Lejun Wang

Characterization of a no-flow underfill encapsulant during the solder reflow process

A challenge in flip-chip technology development is to improve the thermo-mechanical reliability of the flip-chip assembly. To increase reliability, an underfill encapsulant is applied to the gap between IC chip and substrate to provide thermal-mechanical protection as well as environmental protection to the assembly. Two processes for applying the underfill encapsulant to the gap between IC chip and substrate can be described as the fast-flow method and the no-flow (reflowable underfill) method. The fast-flow method is currently the most widely used method. The no-flow method is a new innovative method that provides cost savings. In order to develop novel underfill encapsulants for the no-flow process, a better understanding of the underfill properties during the solder reflow is needed. This paper studies two aspects of the No-Flow underfill: fluxing activity and viscosity during reflow. These two aspects are important for proper interconnect formation. Solder wetting studies were conducted by applying the no-flow underfill on top of solder beads on substrates of different metallizations. The samples were then placed in a 7-zone reflow oven on different eutectic type heating cycles. Cross sections of the samples were taken and the angle the solder makes with the substrate was determined. The viscosity of the underfill during reflow is important to allow proper solder interconnects. To acquire the viscosity of the underfill just before, during, and shortly after the solder reflow temperature, a no-flow underfill encapsulant developed at the Georgia Institute of Technology was studied. Samples of this underfill were placed in a 5-zone reflow oven on a standard eutectic cycle and taken out at different points. The samples were then analyzed by differential scanning calorimetry (DSC) to find the % conversion (amount of cure) of the underfill material. These % conversions were then used to find the complex viscosity at different points in the reflow process. In this paper, we present the experimental procedures and results of the No-Flow underfills fluxing abilities and viscosity during reflow heating conditions.

Epoxy-additive interaction studies of thermally reworkable underfills for flip-chip applications

Reworkable underfill encapsulant is the key to address the non-reworkability of the flip-chip-on-board packages. This paper presents work on epoxy-additive interaction studying to develop reworkable additive-modified underfills. Four additives that have the potential to provide epoxy reworkability are selected. Their interactions with epoxy composition before, during, and after epoxy curing are studied. The results show that all additives retain their thermal decomposition feature after they are incorporated into the epoxy formulation, and they do not adversely affect the epoxy properties. However, when the additive decomposition temperature is reached, the decomposition of the additive caused a big CTE jump in the epoxy matrix. Additionally, the adhesion of the epoxy matrix is greatly reduced. These two changes are considered important to provide reworkability to the epoxy. Based on the epoxy-additive interaction study, two additives are selected for developing reworkable additive-modified epoxy underfills.

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.

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.

Reworkable underfills for flip chip, BGA, and CSP applications

Underfill is a polymeric material used in the flip-chip devices that fills the gap between an IC chip and an organic board, and encapsulates the solder interconnects. This underfill material can dramatically enhance the reliability of the flip-chip devices as compared to the non-underfilled devices. Current underfills are mainly epoxy-based materials that are not reworkable after curing, which places an obstacle in Flip-Chip on Board (FCOB) technology developments. Reworkable underfill is not only the key material to address the non-reworkability of the FCOB packages, but it can also be used to enhance the board-level reliability of BGA, CSP devices without losing their good reworkability feature. The objectives of this study are to determine the process viability, material performance, and reliability of reworkable underfills for advanced area array packaging and assembly technologies. Several reworkable underfills along with standard non-reworkable underfills (baseline materials) are evaluated in the study. Rework processes for BGA, CSP and flip chip devices are developed. Reworkable underfills allow flip chip, BGA, and CSP packages to be reworked using developed rework processes, while standard underfills show no reworkability. Reliability assessment of these reworkable underfills is ongoing.

Xerographic printing of textiles : Polymeric toners and their performance

Xerographic printing of a number of common fabrics was investigated. The role of the polymeric binder used for the formulation of the commercially available and custom-made toners was investigated. Fabric performance tests (crockfastness), friction tests, and morphological investigations using scanning electron microscopy were performed. The intricate relations of toner and fabric properties with the results of an important overall industrial performance test for fabrics (crockfastness) are discussed. Both cohesive and adhesive toner failure can be important. Improved toner performance was achieved with a thermoset polymer as the toner binder. However, curing times for the thermoset polymer used are not sufficiently short for high-speed industrial printing.

Studies of latent catalyst systems for pot-life enhancement of underfills

Underfill encapsulant is the material used in flip-chip devices that fills the gap between the IC chip and the organic board, and encapsulates the solder interconnects. This underfill material can dramatically enhance the reliability of flip-chip devices as compared to nonunderfilled devices. Current underfill encapsulants generally consist of epoxy resin, anhydride hardener, catalyst, silica filler, and other additives to enhance the adhesion, flow, etc. Underfill properties that are mainly determined by the catalyst include pot-life, cure speed, and cure temperature. Long pot-life and fast cure at relatively low temperature (/spl sim/150/spl deg/C) are desired, such that it requires the catalyst to be latent at room temperature, but able to catalyze the epoxy curing efficiently at desired temperature. Currently, the pot-life of commercial underfills is normally less than 1 day. The underfills must be put in the freezer at -40/spl deg/C for storage and in dry ice for shipping. The goal of this work was to test various catalyst systems that have the potential to enhance the pot-life of the underfill without adversely affecting its curing. The viscosities of the underfill with various catalysts were measured periodically by using a stress-controlled rheometer in order to establish their viscosity-time relationships. The curing of the underfills was studied using DSC. The pot-life and curing data of the underfill mixed with each of these catalysts are presented in this paper.

