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Reflow of Solder Bumps on Substrate by Thermal Conduction Through a Copper Pattern

IP.com Disclosure Number: IPCOM000103402D
Publication Date: 2005-Mar-17
Document File: 3 page(s) / 240K

Publishing Venue

The IP.com Prior Art Database

Abstract

Disclosed is a method for reflowing solder-bumps on a substrate by moving heat from a hotplate through copper patterns to prevent delamination of the Inter Layer Dielectric (ILD) materials on the die.

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Reflow of Solder Bumps on Substrate by Thermal Conduction Through a Copper Pattern

Disclosed is a method for reflowing solder-bumps on a substrate by moving heat from a hotplate through copper patterns to prevent delamination of the Inter Layer Dielectric (ILD) materials on the die.

Background

Currently, most substrates used for FCXGA packages have a higher coefficient of thermal expansion (CTE), compared with the die (e.g. 15.0ppm/K of substrate against 3.5ppm/K of die at room-temperature). This means that the substrate expands more than the die during the chip attach process used to melt solder-bumps in the reflow oven. Due to the mismatching of CTEs, bump-pitches on substrates are designed to be slightly narrower than the die’s at room temperature, so that they match at the peak temperature of reflow (e.g. 260oC for lead-free bumps) See Figure 1a and 1b. 

After the peak temperature, as the substrate and die cool, stresses build at the bump connections because they are returning to their normal sizes at different rates. This stress leads to delamination of the ILD layers on the die, made of very brittle material (see Figure 1c and 1d).

Low-k (dielectric constant) ILD materials are used as the clock frequency of the CPU grows, but generally ILD materials with lower dielectric constants are more brittle. Furthermore, stresses on the ILD material increases when converting solder-bumps to lead-free Alternate Bumping Metallurgy (ABM) of copper on the die.

Thermal expansion of the substrate is dominated by a core material made of epoxy, since it amounts to ~75% of the total thickness (e.g. about 8...