA Package. It was found that the shape and size of the phase form are affected by the processing parameters. The material have used to fill in the via was Sn/36Pb/2Ag and Sn/0.75Cu type solder paste. Sn/36Pb/2Ag and Sn/0.75Cu paste were fabricated on Tape-BGA substrates by screen printing process, and via ball mount data were characterized with variations of dwell time of 85 seconds at reflow peak temperature at 22<TEX>$0^{\circ}C$</TEX> or 24<TEX>$0^{\circ}C$</TEX>. The test condition was MRT 30 <TEX>$^{\circ}C$</TEX>/60 %RH/96 HR. Failures formed of a ball-off in solder paste process were observed by using a Optical Microscope and SEM(Scanning Electron Microscope). It was concluded that intermetallic layer growth played important roles in increasing solder fatigue strength for addition of Ag composition. The degradation of shear strength of solder composition is discussed.

Development of the green plastic encapsulation for high density wirebonded leaded packages

The push in the electronics industry toward miniaturization and high density wirebonds is a major driving force in integrated circuit (IC) package design. One problem has been the use of conventional mold compounds to encapsulate the high density wirebonded packages, due to performance issues, such as wire sweep and coplanarity. Additionally, in order to match lead-free solutions in the near future, antimony- and halogen-free molding compound must be developed because antimony (Sb) and halogens in current flame retardant systems pose environmental hazardous. This article, discusses a “green” compound that eliminated these environmentally hazardous elements, resolving moldability and reliability issues for high density wirebonded packages. The reliability assessment was conducted at the maximum reflow peak temperature of 240 °C after moisture soaking at 60 °C/60%RH for 40 h, followed by temperature cycle tests. The study aimed for good package integrity, process, and performance to meet the requirements of high volume production and an acceptable moisture sensitivity (level 3) with a reflow temperature of 240 °C. The study indicates that moldability and reliability involved separate issues and offers a solution for high volume production and field application.

Assessment of board level solder joint reliability for PBGA assemblies with lead‐free solders

Evaluates the board level reliability of plastic ball grid array (PBGA) assemblies under thermal and mechanical loading, with the objective of characterizing the reliability of lead‐free solder joints for various assembly conditions. A five‐leg experiment was designed which included various combinations of solder materials and peak reflow temperatures. It was found that the lead‐free solder joints have a much longer thermal fatigue life than the 63Sn–37Pb solder. The 63Sn–37Pb solder joints seem to perform slightly better than the lead‐free solder under mechanical loading.

Effect of reflow profile on wetting and intermetallic formation between Sn/Ag/Cu solder components and printed circuit boards

The wetting performance and intermetallic formation of a Sn/Ag/Cu alloy on printed circuit board (PCB) surfaces and on component terminations were studied in this work. Two different PCB surface finishes, immersion gold over electroless nickel (Ni/Au) and an organic solderability preservative (OSP), were studied. Chip components with Sn/Pb coating and a gull‐wing type component with 100% Sn coating were used in these experiments. Different reflow profiles were tested, and the dependence of the wetting performance, intermetallic layer thickness and the microstructure of the solder joints on the reflow profile were investigated.It was found that reflow process conditions did not significantly influence the spreading or intermetallic formation on either of the surfaces. Neither the wetting onto the component nor the general microstructure of the solder joints varied significantly with the reflow profile. When a Sn/Pb ‐coated component was used, the content and size of Pb‐rich phases in the solder joint increased with a longer time above liquidus or a higher reflow peak temperature.

The impact of lead‐free soldering on electronics packages

This paper will review the challenges brought by lead‐free soldering and some preliminary experimental evaluation results will be discussed. The initial results show that the lead‐free soldering process with 260°C reflow peak temperature does not directly cause failures for bismaleimide‐triazine (BT)‐based fine pitch ball grid array (FPBGA) packages. However, the strict lead‐free soldering condition could degrade the integrity of weak interface joints and potentially damage the package in subsequent unbiased highly accelerated stress test (unbiased HAST) evaluation. The impacts of lead‐free soldering with high reflow temperature on concurrent available electronics components could be more severe than previously believed. In the future, new materials and design concepts should be applied to enhance the package reliability under strict lead‐free soldering conditions.

Lead‐free reflow soldering for electronics assembly

Results for reflow soldering are presented from a three‐year EC funded project “IDEALS” to develop lead‐free soldering solutions. On the basis of fundamental data from the literature, a shortlist of candidate lead‐free solders was selected, and results from tests on physical and soldering characteristics, and wetting balance testing, led to the choice of SnAg3.8Cu0.7, melting at 217°C. Implications for solder paste medium development are discussed. Differences in alloy density, melting point, and surface tension relative to conventional solders were found to give higher levels of internal voids, reduced spread on copper, and rougher, duller joints. Reflow process window studies showed that sound reliable joints could be obtained with a peak temperature as low as 225°C. Reliability was tested on soldered test boards using thermal shock cycling, power cycling, and vibration. Overall the SnAg3.8Cu0.7 gave results approximately equivalent to conventional solders, and different board finishes had no significant effect. The effects of Sb and Bi were also evaluated. No justification was found for minor additions of Sb, but 2‐5 per cent Bi was found to allow a reduction of the peak reflow temperature, though at the cost of reduced reliability if any Pb was present.

