Reworkable Underfill Research Articles

Advanced Mechanical/Optical Configuration of Real-Time Moiré Interferometry for Thermal Deformation Analysis of Fan-Out Wafer Level Package

The mechanical/optical configuration of moire interferometry for real-time observation of thermal deformations is advanced to quantify the thermomechanical behavior of fan-out wafer level packages (FO-WLPs). Two most notable advancements are on the: 1) mechanical front: conduction-based thermal chamber for a wide range of ramp rates with accurate temperature control, and 2) optical front: microscope objectives to observe a microscopic field of view. The advancements enable the method to document the global and local behavior of an FO-WLP while it is subjected to a controlled thermal excursion. After describing the details of two advancements, a procedure of actual testing is presented. The advanced configuration is implemented to analyze an FO-WLP in a package-on-package with and without a reworkable underfill. The results obtained from the global and local analyses are presented, and reliability implications are discussed.

Process and Evaluation of High Reliability Reworkable Edge Bond Adhesives for Large Area BGA Applications

Reworkable underfills and edge bond adhesives are finding increasing utility in high reliability and harsh environment applications. The ASICs and FPGAs often used in these systems typically require designs incorporating large BGAs and ceramic BGAs. For these high reliability and harsh environment applications, these packages typically require underfill or edge bond materials to achieve the needed thermal cycle, mechanical shock and vibration reliability. Moreover, these applications often incorporate high dollar value printed circuit boards (on the order of thousands or tens of thousands of dollars per PCB) hence the need to rework these assemblies and maintain the integrity of the PCB and high dollar value BGAs. This further complicates the underfill requirements with a reworkability component. Reworkable underfills introduce a number of process issues that can result in significant variability in reliability performance. In contrast, edge bond adhesives provide a high reliability solution with substantial benefits over underfills. One interesting question for the large area BGA applications of reworkable underfills and edge bond materials is the comparison of their reliability performance. This paper presents a study of reliability comparison between two robust selected reworkable underfill and edge bond adhesive in a test vehicle including 11mm, 13mm, and 27mm large area BGAs. Process development for those large area BGA applications was also conducted on the underfill process and edge bond process to determine optimum process conditions. For underfill processing, establishing an underfill process that minimizing/eliminates underfill voids is critical. For edge bond processing, establishing an edge bond that maximizes bond area without encapsulating the solder balls is key to achieving high reliability. In addition, this paper also presents a study of new high performance reworkable edge bond materials designed to improve the reliability of large area BGAs and ceramic BGAs assemblies while maintaining good reworkablity. Four edge bond materials (commercially available) were studied and compared for a test vehicles with 12mm BGAs. The reliability testing protocol included board level thermal cycling (−40 to 125°C), mechanical drop testing (2900 G), and random vibration testing (3 G, 10 – 1000 Hz).

Thermomechanical Fatigue Performance of Lead-Free Chip Scale Package Assemblies with Fast Cure and Reworkable Capillary Flow Underfills

In this paper, we present the results of temperature cycling test for full and partial capillary flow underfilled lead-free chip scale packages (CSPs), the tests were carried out on the basis of JEDEC standard. Two kinds of representative fast cure and reworkable underfill materials are used in this study, and CSPs without underfills were also tested for comparison. The test results show that the two underfill materials reduce the thermomechanical fatigue performance of CSP assemblies. The underfill with high Tg and storage modulus yielded better performance; indeed, the coefficient of thermal expansion (CTE) is also very critical to the thermomechanical fatigue performance, but its effects is not so obvious in this study owing to the similar CTEs of the underfills used. In addition, the negative effect of a partial underfill pattern is smaller than that of a full underfill pattern. Failure analysis shows that the dominant failure mode observed is solder cracking near the package and/or printed circuit board pads.

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.

On the Incorporation of Fine Pitch Lead Free CSPs in High Reliability SnPb Based Microelectronics Assemblies

The thermal cycling performance of lead free soldered ball grid arrays (BGAs) and chip size packages (CSPs) soldered with eutectic SnPb solder paste, so-called backward compatible assemblies, tends to drop with increasing Pb concentration in the joints. This is a particular concern for fine pitch CSPs where concentrations are invariably rather large. Flip chip assembly life is almost always improved by underfilling, although lead free soldered assemblies are more sensitive to the underfill material properties. In the case of BGAs and CSPs, however, underfilling with the wrong material may actually reduce life in thermal cycling substantially. In general, underfills with low coefficients of thermal expansion (CTE) and high glass transition temperatures (Tg) are preferred, especially for lead free solder joints. However, recent work showed a reworkable underfill with a high CTE and a very low Tg to still improve the life of SnAgCu joints with 3% Pb mixed in. A variety of 0.4- and 0.5-mm pitch CSPs with SnPb, SAC305, or SAC405 solder bumps were assembled onto a six layer PCB with a SnPb solder paste. Assemblies were tested in 0/100 °C thermal cycling both with and without a reworkable underfill with a reasonably high modulus and a Tg of 105°C. Results deviated systematically from trends previously observed for larger pitch BGAs.

