Advanced Technology Symposium Conference Proceedings


Authors: Rita Mohanty, PhD
Company: Ark-Les Electronics
Date Published: 6/11/2002   Conference: Advanced Technology Symposium

Abstract: Tin lead-based solder is the most commonly used joining material in the electronics industry due to its low cost and convenient material properties. In recent years, lead-based joining materials (Sn-Pb solder) have become a pressing issue due to their adverse effects on the environment. As a result, many alternatives are in the works. One of these alternatives is Organic Electrically Conductive Adhesive. Organic adhesives were used as joining materials long before solder appeared on the scene. Organic Conductive Adhesives not only offer an environmentally friendly solution but also offer other attractive technical advantages such as low temperature processing, flexible structure and fine pitch capability. These properties are highly critical for some applications such as flexible substrate application.

Polymeric based flexible substrates are widely used as bendable printed circuits in automotive, appliance, computer peripherals and medical applications. One of the most commonly used polymer substrates is polyester (PET, Mylar) with an upper temperature limit of 1650C. This is clearly an unsuitable candidate for Sn-Pb solder with a reflow temperature of 210-2200C. This paper will focus on evaluating two distinct methods of applying CA (Conductive Adhesive) for SMD (Surface Mount Device) attachment as an alternative to solder.

Stencil/Screen printing is, by far, the most popular technology to deposit conductive adhesive on PTF (Polymer Thick Film) circuits for SMD attachment. This process has been adopted from solder paste applications. Among the many process issues, screen life and costs associated with material waste are some of the major concerns. Silver-filled epoxy, the most commonly used conductive adhesive, costs more than one dollar a gram compared to solder which costs pennies a gram. Material left on the screen at the end of its screen life can vary from 10-95% of total use. Clearly, this is money down the drain.

An alternate process is dispensing. Many factors must be considered in such a process change, such as adhesive in use, impact on process cycle time, product performance, and manufacturing yields. Most of the silver-filled epoxy for SMT assembly was originally designed for printing, not dispensing. Formulation modification is necessary to alter rheology for consistent dispensing. It is important to understand the effects of these materials and process change on the total system performance.

This paper will describe the basic difference between leadbase and ICA (Isotropic Conductive Adhesive), along with some of the processing challenges associated with each material. The results of a series of experiments aimed at identifying optimal dispensing and printing parameters for ICA are also reported in this paper. Finally, a novel process for device attachment along with economical impact associated with each process change will be discussed briefly.

Keywords: Conductive Adhesive, Lead free alternative, SMT process, PET substrate.

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