Lead-free solder flip chip-on-laminate assembly and reliability

This paper examines the assembly process for flip chip die with SnAgCu solder bumps and the results of liquid-to-liquid thermal shock testing. The SnAgCu alloy required a thicker dip layer of flux to achieve good wetting compared to the SnPb eutectic alloy. A liquid spray flux yielded more consistent solder wetting with the SnAgCu alloy. With both fluxes, a nitrogen reflow atmosphere was necessary with the SnAgCu alloy. A peak reflow temperature of 246/spl deg/C was used for the assembly of the SnAgCu thermal shock test vehicles. A lower peak temperature of 235/spl deg/C did not yield sufficient solder wetting. Liquid-to-liquid thermal shock testing was performed from -40/spl deg/C to +125/spl deg/C. The SnPb alloy performed slightly better than the SnAgCu and the dip flux was better that the spray flux. The degree of delamination with the SnAgCu alloy was significantly higher than with the SnPb alloy. Cracks in the underfill between adjacent solder balls were observed. The SnPb alloy extruded into these cracks more readily than the SnAgCu and created electrical shorts.

Reflow soldering and isothermal solid-state aging of Sn-Ag eutectic solder on Au/Ni surface finish

The reaction between the eutectic Sn-3.5Ag solder and the Au/Ni surface finish during reflow as well as during isothermal aging was studied. The Au layer was electroplated and had a thickness of the one μm. The peak reflow temperature was fixed at 250 C while the reflow time was varied between 10 sec and one h. Samples that went through 90 sec reflow time were then subjected to 160 C isothermal aging for up to 875 h. It was found that during reflow the Au layer reacted very quickly with the solder to form AuSn4. One μm of Au layer was consumed in less than 10 sec. As the aging time increased, AuSn4 grains began to separate themselves from the Ni layer at the roots of the grains and started to fall into the solder. When, the reflow time reached 30 sec, all the Au intermetallic head left the interface, and Ni3Sn4 started, to form at the interface. The Ni3Sn4 growth rate followed linear kinetics initially (<240 sec), but the growth rate slowed down afterward. During the isothermal aging, only a small amount of (AuxNi1-x)Sn4 resettled back to the interface, and a continuous (Au0.45Ni0.55)Sn4 layer did not form at the interface, unlike the case for the Sn-37Pb solder. This is an important advantage for Sn-3.5 Ag over Sn-37Pb because a continuous (Au0.45Ni0.55)Sn4 layer inevitably will weaken a solder joint. Our observation indicated that many (AuxNi1-x)Sn4 particles were trapped by the Ag3Sn particles, and were hindered from resettling back to the interface.

Mechanical Properties and Fracture Behavior of the Cu and Cu6Sn5–Dispersed Sn-Pb Solder Bumps Processed by Screen Printing

ABSTRACTThe mechanical properties of the 1-9 vol % Cu and Cu6Sn5-dispersed 63Sn-37Pb solder alloys were characterized with tensile test. Also, the Cu and Cu6Sn5-dispersed 63Sn-37Pb solder bumps of 760 μm size were fabricated on the Au(5 μm)/Ni(5 μm)/Cu(27 μm) BGA substrates by screen printing process, and their shear strength were characterized with variations of the dwell time at the reflow peak temperature(220°C) and aging time at 150°C. The yield strength and ultimate tensile strength of the 63Sn-37Pb solder alloy increased with dispersion of 1-9 vol % Cu and Cu6Sn5. In general, however, the Cu and Cu6Sn5-dispersed solder bumps exhibited lower shear strengths than those of the 63Sn-37Pb solder bumps The failure surface of the solder bumps after ball shear test could be divided into two regions of slow crack propagation and critical crack propagation, and the shear strength of solder bumps was inversely proportional to the slow crack propagation length.

