Repairability of underfill encapsulated flip-chip packages

IEEE Trans. Electron. Packag. Manufact.

2001

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 C, called underfill 2. Comparing to underfill 0, these two underfills have similar properties including glass transition temperature (), coefficient of thermal expansion (CTE) and modulus. Underfill 1 has similar curing and fluxing capability as to 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. Index Terms-Compress-flow underfill, flip chip, flux underfill, no-flow underfill, reworkable underfill.

Section: B Reworkable Underfillmentioning

confidence: 99%

Section: B Reworkable Underfillmentioning

confidence: 99%

Reworkable no-flow underfills for flip chip applications

IEEE Trans. Electron. Packag. Manufact.

2001

“…However, the use of chemicals makes localized repair difficult. Furthermore, it is time consuming for the chemicals to penetrate into the underfilled flip‐chip package and dissolve the underfills between the chip and the substrate 7. On the other hand, thermally reworkable materials offer the possibility of a fast, clean, and localized rework process.…”

Section: Introductionmentioning

confidence: 99%

Study of additive‐epoxy interaction of thermally reworkable underfills

J of Applied Polymer Sci

2001

ABSTRACT:Underfill 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.

“…But the use of chemicals makes localized repair difficult. Furthermore, it could be time consuming for the chemicals to penetrate into and dissolve the underfills between the chip and the substrate [8]. On the other hand, thermally reworkable materials offer the possibility of a fast, clean, and localized rework process.…”

Section: Introductionmentioning

confidence: 99%

“…Gutierrez and Yu [9] reported development of a reworkable flip-chip type of circuit module using a nonstick release coating (parylene) on all surfaces intermediate of the chip and the substrate. Although the use of release coating enables the chips to be removed from the underfill, the exposed underfill as well as parylene require expensive cleaning processes by depotting solvents [8].…”

Section: Introductionmentioning

confidence: 99%

Novel thermally reworkable underfill encapsulants for flip-chip applications

IEEE Trans. Adv. Packag.

1999

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/ 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 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 epoxybased materials that are normally not reworkable after curing. This places an obstacle in 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.