Date of Award

9-22-2017

Document Type

Thesis

Publisher

Santa Clara : Santa Clara University, 2017.

Degree Name

Master of Science (MS)

Department

Mechanical Engineering

First Advisor

Panthea Sepehrband

Abstract

Intermetallic Metallic Compounds (IMCs) formation is a common cause for wire bond failures. This research studied the effect of US vibration duration on IMC formation and growth in Copper-Aluminum (Cu-Al) wire bonded samples. Wire bonded samples, using 2.5 mil (63.5 μm) thick Palladium coated Copper wire, is ultrasonically bonded on a 2 cm thick Aluminum (1”x1”) coupon. Segmented bonding technique using 200 gf force and 220 gf force are applied during segment 1 and segment 2 of the bonding respectively. Ultrasonic (US) vibration frequency of 115-117 khz and a bonding temperature of 175°C is used. A pair of 5 samples with bonding duration: 20 milliseconds (ms), 40ms, 60ms, 80ms, 100ms is created. Keeping the temperature constant at 250 °C, a tube furnace is used to annealing one set for 2 hours and the other set for 4 hours.

Backscattered Electrons (BSE) images were used to detect IMC growth. Backscattered images revealed formation of IMC at the Cu-Al bond interface, mostly around the center of and bond periphery. Using BSE images to identify location of IMC, Energy Dispersive Spectroscopy (EDS) linescans were then performed. Only EDS analysis was taken into account for final results assuming it was more accurate than visual inspection of BSE images. EDS linescan analysis for 2 hour heat treated samples showed IMC thickness growing from 0.6 μm to 1 μm as bond duration increased from 20ms to 100ms. Linescan results for 4 hour samples had IMC thickness ranging from 0.8μm to 1.5 μm, and hence showed an increase xiv with bond duration from 20ms to 100 ms.

Using micro indentations, hardness of both Cu ball and Al was measured. Change in hardness for Cu and Al was compared with bond duration and annealing time. Cu hardness decreased from 20ms to 60ms bond time and then increased in value from 80ms to 100ms bond time. When compared to anneal time, overall hardness in Cu increased with increase in annealing time. Overall hardness in Aluminum increased with increasing bond duration but decreased with increase in anneal time, such behavior is related to the concurrent effect of annealing and IMC growth.

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