Corner Bond Research Articles

A HIGH THROUGHPUT, LOW COST ASSEMBLY APPROACH FOR LED IMS PACKAGES TO HEAT SINK USING ADVANCED THERMAL INTERFACE MATERIALS

The objective of this study is to design and fabricate a high reliability LED Insulated Metal Substrate (IMS) package to complex heat sink attachment using an advanced thermal interface material (TIM). The assembly consists of LED IMS parts bonded to a heat spreader/sink using an advanced TIM and a corner bond material to quickly and accurately secure the LEDs in position. The corner bond adhesive is snap cured for fast machine cycle times while the high performance, high adhesion TIM materials cure throughout the rest of the assembly operation. This approach allows high accuracy LED bonding without the need for alignment pins or fasteners to anchor to the IMS. The IMS attached to the heat sink is then electrically interconnected with a thin flex substrate on top of the IMS. This approach is expected to replace the current mechanical fastners and low strength silicone TIM materials and reduce the cycle time and overall placement cost which are key drivers especially for the automotive industry.

Analysis of edge and corner bonded PSvfBGA reliability under thermal cycling conditions by experimental and finite element methods

In this paper, we present the thermal cycling test results for edge and corner bonded lead-free PSvfBGAs on a high-Tg test board. The test results show that the characteristic lives of PSvfBGAs with edge bond acrylic, edge bond epoxy and corner bond epoxy are respectively about 0.6, 0.9 and 1.3 times of the PSvfBGAs without bonding. The epoxy with lowest CTE, highest storage modulus and smallest adhesive volume yielded best performance. The 3D quarter finite element model was also built to further study the failure mechanism.

Effects of Corner/Edge Bonding and Underfill Properties on the Thermal Cycling Performance of Lead Free Ball Grid Array Assemblies

Underfilling will almost certainly improve the performance of an area array assembly in drop, vibration, etc. However, depending on the selection of materials, the thermal fatigue life may easily end up worse than without an underfill. This is even more true for lead free than for eutectic SnPb soldered assemblies. If reworkability is required, the bonding of the corners or a larger part of the component edges to the printed circuit board (PCB), without making contact with the solder joints, may offer a more attractive materials selection. A 30 mm flip chip ball grid array (FCBGA) component with SAC305 solder balls was attached to a PCB and tested in thermal cycling with underfills and corner/edge bonding reinforcements. Two corner bond materials and six reworkable and nonreworkable underfills with a variety of mechanical properties were considered. All of the present underfills reduced the thermal cycling performance, while edge bonding improved it by up to 50%. One set of the FCBGAs was assembled with a SnPb paste and underfilled with a soft reworkable underfill. Surprisingly, this improved the thermal cycling performance slightly beyond that of the nonunderfilled assemblies, providing up to three times better life than for those assembled with a SAC305 paste.

Experiment and numerical analysis for edge and corner bonded PoP bottom package assemblies under four-point bending

This paper presents the four-point bend test results for edge and corner bonded 0.5mm pitch lead-free package stackable very thin fine pitch ball grid arrays (PSvfBGAs) as package-on-package (PoP) bottom packages on a standard IPC/JEDEC bend test board. The tests were carried out based on the above standard with a 30mm loading span, 90mm support span and 7.5mm/s crosshead speed. The daisy chain resistance, strain, crosshead displacement and load of each PSvfBGA were measured and a 20% increase in the resistance was used as the failure criterion. Materials used in this study were a UV-cured acrylic edge bond adhesive, a thermal-cured epoxy edge bond adhesive and a thermal-cured epoxy corner bond adhesive. The test results show that all of them can improve the bend performance significantly; especially the edge bond high module epoxy increased the crosshead displacement, strain and bending force when mechanical damage occurred by 33.46%, 26.74% and 3.05% respectively. Failure analysis indicated that the predominant failure site was PCB pad lift/cratering regardless of with or without adhesives. The 3D quarter finite element model was also built to further study the improvement mechanism of bend performance by these adhesives.

Lead-Free Chip Scale Packages: Assembly and Drop Test Reliability

Three underfill options compatible with lead-free assembly have been evaluated: capillary underfill, fluxing underfill, and corner bond underfill. Chip scale packages (CSPs) with eutectic Sn/Pb solder were used for control samples. Without underfill, lead-free and Sn/Pb eutectic drop test results were comparable. Capillary flow underfills, dispensed and cured after reflow, are commonly used in CSP assembly with eutectic Sn/Pb solder. With capillary flow underfill, the drop test results were significantly better with lead-free solder assembly than with Sn/Pb eutectic. Fluxing underfill is dispensed at the CSP site prior to CSP placement. No solder paste is printed at the site. The CSP is placed and reflowed in a standard reflow cycle. A new fluxing underfill developed for compatibility with the higher lead-free solder reflow profiles was investigated. The fluxing underfill with lead-free solder yielded the best drop test results. Corner bond underfill is dispensed as four dots corresponding to the four corners of the CSP after solder paste print, but before CSP placement. The corner bond material cures during the reflow cycle. It is a simpler process compared to capillary or fluxing underfill. The drop test results with corner bond were intermediate between no underfill and capillary underfill and similar for both lead-free and Sn/Pb eutectic solder assembly. The effect of aging on the drop test results with lead-free solder and either no underfill or corner bond underfill was studied. Tin/lead solder with no underfill was used for control. This test was to simulate drop performance after the product has been placed in service for some period of time. There was degradation in the drop test results in all cases after 100 and 250 h of storage at 125/spl deg/C prior to the drop test. The worst degradation occurred with the lead-free solder with no underfill.