Characteristics and reliability of fast-flow, snap-cure, and reworkable underfills for solder bumped flip chip on low-cost substrates

In this study, the fast-flow, fast-cure, and reworkable underfill materials from two different vendors are considered. Emphasis is placed on the determination of the curing conditions such as temperature and time, and the material properties such as the thermal coefficient of expansion (TCE), storage modulus, loss modulus, glass transition temperature (T/sub g/), and moisture uptake of these underfill materials. Also, the key elements and steps of the solder-bumped flip-chips on low-cost substrates with these underfill materials such as the chip, printed circuit board (PCB), flip chip assembly, and underfill application are presented. Furthermore, the key elements and steps of the rework of the solder-bumped flip-chip assemblies with these underfill materials such as chip removal, chip reballing, substrate cleaning, and new chip placement are discussed. Finally, shear test results of the assemblies with one-time rework and no-rework are presented.

Syntheses and characterizations of thermally degradable epoxy resins. III

AbstractIn flip‐chip technology, the development of reworkable underfill materials has been one of the keys to the recovery of highly integrated and expensive board assembly designs through the replacement of defective chips. This article reports the syntheses, formulations, and characterizations of two new diepoxides, one containing secondary ester linkages and the other containing tertiary ester linkages, that are thermally degradable below 300 °C. The secondary and tertiary ester diepoxides were synthesized in three and two steps, respectively. Both compounds were characterized with NMR and Fourier transform infrared spectroscopy and formulated into underfill materials with an anhydride as the hardener and an imidazole as the catalyst. A dual‐epoxy system was also formulated containing the tertiary ester diepoxide and a conventional aliphatic diepoxide, 3,4‐epoxy cyclohexyl methyl‐3,4‐epoxycyclohexyl carboxylate (ERL‐4221E), with the same hardener and catalyst. The curing kinetics of the formulas were studied with differential scanning calorimetry (DSC). Thermal properties of cured samples were characterized with DSC, thermogravimetric analysis, and thermomechanical analysis. The dual‐epoxy system showed a viscosity of 18.7 and 0.87 P at 25 and 100 °C, respectively. The cured secondary, tertiary, and dual‐epoxy formulas showed decomposition temperatures around 265, 190, and 220 °C, glass‐transition temperatures around 120–140, 110–157, and 140–157 °C, and coefficients of thermal expansion of 70, 72, and 64 ppm/°C below their glass‐transition temperatures, respectively. The shear strength of the cured dual‐epoxy system decreased quickly with aging at 230 °C. The reworkability test showed that the removal of a chip underfilled with this material from the board was quite easy, and the residue on the board could be thoroughly removed with a mechanical brush without obvious damage to the solder mask. In summary, the synthesized tertiary epoxide can be used as a reworkable underfill for flip‐chip applications. © 2002 Wiley Periodicals, Inc. J Polym Sci Part A: Polym Chem 40: 1796–1807, 2002

Development of reworkable underfill from hybrid composite of free radical polymerization system and epoxy resin

The application of the underfill encapsulant is to enhance the solder joint fatigue life in the flip chip assembly, typically up to an order of magnitude, as compared to the nonunderfilled devices. Most of the current underfills, however, are primarily thermosetting epoxy resin curing system based materials, which transform into an infusible three dimensional network structure, and exhibit appreciable adhesion and reliability, but lack of desirable reworkability after curing. From the standpoint of polymeric material chemistry, other thermoplastic or thermosetting polymer materials could be of great economic/cost interest as encapsulants for some microelectronic packaging applications. In this paper, the experimental focus was devoted to the study of adhesion, reliability and reworkability of the free radical polymerization (FRP) system, as well as its hybrid composites or blends with phenoxy resin or epoxy resin (EPR), which could be potential underfill materials. The study encompassed formulation screening based on adhesion measurement, and assessment on reliability and reworkability performance for selected compositions developed so far.

