Epoxy die-attach

Residual stress in the die after assembly remains a concern when using epoxy die attach adhesives, especially with a large die. Various formulation techniques to reduce stress have been reported that include lowering the Tg of the matrix [47,48], formulating the matrix to exhibit two Tg values [49], reducing the modulus of the adhesive by minimizing silver content, and lowering cure temperature [47]. 

Thermally conductive and high-purity dual Tg epoxy die-attach paste Epo-Tek H65-175MP Epoxy Technology 80, 150 55 165... 

Electrically conductive, high-purity, snap-cure epoxy die attach Hysol KO no Loctite 78 57 -... 

Table 2. Typical Epoxy Die-Attach Adhesive Properties.

Epoxy B in Table 3 is cured with a Lewis acid salt, the monoethylamine salt of boron trifluoride (BF3MEA). The first gold- and silver-filled one-component epoxy die attach adhesives, which were sold in the U.S. in the early 1970s, contained this curing agent. When heated to 150 C, BF3MEA liberates BF3 gas, which serves as an acid catalyst to cure the epoxy. 

Table 3. Composition of First-Generation One-Component Epoxy Die Attach...

The ideal die attach material has a short cure time at a relatively low-cure temperature, exhibits low moisture absorption, provides good adhesion to multiple surfaces, contributes low stress to the die and can be processed within a very tight tolerance. Many epoxies are now available meet some of these demands to a high degree, yet each material has limitations. 

Pietrucha BM, Reiss EM. The reliability of epoxy as a die attach in digital and linear integrated circuits. In Proc. 12th Annual Reliability Phys. Symp. Apr. 1974 234-238. Shenfield DM. Microcircuit adhesives tutorial. Hybrid Circuit Technol. Oct. 1987. 

Hannafin JJ, Pemice RF, Estes RH. Die Attach Adhesives A Guide to Material Properties and Applications. Epoxy Technology, Technical Paper GB-48. 

Mooney CT, Bolger J. Die attach in Hi-Rel PDIPS polyimide or low-chloride epoxies. 

Comstock R. Die attach epoxy s role in thin-package delamination. Semicond Int Jul. 

Electrical-stability testing is essential for conductive adhesives used for electrical connections. Electrical conductivity can degrade at elevated temperatures, on aging with or without power, and on exposiue to humidity and temperature. The specific test method used depends on the application. One test used for die-attach adhesives specified in NASA MSFC-SPEC-592 (now inactive) involves a series of gold-plated Kovar tabs attached with conductive epoxy to metal pads on an interconnect substrate. In the test vehicle, a bias of 5 V and cmrent density of 139 3.9 A7cm (900 A/in ) are applied to a series of wire-connected tabs, and the resistance change is measured after exposure to 150 °C periodically up to 1,000 horns. The maximum allowable resistance change is 5%. 

The process steps are depicted in Figure 2.38. They begin with the manufacture of expendable wax patterns, by injecting molten wax into an aluminium or epoxy die to form a pattern that is virtually an exact replica of the desired casting. The wax may contain fillers. For smaller castings, several wax patterns are attached to a wax gating system. Water soluble die release agents are used to facilitate the wax model take-out. 

Pietrucha, B. M., and Reiss, E. M., The Reliability of Epoxy as a Die Attach in Digital and Linear Integrated Circuits, Proc. 12 Annual Reliability Physics Symp., pp. 234—238 (Apr. 1974)... 

Electrically conductive epoxy paste adhesive for low-stress and in-line die attach ME8418-DA AIT echnology 80 40 -... 

Electrically conductive polyimide die-attach adhesive (screen printable) Epo-Tek P-1011 Epoxy Technology 180 37 -... 

Mooney, C. T., and Bolger, J., Die Attach in Hi-Rel PDIPS Polyimide or Low-Chloride Epoxies, Trans. IEEE 9iA)... 

---