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Method for solder joint microstructure manipulation to increase reliability through particulate additions to solder alloy paste

IP.com Disclosure Number: IPCOM000032217D
Publication Date: 2004-Oct-26
Document File: 5 page(s) / 145K

Publishing Venue

The IP.com Prior Art Database

Abstract

Disclosed is a method for solder joint microstructure manipulation to increase reliability through particulate additions to solder alloy paste. Benefits include improved functionality, improved performance, and improved reliability.

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Method for solder joint microstructure manipulation to increase reliability through particulate additions to solder alloy paste

Disclosed is a method for solder joint microstructure manipulation to increase reliability through particulate additions to solder alloy paste. Benefits include improved functionality, improved performance, and improved reliability.

Background

      Low melting temperature solder alloys must be strengthened through the control of solder-joint microstructure formation during reflow. Temperature-sensitive devices can have typical operating temperatures that range from 50-80°C but require assembly at temperatures less then 125°C. Due to the low melting points and high homologous temperature of these alloys, they can be highly susceptible to creep and fatigue at standard operating temperatures, which severely reduces solder joint reliability. Multiple reflow steps during the assembly process can further accelerate solder joint failure through microstructural coarsening and intermetallic growth.

      Conventionally, the problem is solved by using low temperature solder alloys, such as tin-bismuth (Sn-Bi) and tin-indium (Sn-In) with a ball-grid array (BGA) process. It requires at least two reflow steps, one for ball attach and the second for board attach. Sn-In is susceptible to thermally accelerated solder-joint reliability failure modes, such as creep and fatigue, which occur more rapidly at the device operating temperature range. Sn-Bi cannot be processed at temperatures less then 125°C.

      Alternative solutions are anisotropic conductive films (ACF), sockets or silver (Ag) filled epoxies. Low-temperature solders are the least expensive option and work well with existing high-volume manufacturing (HVM) infrastructure. However, reliability is of great concern. ACFs are expensive and limit the form factor and require significant force to establish interconnection. Sockets are also cost and form-factor limiting. Ag-filled epoxies have poor mechanical properties.

      Conventionally, low-temperature solder alloy paste is printed using an automated stencil-print process. As printed, solder paste takes the form of a rectangular brick (see Figure 1, item1). Flux in the solder paste reacts chemically at rising temperatures to release acids, which reduce metal-oxides that are present (1.2). As the temperature reaches and surpasses the liquidus temperature of the solder alloy, the metal powder particles in the solder paste liquefy, coalesce, form the bump shape, and react chemically with the under-bump metallization layer to create a metallic bond (1.3). During cooling, the solder bump solidifies (1.4).

      The addition of dopants/additives to control the microstructure formation of a material is a well-practiced concept (Zener-Smith relation). The addition of submicron ZrO2 particles to Al2O3 produces a fine grain microstructure through grain-boundary pinning during sintering. The same effect i...