Process ControlsProcess controls are needed to assure a reliable hybrid module manufacturing production line. At GED with we do both wire pull testing and sample die shear testing in accordance with Mil-Std-883 and follow along with the process control guidelines as specified in MIL-PRF-38534
WIRE PULL TESTING
Wire bond strength and the reliability of wire-bonded interconnects can be evaluated using either a destructive pull test or a non-destructive pull test. The purpose of these tests is to measure bond strengths, evaluate bond strength distributions, or determine compliance with specified bond strength requirements of the applicable specifications. These tests may be applied to the wire-to-die bond, wire-to-substrate bond, or the wire-to-package lead bond inside the package of wire-connected microelectronic devices bonded thermocompression, thermosonic, ultrasonic, or any related techniques.
The two standards MIL-STD-883 pull test methods are:
Test Method 2023 NONDESTRUCTIVE BOND PULL
Test Method 2011 BOND STRENGTH (DESTRUCTIVE BOND PULL TEST)
These standards describe the procedure and equipment for each type of test and the accept/reject criteria for different wire diameter and material (gold or aluminum) type. For the destructive bond pull test a small wire is placed under the wire bond and a force is slowly applied until the wire breaks as shown in Figure 1. For non-destruct pull test a force equal to the minimum level specified in MIL-STD-883, Test Method 2023 is applied.
Figure 1 Wire hook used in Pull Test Methods
The actual force applied is a complicated function of the wire bond geometry as shown in Figure 2 below.
Figure 2 Wire Force versus Wire Bond Geometry
The acceptance criterion is based on the strength and wire break location during destructive bond pull tests. There are eight wire modes and locations as show in Table I per MIL-STD-883, Method 2011. The minimum destruct pull test value for a given wire size are shown in Table II.
There are additional methods for evaluating wire bonds, which include visual inspection, ball shear strength, accelerated temperature and temperature cycling tests.
The sample size and process controls are defined in MIL-PRF-38534 and the simplified flow chart is shown in Figure 3.
Figure 3 Process Control Flow Chart for Wire Bonding
Table I MIL-STD-883, Test Method 2011 BOND STRENGTH
(DESTRUCTIVE BOND PULL TEST) Criteria
DIE SHEAR TEST
Table II MIL-STD-883, Test Method 2011 BOND STRENGTH
DESTRUCTIVE BOND PULL TEST) Pull Strengths.
MIL-STD-883 TEST METHOD 2019 DIE SHEAR STRENGTH
The purpose of this test is to determine the integrity of materials and procedures used to attach semiconductor die or surface mounted passive elements to package headers or other substrates. This determination is based on a measure of force applied to the die, the type of failure resulting from this application of force (if failure occurs) and the visual appearance of the residual die attach media and substrate/header metallization.
The die shear tester consists of a load-applying instrument with a linear motion force and a die contact tool, which applies a uniform distribution of the force to an edge of the die. (See Figure 4)
Table III shows the die shear requirement as a function of the die area.
Failures from die shear testing include: 1) failure to meet the specified die shear strength requirements; 2) a separation that occurs at less than 1.25X the minimum die shear strength and evidence of less than 50% adhesion of the die attach material; and 3) a separation that occurs at less than 2X the minimum die shear strength and evidence of less than 10% adhesion of the die attach material.
The mode of separation must also be classified into and recorded as any of the following: 1) shearing of the die itself with silicon remaining; 2) separation of the die from the die attach material; and 3) separation of both the die and die attach material from the package substrate.
Figure 4 Die Shear tester
Figure 5 Die Shear requirement versus Die area