BY KRISHNA G. SACHDEV, Ph.D. IBM

Silicone adhesives have a wide array of applications in microelectronic products. Additionally, thermally and electrically conductive silicones are used as adhesives for direct-bonding device chips to substrates, attaching heatsinks or heat spreaders to chips, and bonding chips to leadframes. Microelectronics fabrication processes often require disassembly of assembled components for diagnostic tests, defect repair, replacement of defective semiconductor devices in multichip modules (MCMs), or to reclaim electrically good substrates. Presence of dense metallurgy and a diverse set of materials requires a selective rework process so there is no detriment to substrate integrity, corrosion of metallurgy, or environmental and chemical safety concerns.

Known rework methods use mechanical scrapping and media blast2, chemicals such as tetramethylammonium hydroxide (TMAH) in alcohol3, or solvents such as CH2Cl2 and DMF, which are not allowed in manufacturing. Other methods include the use of M-rich silicone with a catalyst4 and acidic ammonium fluoride (NH4F.HF), or potassium fluoride (KF.HF) with TiO2 and/or acrylic resin powder for removing silicone films from the surface of automobile and aircraft windshields.5 None of these methods are suitable for high-performance electronic modules.

Figure 1. MCM with a ceramic chip carrier or substrate chips attached through flip chip solder joints encapsulated with epoxy.Click here to enlarge image

Typical silicones used in adhesives/sealants comprise a vinyl-functionalized (CH2=CH) poly (dimethylsiloxane) and dihydro-dimethyl siloxane along with a curing catalyst and, optionally, inorganic fillers such as silica. Lack of a known practical method for removing cured silicone bonding materials limits the rework or repair processes that prevent rejection of volumes of multichip modules and other component parts. This adds to overall product cost.

A method exists for removing silicone polymer residues from electronic assembly component surfaces, interfaces, and under-the-chip regions of solder-joined devices to substrate pads in flip chip bonding. Exposure of cured silicone coatings to a dilute solution of tetrabutyammonium fluoride reagent (TBAF) in non-hydroxylic aprotic solvent, such as tetrahydrofuran (THF) or propylene glycol methyl ether acetate (PMA), causes rapid disintegration of the polymer matrix resulting in its removal and dissolution of the polymer in the solvent. A key feature of the chemistry used is its compatibility with various materials including Cu, Cr, Ni, Au, Pb/Sn, Ag/Sn , and other sensitive metallurgies, lead-free solders, polyimide passivation, cured epoxy underfill, ceramic chip carriers, and silicon device chips. In addition to TBAF, other quaternary ammonium fluoride (QAF) reagents that can be used include tetramethylammonium fluoride (TMAF), tetraethylammonium fluoride (TEAF), or tetra-n-octylammonium fluoride (TOAF). The quaternaryammonium fluoride (QAF) can be in the form of a hydrate (QAF. xH2O) or as anhydrous solution in THF.

Mechanism of Cured Silicone Adhesive Removal with TBAF

The rapid reaction of TBAF with silicone adhesive seems to be due to fluoride-ion-assisted depolymerization/chain scission of the silicone matrix to monomers/ oligomers, followed by its dissolution in the solvent. Fluoride ion TBAF in an aprotic solvent, such as THF, has high affinity for Si, which is a key factor in its use as a desilylation reagent in the removal of silyl ether and silyl ester protecting groups in organic synthesis.6

Propylene glycol methyl ether acetate (PMA), bp.145-146°, fp.110°F was considered a more suitable solvent for TBAF than THF, bp 65-67°, fp.1°F, used in organic synthesis. Other high-boiling solvents that could also be used are DI(propylene glycol) methyl ether acetate, bp. 200°, propylene glycol methyl ether propionate, and ethoxy ethyl propionate, bp. 166°. PMA is preferred for its ease of availability in electronic-grade purity and for environmental, health, and safety reasons.

Table 1. Process sequence for silicone adhesive removal with TBAF/PMA rework chemistry.Click here to enlarge image

A solution of tetrabutylammonium fluoride trihydrate (TBAF.3H2O), at as low a concentration as 0.5-1.0% in 99% purity semiconductor-grade PMA, provides rapid removal of silicone polymer from metal surfaces. A representative rework process is summarized in Table 1.

Figure 2. MCM with a ceramic chip carrier with device chips attached through solder joints that are not encapsulated.Click here to enlarge image

Two types of typical multichip module (MCM) assemblies that were exposed to rework solutions are shown in Figures 1 and 2. For a functional test, the reworked substrate and protective cap were re-bonded by applying the silicone adhesive/sealant onto the bonding surfaces at the seal band areas of both parts. The parts were assembled and the adhesive cured at 170-175°C for 55-60 min. Shear-strength measurements showed no significant difference between a reworked and a new module assembly.

The reworked module assembly parts were then examined visually and microscopically, and in selected cases surfaces were analyzed by x-ray photoelectron spectroscopy (XPS) before and after the rework process. Tests were carried out with silicon device chips having Pb/Sn C4 solder ball array, ceramic SCMs, MCMs, epoxy-underfilled DLA modules, terminal via passivation polyimide coatings, plated Cu coupons, and Cr/Cu coating on silicon wafers.

As an example of rework process compatibility, the heat sink of MCM module shown in Figure 1 was sheared off to expose the silicone adhesive on the components, which was subjected to the rework process (Table 1). Examination of various surfaces, and wettability test by water contact angle after cleaning, showed no evidence of hydrophobicity, indicating complete removal of adhesive residue.

Figures 3A-D. SEM Photographs of Pb/Sn C4 array before and after subjecting to rework process.Click here to enlarge image

In the case of non-underfilled MCM modules (Figure 4), the C4 (Pb/Sn) solder interconnections are exposed to all rework process chemicals. A detailed surface-chemistry analysis with XPS and SEM revealed no significant changes after rework on C4s on bare chips or on chips pulled from ceramic carriers by high-temperature reflow.

Figures 3a and b show C4 arrays on a Si chip after rinse sequence only and after the entire rework process sequence (Table 1). Figures 3b and c show a C4 array on a pulled chip from a substrate after chip-join and flux-clean, and after disassembly of the module. From these pictures of the C4 solder ball array, there is no discernible change in C4-size, shape, or surface morphology.

Summary

Quaternary ammonium fluoride reagent TBAF in PMA is effective in removing cured silicone adhesives from electronic components. This rework process is simple and efficient for all types of silicone adhesives, and compatible with all module materials and contacting surfaces. Additionally, it adapts to manual or automated cleaning modes with no major safety or regulatory issues, and the water-soluble organic residue after filler filtration and solvent recovery enables low-cost waste disposal.

References and Acknowledgements

1. U.S. Patent No. 6,652,665 (Sachdev et al)
2. U.S. Patent No. 3,969,813 (Minetti et al)
3. U.S. Patent No. 4,089,704 (Heiss et al)
4. U.S. Patent No. 5,747,624 (Rubinsztajn et al)
5. U.S. Patent No. 5,772,788 (Ikeda et al)
6. E.J. Corey and B.B. Snider, Am. Soc. 94, 2549 (1972)

The author thanks Ranee Kwong and Chon Lei.

This posthumously published feature was contributed by the author’s husband, HARBANS SACHDEV, who may be contacted at 23 Farview Dr. Hopewell Junction, NY 12533; Tel. 845/ 226-8303; E-mail: [email protected].