Underfilling fine pitch BGAs

Fine pitch BGAs and chip scale packages have been developed as an alternative to direct flip chip attachment for high-density electronics. The larger solder sphere diameter and higher standoff of CSPs and fine pitch BGAs improve thermal cycle reliability while the larger pitch relaxes wiring congestion on the printed wiring board. Fine pitch BGAs and CSPs also allow rework to replace defective devices. Thermal cycle reliability has been shown to meet many consumer application requirements. However, fine pitch BGAs and CSPs have difficulty passing mechanical shock and substrate flexing tests for portable electronics applications. The fine pitch BGA used in the study was a 10 mm package with the die wire bonded. The package substrate was bismaleimide-triazine (BT) and the solder sphere diameter was 0.56 mm. Two types of underfill were examined. The first was a fast flow, snap cure underfill. This material rapidly flows under the package and can be cured in five minutes at 165/spl deg/C using an in-line convection oven. The second underfill was a thermally reworkable underfill for those applications requiring device removal and replacement. The paper discusses the assembly and rework process. In addition, liquid-to-liquid thermal shock data is presented along with mechanical shock and flexing test results.

Study of additive‐epoxy interaction of thermally reworkable underfills

Study of additive‐epoxy interaction of thermally reworkable underfills

Study of additive‐epoxy interaction of thermally reworkable underfills

AbstractUnderfill is the material used in a flip‐chip device to dramatically enhance its reliability as compared to a nonunderfilled device. Current underfills are mainly epoxy‐based materials that are not reworkable after curing, which is an obstacle in flip‐chip technology developments, where unknown bad die is a concern. Reworkable underfill is the key to address the nonreworkability of the flip‐chip devices. The ultimate goal of this study is to develop epoxy‐based thermally reworkable underfills. Our previous work showed that when incorporated into epoxy matrix, special additives could provide the epoxy formulation with die‐removal capability around solder reflow temperature. The additive‐epoxy interactions were studied and the results show that the additives do not adversely affect the epoxy properties. Moreover, when the additive decomposition temperature is reached, the decomposition of the additive causes a microexplosion within the epoxy matrix. Subsequently, the adhesion of the epoxy matrix is greatly reduced. Among the four additives studied, Additive1 and Additive2 may be used in reworkable underfills that can be reworked around solder reflow temperature, Additive3 cannot be used in underfill because it greatly reduces the shelf life of the underfill, and Additive4 may be used to develop reworkable underfill that withstands multiple reflows. © 2001 John Wiley & Sons, Inc. J Appl Polym Sci 81: 1868–1880, 2001

Reworkable no-flow underfills for flip chip applications

Underfill is a polymeric material used in the flip-chip devices that fills the gap between the integrated circuit (IC) chip and the substrate (especially on the organic printed circuit board), and encapsulates the solder interconnects. This underfill can dramatically enhance the reliability of the flip-chip devices as compared to the nonunderfilled devices. No-flow (compress-flow) underfill is a new type of underfill that allows simultaneous solder bump reflow and underfill cure, which leads to a more efficient no-flow underfilling process as compared to the standard capillary-flow underfilling process. Reworkable underfill is another type of underfill that allows the faulty chips to be replaced individually. It is the key material to address the nonreworkability issue of the current flip-chip devices. Reworkability is especially important to the no-flow underfill because electrical test of the assembled chips can only be done at the end of the no-flow underfilling process. The goal of this study is to demonstrate the feasibility of a no-flow reworkable underfill. Two approaches are taken to develop this new type of underfill. The first one is to add a special additive into a standard no-flow underfill formulation (underfill 0) to make it reworkable, called underfill 1. The second approach is to develop a no-flow underfill based on a new thermally degradable epoxy resin that decomposes around 240/spl deg/C, called underfill 2. Comparing to underfill 0, these two underfills have similar properties including glass transition temperature (T/sub g/), coefficient of thermal expansion (CTE) and modulus. Underfill 1 has similar curing and fluxing capability as underfill 0. Underfill 2 cures faster than underfill 0, and it has slightly weaker fluxing capability than underfill 0, but it still allows 100% of solder bumps wetting and collapsing on the copper board. Moreover, underfill 1 and underfill 2 allow the flip chips to be reworked using a developed rework process while underfill 0 does not.

