New die attach adhesives that stick

A new generation of materials offers low modulus and low moisture absorption, and reduces warpage, cracking and delamination.

MARK BONNEAU

With the increasing need for high-density devices for use in lightweight, portable electronics, there has been a gradual shift in integrated circuit (IC) sizes and their package configurations. This gradual shift has resulted in a wide range of die sizes being packaged in various package types, such as the small-outline integrated circuit (SOIC), thin small-outline package (TSOP), miniature quad flat pack (MQFP), plastic leaded chip carrier (PLCC) and thin quad flat pack (TQFP). This myriad of packages has consequently resulted in the development and usage of various die attach adhesives. There have been growing needs to develop low-stress die attach adhesives to help reduce chip warpage, chip cracking and adhesive popcorn delamination for larger die; this usually requires a low modulus, low moisture absorption adhesive. This paper discusses the concept of an ideal modulus die attach adhesive and the development of an adhesive to meet the conflicting requirements for bonding small and large die.

Epoxy Die Attach Adhesives

High modulus die attach adhesives are being used to assemble the majority of small- to medium-sized die today. These epoxy-based die attach materials provide high adhesion and excellent wire bondability for small- to medium-sized die. When larger die are employed on copper-based lead frames, higher bondline stress resulting from the severe coefficient of thermal expansion (CTE) mismatch between silicon die and copper lead frame leads to a higher chip warpage. This can lead to delamination after wire bonding or post mold bake. In some cases, delamination is not observed until a joint electronic devices engineering council (JEDEC) reflow conditioning is performed. Delamination becomes extremely critical when electrical conductivity or heat dissipation is required.

To resolve this problem of warpage-related delamination with large dies, low-stress die attach materials are used. These epoxy die attach materials usually have a very low modulus to help absorb the stress and, consequently, a very low modulus at wire bonding temperatures (220°C to 250°C). This results in wire bonding problems when small die are used. The thermosonic bonding process does not function properly because the die tends to “float” and the ultrasonic energy is absorbed by the low modulus die attach. This results in “bond no-stick” or low ball shear values.

Higher chip warpage or package stress can be a negative factor during JEDEC conditioning reflow testing. Package stress alone is not the only factor in determining popcorn resistance during reflow. Many have addressed this complex equation, including the factors of chip warpage, moisture absorption and de-absorption, hot/wet die shear strength, and failure mode of the die attach.1,2,3 The package configurations, such as lead frame design, lead frame plating, type of mold compound and thickness, are some of the most dominant factors in JEDEC reflow popcorn performance.

A new generation of die attach materials has been developed that addresses the need for bonding large die reliably without sacrificing the wirebondability on small die. Properties of these adhesives include low chip warpage on large die, low moisture absorption, improved hot/wet die shear strength and sufficiently high modulus that allows wirebondability on small die.

Next Generation Adhesive Background

New epoxy die attach adhesives are available that give low chip warpage with large die, in addition to facilitating the wire-bonding of smaller die, and they have been designed to pass JEDEC Level II popcorn testing during solder reflow. The chemistry of these systems is based on a combination of epoxy resins and hardeners, some of which have been custom-manufactured. These raw materials typically contain flexible hydrophobic backbones or pendant groups. These chemistries facilitate the reduction of moisture absorption in the final cured polymer. The backbone structure and functionality also determine the physical properties, such as modulus and glass transition temperature. The catalysts allow for fast oven curing profiles (15 minutes at 175°C in a pre-heated oven) depending upon oven load and size. In addition, a snap curable version has been formulated. The additive chemistries and adhesion promoters are also critical to the end performance of the material.

Modulus and Wirebondability

These new-generation adhesives have been formulated to yield low modulus and low glass transition temperatures. To avoid wirebonding problems that are observed with low-modulus adhesives, the modulus of the adhesives at wirebonding temperatures (220°C to 250°C) was raised. Small die (2.5 mm2) were observed to be wirebondable when adhesives with a modulus greater than 44 MPa between 200°C and 250°C are used. With the value of 88 MPa at 250°C, the dynamic tensile modulus is approximately eight times that of a typical low modulus adhesive. The modulus comparison of a standard die attach adhesive used for small to medium sized die, a typical low-modulus adhesive and the epoxy die attach adhesive is shown in Figure 1; glass transition temperatures are shown in Table 1.

