SMTA International Conference Proceedings


Author: Fulvio Fontana
Company: CELESTICA Pkg & Tech Dvpmt
Date Published: 9/21/2003   Conference: SMTA International

Abstract: RF application includes different products with some dies attached and wire-bonded on different carrier, organic and ceramic with other SMT components. Usually these products are expensive, so the possibility to test and replace a failing die allows low scrap cost.

Up to now the focus has been on special reworkable adhesives, but the results were not very satisfactory due to poor characteristic of materials not able to meet a satisfactory compromise between reworkability (thermoplastic-high CTE, thermoset-lowTG) reliability (low CTE - high TG) process requirements (low bleeding, low degaussing) and customer requirements (high thermal conductivity). So, Celestica’s approach was quite different.

Starting from standard high thermal conductive adhesives, both electrical conductive and electrical insulant (to eliminate the bleeding problem on small scale applications), some feasibility tests have been done with the purpose to develop a new rework process. A dedicated test vehicle has been developed by Celestica based on single layer low TG FR4 Cu-Ni-flash gold carrier on chip sites and bonding pad sites. To stress processes chips assembled are larger (15x15mm) and gold wires are thin (0.18 microns). A standard ball bonding process has been used to bond wires.

First rework step is removing wires without damage to carrier or die. For this purpose a high pressure DI water spray equipment has been modified. To remove die and adhesive, three different types of process has been investigated; testing thermal and mechanical removal of chip, thermal Reactive Ion Etching and Plasma cleaning to remove adhesive residual and to reduce surface oxidation. Viscosimeter, DTMA , DSC to characterize materials SEM and XPS-ESCA have been used to analyze carrier surface after adhesive removal.

Key words: die attach, rework, adhesives, RIE.

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