Field reworkable underfill materials for flip chip on board assemblies

Studies have shown that underfill encapsulation dramatically improves the solder joint fatigue reliability of flip chip on board (FCOB) assemblies. The lack of reworkability of the underfill after the product is in the field has limited the integration of FCOB into cost sensitive electronic products and the continued proliferation of the FCOB technology will depend on the development of reworkable underfill materials systems. This paper presents data that correlates reliability performance to mechanical properties for twelve field reworkable underfill materials from three different suppliers. Their respective properties, processing parameters, and reliability performances are compared to the qualified, commercially available high performance underfills. Techniques were developed to redress the printed wiring board (PWB) site to enhance the reworked FCOB assembly yield. In addition, reliability performance results and failure analysis observations were compared to the first time nonreworked assemblies.

Syntheses and characterizations of thermally reworkable epoxy resins II

Flip-chip technology is a face-down attachment of the active side of the silicon device onto the substrate. It is the ultimate packaging solution to integrated circuit devices used in 21st century electronic systems to meet the requirements of small size, high performance, and low cost. Underfill technology enhances the flip chip on board cycle fatigue life and thus dramatically extends the application of flip-chip technology in electronics from high-end to cost-sensitive commodity products. Reworkable underfill is the key to addressing the nonreworkability of the underfill, so it is very important to electronic packaging. To meet the need for reworkable epoxy resins, four cycloaliphatic epoxides containing thermally cleavable carbonate linkages have been synthesized and characterized. These materials are shown to undergo curing reactions with cyclic anhydride similarly to a commercial cycloaliphatic diepoxide. Furthermore, these cured epoxides start to decompose at temperatures lower than 350 °C, the decomposition temperature for the cured sample of the commercial cycloaliphatic diepoxide. Two formulations based on two carbonate-containing diepoxides start network breakdown around 220 °C, which is the targeted rework temperature. Moreover, these two formulations have similar properties, including the glass-transition temperature, coefficient of thermal expansion, storage modulus, viscosity, and adhesion, compared to the standard commercial diepoxide formulation. As such, these two formulations are potential candidates for a successful reworkable underfill. © 2000 John Wiley & Sons, Inc. J Polym Sci A: Polym Chem 38: 3771–3782, 2000

Syntheses and characterizations of thermally reworkable epoxy resins. Part I

To meet the need for reworkable epoxy resins, a series of cycloaliphatic diepoxides containing thermally cleavable carbamate linkages were synthesized and characterized. These materials were shown to undergo curing reactions with cyclic anhydride in a similar fashion as a commercial cycloaliphatic epoxide, except that the carbamate group within the diepoxides can act as the internal catalyst. Furthermore, cured samples of the formulations from these diepoxides started to decompose at lower temperatures, i.e., between 200–300°C as compared with 350°C for the cured sample of the commercial cycloaliphatic epoxide, which showed their potential to be used as reworkable underfill encapsulants in the electronic packaging area. © 1999 John Wiley & Sons, Inc. J Polym Sci A: Polym Chem 37: 2991–3001, 1999

Development of reworkable underfills, materials, reliability and processing

The rework of packages is a standard practice in surface mount technology (SMT) assembly. The reasons for rework include process yields, failed parts, and field returns. The implementation of flipchip has been hindered by the lack of reworkability following underfill cure. Current commercial underfills do not allow for the rework of the flipchip. The development of a reworkable underfill is described in this paper. The materials selection process, information on the process for rework as well as pertinent reliability data are described.

Novel thermally reworkable underfill encapsulants for flip-chip applications

The flip-chip technique of integrated circuit (IC) chip interconnection is the emerging technology for high performance, high input/output (I/O) IC devices. Due to the coefficient of thermal expansion mismatch between the silicon IC (CTE=2.5 ppm//spl deg/C) and the low cost organic substrate such as FR-4 printed wiring board (CTE=18-22 ppm//spl deg/C), the flip-chip solder joints experience high shear stresses during temperature cycling. Underfill encapsulant is used to couple the bilayer structure and is critical to the reliability of the flip-chip solder interconnects. Current underfill encapsulants are filled epoxy-based materials that are normally not reworkable after curing. This forms an obstacle to flip-chip on board (FCOB) technology development, where unknown bad dies (UBD) are still a concern. Approaches have been taken to develop the thermally reworkable underfill materials in order to address the nonreworkability problem of the commercial underfill encapsulants. These approaches include introduction of thermally cleavable blocks into epoxides and addition of additives to the epoxies. In the first approach, five diepoxides containing thermally cleavable blocks were synthesized and characterized. These diepoxides were mixed with hardener and catalyst. Then the mixture properties of Tg, onset decomposition temperature, storage modulus, CTE, and viscosity were studied and compared with those of the standard formulation based on the commercial epoxy resin ERL-4221E. These mixtures all decomposed at lower temperature than the standard formulation. Moreover, one mixture, Epoxy5, showed acceptable Tg, low viscosity, and fairly good adhesion. In the second approach, two additives were discovered that provide die removal capability to the epoxy formulation without interfering with the epoxy cure or properties of the cured epoxy system. Furthermore, the combination of the two approaches showed positive results.