The data indicate that the epoxy die attach adhesive sufficiently retains its modulus at elevated temperatures for small die wirebonding; ball shear strength was within 80 percent of the standard die attach adhesive.

Package Reliability Testing

Package reliability in JEDEC reflow moisture tests can be correlated to a number of die attach adhesive properties. Some of the key factors of an adhesive that affect JEDEC popcorn performance are moisture absorption, die warpage and hot/wet die shear adhesion.

Moisture Absorption

Moisture absorption is inherent in polymeric adhesives. The backbone of one resin used in the epoxy die attach adhesive has been modified to improve the resin`s resistance to moisture. Percent weight gain after 85°C/85%RH/48 hour exposure curves for standard die attach, low-modulus, and epoxy die attach adhesives are shown in Table 3. The epoxy adhesive showed a significantly lower moisture pick-up of 0.35 percent, as compared to 0.6 percent and 0.75 percent for the standard die attach adhesive and the low modulus adhesive, respectively.

Die Warpage

The structure of the resins used in the epoxy adhesive give flexibility, which in turn results in lower die warpage. Die warpage was tested using 12.7 x 12.7 x 0.38 mm (500 x 500 x 15 mil) die on 0.2 mm (8 mil) thick bare Cu lead frame (Figure 2). At 19 µm, the die warpage of an epoxy adhesive is approximately the same as that of a low modulus adhesive.

Hot/Wet Die Shear Strength and Failure Mode

Typically, die shear strength is sacrificed with low modulus die attach adhesives. However, an epoxy adhesive can improve hot/wet die shear strength. Hot/wet die shear strength was tested by assembling 12.7 x 12.7 mm (500 x 500 mil) die onto Ag/Cu lead frames and exposing the parts to 85°C/85% until moisture saturation was complete. The specimens were typically exposed for 48 to 72 hours. The exposed parts were then die sheared at 250°C before they could dry out (Figure 3). The hot/wet die shear strength of the epoxy adhesive was five times that of the standard adhesive and three times that of the low-modulus adhesive.

In addition to the high hot/wet die shear strength, the failure mode is another important aspect. A die attach adhesive yielding a cohesive failure mode is more desirable, because it makes propagation of the delamination more difficult. The epoxy die attach adhesives showed a cohesive failure mode.

JEDEC Moisture Test Results

Building a TQFP package and exposing it to various levels of JEDEC moisture conditioning tested the effectiveness of these improvements made in the epoxy die attach adhesives. The TQFP packages were built per these specifications:

- 100L 14 x 14 mm

- 9 x 9 mm die, 12 mil. thick

- 10 x 10 mm Ag/Cu pad

- Sumitomo 7320 mold compound

These packages were exposed to levels II and III moisture conditioning: 85°C/ 60%RH/168 hours and 30°C/60%RH/ 192 hours, respectively. The epoxy adhesive parts were exposed to VPS or IR reflow, then acoustic scans were performed to determine if delamination occurred during the reflow simulation. No delamination was observed at JEDEC level II (Figure 4).

Conclusion

The evolution of conventional lead frame packaging has resulted in a broad range of package types. Consequently, various adhesives with different properties are being used to meet the performance requirements. An optimum epoxy modulus die attach adhesive, with its hydrophobic, good adhesion and stress-absorbing properties and its ability to handle elevated temperatures, is suitable for wire bonding of small die. AP

References:

1. G.S. Ganesan and H. Berg. “Model and Analysis for Reflow Cracking Phenomenon in SMT Plastic Packages,” Proceedings of 42nd Electronic Components and Technology Conference, 1993, pp. 653-660.

2. K. Sawada, T.Nakasawa, N. Nawamura and T. Sudo, “Package Deformation and Cracking Mechanism Due to Reflow Soldering,” 1993 Proceedings of the Japan International Electronic Manufacturing Technical Symposium, pp. 295-298.

3. M. Konarski, “The Breaking Point,” Advanced Packaging, Feb. 1998, pp. 36-39.

MARK BONNEAU, research associate, can be contacted at Ablestik, 20021 Susana Road, Rancho Dominguez, CA 90221; 310-761-4833; Fax: 310-764-2545; E-mail: mark.bonneau@nstarch.com.

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Figure 1. Tensile modulus curve comparison.

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Figure 4. Acoustic scan after JEDEC Level II IR relow at 225°C.

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