Browse Prior Art Database

Metal Interconnect with Self-Limiting Flow

IP.com Disclosure Number: IPCOM000004826D
Original Publication Date: 2001-Jun-19
Included in the Prior Art Database: 2001-Jun-19
Document File: 3 page(s) / 29K

Publishing Venue

Motorola

Related People

Daniel R. Gamota: AUTHOR

Abstract

Metal Interconnect with Self-Limiting Flow

This text was extracted from a WORD97 document.
This is the abbreviated version, containing approximately 96% of the total text.

Metal Interconnect with Self-Limiting Flow

Daniel R. Gamota

BACKGROUND

Interconnection of components to bond pads on printed wiring boards (PWB) in standard surface mount assembly is typically accomplished using a lead free or lead based metallurgical solder alloy. Above the liquidus temperature, the solder melts and flows to wet and engulf any high-energy surfaces, such as interconnect pads on components and Ni/Au or OSP finished traces on PWB. Solder wetting and flow can continue until stopped by a low energy surface interface that serves as an impediment to flow. Solder mask is used on PWBs to confine the flow of solder during the reflow process. The mask is usually applied in a subtractive process after completion of the copper circuit pattern fabrication. During the PWB processing careful alignment of the solder mask openings with PWB components is required. Poor alignment may result in an unmasked trace that will allow the free flow of solder along its length (un-masked trace wet-out) during reflow, depleting the solder volume at the component interconnection. This can lead to reduced reliability performance of the interconnection.

PROBLEM

Recent development of components requiring fine pitch solder bumps for interconnection to PWBs is driving assembly machine alignment and PWB etching technologies to its limits, and subsequently increasing costs due to decreasing PWB yields. In addition, adhesion of the solder mask to the PWB core, essential to contain the flow of the solder during reflow, is becoming a concern as the solder mask features decrease. Moreover, high accuracy solder mask alignment in high volume PWB manufacturing is a major impediment to implementation of fine pitch, solder alloy interconnections (e.g. direct chip attach, chip scale packages, micro ball grid array) in surface mount technology (SMT) assembly lines.

SOLUTION

The invention is a combination of interconnect design and metal alloy composition that makes the flow of the molten metal self-limiting, eliminating the need for a solder mask/resist. One metal or metal alloy (Figure1: Alloy A), upon reaching its liquidus temperature, would flow and interconnect the components to the

Figure 1. Assembly FCOB assembly prior to reflow.

Figure 2. FCOB assembly after reflow.

PWB, at the same time encountering and alloying with a second metal or metal alloy (Figure 1: Alloy B). The composition of the two metals or metal alloys would be designed such that their intermingling would cause rapid alloying, creating a third allow (Figure 2: Alloy C) whose liquidus temperature is above the ambient temperature. This would cause self-solidification of the interconnect, thus limiting wet-out along the PWB trace. The disclosed invention enables the formation of an interconnection of the critical volume of metal required to form a fatigue resistant interconnect. Furthermore, the requirement for...