Reaction of solder with Ni/Au metallization for electronic packages during reflow soldering

Gold over Ni is one of the most common surface finishes for Cu soldering pads in ball-grid-array (BGA) and other electronic packages. The Au layer is for oxidation protection, and the Ni layer serves as a solderable diffusion barrier. In this study, eutectic Pb-Sn solder-balls were reflowed on the Au/Ni/Cu pads, and the chemical interactions between the solder and the surface finish were studied. Quenched-in microstructures at different stages of the reflow were carefully examined using the scanning electron microscopy. It was found that the solder melted locally along the solder/pad interface at the very early stages of the reflow before the whole solder ball had reached the Pb-Sn eutectic temperature. This was because a ternary eutectic reaction L=(Pb)+(Sn)+AuSn/sub 4/ occurred at 177/spl deg/C, six degrees below the Pb-Sn eutectic temperature. Four distinct stages were identified for the reflow process. The four stages are: (1) partial melting of solder balls and the initial reaction of Au with Sn; (2) complete reaction of An with Sn; (3) separation of (Au/sub x/Ni/sub 1-x/)Sn/sub 4/ from the pad; (4) complete melting of solder balls and the reaction of Ni with Sn. After a typical reflow, with a 225/spl deg/C peak reflow temperature and 115 s reflow time, all the An and Au-bearing intermetallic compounds left the interface and the only intermetallic compound at the interface was Ni/sub 3/Sn/sub 4/ with a thickness of about 2 /spl mu/m.

Formation and resettlement of (AuxNi1−x)Sn4 in solder joints of ball-grid-array packages with the Au/Ni surface finish

The interfacial reactions between eutectic PbSn solder and the solder ball pads with the Au/Ni surface finish were studied. Solder joints subjected to up to three repeated reflow-and-aging treatments were examined. For the reflow, the peak reflow temperature was 225°C, and the reflow time was 115 s. Each aging process was performed at 160°C for 500 h. After the first reflow, all the Au would disappear from the interface, and formed many (AuxNi1−x)Sn4 particles inside the solder joints. The value of x was between 0.99 and 0.75. In addition, there was a thin layer of Ni3Sn4 (1.4 µm) at the interface. After one reflow and one subsequent aging, most of the (AuxNi1−x)Sn4 would relocate from inside the solder joint to the interface, and the value of x for (AuxNi1−x)Sn4 at the interface decreased to 0.45. This (AuxNi1−x)Sn4 resettlement process repeated itself for additional reflow-aging cycles. More reflow-aging treatments, however, made the microstructure of (Au0.45Ni0.55)Sn4 at the interface become more non-planar. It was shown that gravitational effect was not the driving force for the resettlement of (AuxNi1−x)Sn4. It is proposed that the driving force is for (AuxNi1−x)Sn4 to seek Ni at the interface so that it can become more Ni-rich. In other words, the driving force is lowering the Gibbs energy of (AuxNi1−x)Sn4 by dissolving more Ni. A decomposition-diffusion mechanism is proposed to explain what happened. Kinetic rationales for this rapid resettlement of (AuxNi1−x)Sn4 at such a low temperature were also discussed.

Thermal fatigue and metallurgical reactions in solder joints of LTCC modules

The fatigue behaviour of solder joints in ceramic LTCC modules on FR-4 boards was investigated using thermal fatigue experiments at temperature intervals of 0 – 100°C and −40 – 125°C, and non-destructive and SEM/EDS investigations of thermally cycled modules. Dissolution reactions and formation of tin-based intermetallics were encountered in the joints and found to cause degradation in them. A high peak reflow temperature in the fabrication of modules had a detrimental effect on performance. Vias filled with silver located under the central regions of the solder pads of the LTCC modules enhanced the reliability of the solder joints.

A lower-melting-point solder alloy for surface mounts

Significant manufacturing cost reductions can be realized with lower-temperature surface mount processing by increasing yields and using less expensive components and boards. A lower-melting-point solder alloy (nominal composition Sn-41.75Pb-8Bi0.5Ag) has been developed that enables significant reductions in peak reflow temperatures during surface-mount assembly. The solder alloy is compatible with standard Pb-Sn surface finishes, melts within the temperature range of ≈166–172°C,andhas promising mechanical properties.

Effect of Solder Reflow Process Variables on the Solder Wettability of Lead Free Solder Alloys

A quantitative dynamic solder wettability measurement was used to evaluate the effects of reflow processing on the wettability parameters associated with two non‐lead bearing solders, 96.5% Sn/3.5% Ag and 58% Bi/42% Sn. An experimental design approach employing full factorial experiments was formulated to investigate the solder wetting dependence of the reflow parameters: atmosphere, peak reflow temperature, time above liquidus and metallisation. Solder wettability was determined with respect to the final degree of spread and the extent of solder wetting onto the lands of surface mount components. The solder alloy composition of 96.5% Sn/3.5% Ag was found to exhibit better wetting characteristics than the 58% Bi/42% Sn alloy. This wetting behaviour was enhanced under the reflow conditions of a nitrogen atmosphere and the use of a gold metallisation. The wetting of the conventional 63% Sn/37% Pb solder alloy was improved over the comparatively processed 58% Bi/42% Sn alloy. However, the 63% Sn/37% Pb solder alloy displayed a greater sensitivity to reflow atmosphere than the 96.5% Sn/3.5% Ag alloy, which generally exhibited better wetting characteristics than the Sn/Pb alloy.