Study of a controlled thermally degradable epoxy resin system for electronic packaging

In flip-chip technology, the reworkable underfill material development has been one of the keys to the recovery of highly integrated and expensive board assembly designs by replacing defected chips. This paper reports the synthesis, formulation and characterizations of two new diepoxides, one contains secondary and the other contains tertiary ester linkages that are thermally degradable below 300/spl deg/C. The secondary and the tertiary ester diepoxides were synthesized in three and two steps, respectively. Both compounds were characterized with NMR and FT-IR spectroscopies, and formulated into underfill materials with an anhydride as hardener and an imidazole as catalyst. A dual-epoxy system was also formulated containing the tertiary ester diepoxide and a conventional aliphatic diepoxide, ERL-4221E, with the same hardener and catalyst. The curing kinetics of the formulas was studied with differential scanning calorimetry (DSC). Thermal properties of cured samples were characterized with DSC, thermogravimetric analysis (TGA) and Thermomechanical analysis (TMA). The dual-epoxy system showed a viscosity of 18.7, and 0.87 Poise at 25/spl deg/C and 100/spl deg/C, respectively. The cured secondary, tertiary and dual-epoxy formulas showed decomposition temperatures around 265/spl deg/C, 190/spl deg/C and 220/spl deg/C, glass transition temperatures (Tg) around 120-140/spl deg/C, 110-157/spl deg/C and 140-157/spl deg/C, and CTE of 70 ppm//spl deg/C, 72 ppm//spl deg/C and 64 ppm//spl deg/C below their Tg, respectively. The shear strength of the cured dual-epoxy system decreased quickly upon being aged at 230/spl deg/C. The reworkability test showed that the removal from the board of a chip underfilled with this material was quite easy, and the residue on the board could be thoroughly removed with a mechanical brush without obvious damage of the solder mask. In summary, the synthesized tertiary epoxide can be used as a reworkable underfill for flip-chip application.

Development of new reworkable epoxy resins for flip chip underfill applications

Underfill is a polymeric material used in the flip-chip devices that fills the gap between an IC chip and an organic board, and encapsulates the solder interconnects. This underfill material can dramatically enhance the reliability of the flip-chip devices as compared to non-underfilled devices. Current employed underfills are mainly silica filled epoxy-based materials that are not reworkable after curing, an obstacle in Flip-Chip on Board (FCOB) and Multi-chip Module (MCM) technology developments where unknown bad dies is still a concern. Reworkable underfill is the key material to address the non-reworkability of the FCOB packages. We have synthesized a series of thermally degradable epoxies that show good reworkability. However, all these epoxies have degradation temperature higher than 250/spl deg/C. Besides, they are cycloaliphatic-only-based which have shown weakness in toughness and thus may not perform well for very high reliability applications. In the search for high performance epoxy resins for reworkable underfills, our approach is to incorporate aromatic moiety and thermally labile group that decomposes around solder reflow temperature. This paper reports part of our recent approach in the study and development of new epoxy resins that may offer better rework properties. Two new diepoxides containing tertiary ester, tertiary carbonate, and aromatic moieties were synthesized. These epoxy compounds exist as liquids at ambient temperature. The curing properties of these two epoxides and thermal properties of cured resins of these epoxy compounds were characterized with DSC, TGA and TMA.

Evaluation of reworkable underfills for area array packaging encapsulation

Underfill is a polymeric material used in flip-chip devices to fill the gap between the IC chip and the organic board, encapsulating the solder joints. It enhances flip-chip device reliability by distributing thermo-mechanical stresses caused by the coefficient of thermal expansion (CTE) mismatch between chip and board evenly over the whole package. Current underfills are mainly epoxy-based materials that are not reworkable after curing, which is an obstacle to flip-chip technology development. Not only is reworkable underfill a key material to address the nonreworkability of flip-chip packages, it can also be used to enhance the reliability of ball grid array (BGA) and chip scale package (CSP) devices without sacrificing their good reworkability feature. The objective of this study is to evaluate process viability, material performance, and reliability of reworkable underfills for board level encapsulation of BGA, CSP and flip-chip packages. Both commercial and in-house developed reworkable underfills are also included in this study. For comparison, three commercial nonreworkable underfills are also included as baseline materials. It is found that certain reworkable underfills can provide similar thermal shock reliability to the flip chip test vehicle as to the baseline underfills. The effect of the underfills on board-level BGA and CSP reliability is largely dependent on the internal structure of the BGA/CSP components, but in general reworkable underfills improve the reliability of these components. In this paper, details of reliability assessment and failure mode analysis are presented and discussed.