Test and Inspection Overview

After processing is complete several quality control standards are used to ensure yield and uniformity:

1) Yield (100% inspection for defects and bump presence)
2) Bump height
3) Bump uniformity
4) Bump Shear (Adhesion)

These techniques all come at a cost, which escalates based on complexity and percentage required. The compromise scenario is to perform a 100% visual inspection to confirm bump presence (yield) and a statistically significant sampling of the bump height, solder shear, and bump uniformity to confirm process reproducibility. The responsibility of a more complete testing would fall back on the originating wafer fab to perform (or out source), i.e. wafer probing.

100% Visual Inspection (Yield)

All wafers which are bumped are inspected for yield and defects. An automated inspection system is used to scan and record all defects on the wafers. The graphic shown below is an example of three defects found by the inspection system: 1) solder bridged bump, 2) one missing solder bump, and 3) an over sized bump.

Each wafer is then ink marked or a wafer map is generated to designate the die position and type of defect found.

Flip Chip Design One (FC1):

Defects = 3 / 79,424
Bump Yield = 99.992%
Die Yield = 99.829
Cumulative die map for 8 wafers

Solder Bridging = 3
Missing bumps = 0
Other = 0

Flip Chip Design Two(FC2):

Defects = 0 / 89,712
Bump Yield = 100%
Die Yield = 100%
Cumulative die map for 4 wafers

Solder Bridging = 0
Missing bumps = 0
Other = 0

The general requirements for solder bump size are such that it they provide sufficient solder height in order to produce adequate chip stand-off, enabling proper underfill and increased reliability. In addition, sufficient bump-to-bump uniformity is required in order to provide a consistent image to the flip chip placement machine image recognition system, to ensure all bumps are adequately fluxed, and to ensure sufficient proximity to substrate traces such that sound solder joints are formed during assembly reflow.

Typical testing protocol would be to test 1 wafer/lot, 5 die/wafer, and 20 bumps/die. Even thought this process is automated, it still requires a significant amount of time to complete with the accuracy desired.

Two examples of  bump height and uniformity measurements  (FC1 and FC2) are shown below.

Solder bump height and uniformity using Cyber-Optic Tool

FC1	FC2
Average Bump Height = 109.8um	Average Bump Height = 138.3um
Standard Deviation = 3.4um	Standard Deviation = 2.9um

Ni Bump Shear

All tests performed on a Dage 2400PC shear tester.
Shear rate = 10 microns/sec (nickel) 25 microns/sec (solder)
Stand-off = 5 microns (nickel) 30 microns (solder)

FC1	FC2
FC1 ( 3.6 mil sq. pads in street) Nickel Thickness (20 um) Pad Area = 12.8 sq. mil Ave. Shear Force (g) 192 (15g/sq. mil) Std Deviation (g) 23	FC2 ( 4 mil rnd bond pads) Nickel Thickness (20 um) Pad Area = 12.57 sq. mil Ave. Shear Force (g) 201 (16g/sq. mil) Std Deviation (g) 18

Solder Bump Shear

FC1 (Octagonal pads) Solder Height  (~110 um) Pad Area = 37.75 sq. mil Ave. Shear Force (g) 113    (3g/sq. mil) Std Deviation (g)        5.6 Failure Mode              Bulk Solder	FC2  (Round pads) Solder Height  (~140 um) Pad Area = 12.57 sq. mil Ave. Shear Force (g)   60  (4.8 g/sq. mil) Std Deviation (g)          5.5 Failure Mode                Bulk Solder

Failure Analysis

	Poor Reflow		Spotting of Zincation
	Undersized Bumps		Rough Sidewalls on Stencil
	Solder Satellites		Missing Bumps
	Poor Paste Print		Poor Paste Shape
	Distorted PI Passivation & Probe Marks Outside of Passivation		Mottled Aluminum
	Solder Bridging		Passivation Defects

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