Novel high performance no flow and reworkable underfills for flip-chip applications

Flip-chip interconnect is the emerging technology for the high performance, high I/O (Inputs/Outputs) IC devices. Due to the thermal mismatch between the silicon IC (CTE=2.5 ppm/0 C) and the low cost organic substrate such as FR-4 printed wiring board (CTE= 18–22 ppm/°C), the flip chip solder joints experience high shear stress during temperature cycling testing. Underfill encapsulant is used to couple the bilayer structure and is critical to the reliability of the flip-chip solder joint interconnects. Novel no-flow underfill encapsulant is an attractive flip-chip encapsulant due to the simplification of the no-flow underfilling process. To develop the no-flow underfill material suitable for the no-flow underfilling process of flip-chip solder joint interconnects, we have studied and developed a series of metal chelate latent catalysts for the no-flow underfill formulation. The latent catalyst has minimal reaction with the epoxy resin (cycloaliphatic type epoxy) and the crosslinker (or hardener) at the low temperature (≤180° C) prior to the solder reflow and then rapid reaction takes place to form the low-cost high performance underfills. The effects of the concentration of the hardener and the catalyst on the curing profile and physical properties of the cured formulations were studied. The kinetics and exothermic heat of the curing reactions of these formulations were investigated by differential scanning calorimetry (DSC). Glass transition temperature (Tg) and coefficient of thermal expansion (CTE) of these cured resins were investigated by using thermo-mechanical analyzer (TMA). Storage moduli (G') and crosslinking density of the cured formulations were measured by dynamic-mechanical analyzer (DMA). Weight loss of these formulations during curing was investigated by using thermo-gravimetric analyzer (TGA). Additionally, some comparison results of our successful novel generic underfills with the current commercial experimental no-flow underfills are reported. Additionally, approaches have been taken to develop the thermally re- workable underfill materials in order to address the nonreworkability problem of the commercial underfill encapsulants. These include introducing the termally cleavable blocks to thermoset resins, and adding additives to thermoset resins. For the first approach, five diepoxides containing thermally cleavable blocks were synthesized and characterized. These diepoxides were mixed with the hardener and the catalyst. Then the properties of these mixtures including Tg, onset decomposition temperature, storage modulus, CTE, and viscosity were studied and compared with those of the standard formulation based on the commercial epoxy: ERL-4221D. These mixtures all decompose at lower temperature than the standard formulation. Moreover, one mixture—Epoxy5—showed acceptable Tg, low viscosity, and fairly good adhesion. For the latter approach, two additives were shown that after added to typical cycloaliphatic epoxy formulation, do not interfere with epoxy curing, and do not affect the typical properties of cured epoxy system, yet provide die removal capability to the epoxy. Furthermore, the combination of the two approaches showed positive results.

Fast-flow underfill encapsulant: flow rate and coefficient of thermal expansion

In the flip-chip on board assembly method, an underfill encapsulant material is applied in the gap between the integrated circuit (IC) chip and substrate to distribute the shear stresses at the solder interconnects. These shear stresses are imposed on the solder interconnects due to a coefficient of thermal expansion (CTE) mismatch between the IC chip and substrate. Different technologies such as fast-flow, no-flow, and reworkable underfills are currently being studied for flip-chip underfill encapsulant materials. This paper looks at the underfill encapsulant used in the fast-flow method of underfilling the IC chip/substrate gap. The effect of filler loading, particle size, and particle size distribution on the flow rate and CTE of the fast-flow underfill material are discussed in this work. The material used for the experiments is an epoxy resin with added silica filler to decrease the CTE. This study focuses on what effect different filler characteristics have on the underfill encapsulant. Also, an underfill encapsulant that can compete with one of industry's faster fast-flow underfills was developed as a result of this work.