Wire Pull Analysis

Recently a customer contacted the EMPF Helpline to conduct a wire pull analysis of Au ribbon wire bonds and to measure the associated bond pad thicknesses. The ribbon wire was 1.3 mil x 10 mil in size and was bonded on a ceramic package. The bond pads were thin film gold on copper traces.

Recently a customer contacted the EMPF Helpline to conduct a wire pull analysis of Au ribbon wire bonds and to measure the associated bond pad thicknesses. The ribbon wire was 1.3 mil x 10 mil in size and was bonded on a ceramic package. The bond pads were thin film gold on copper traces.

In today’s world of advanced packaging, ribbon wire bonding is starting to emerge in many applications traditionally dominated
by round wire. Figure 2-1 shows examples of Au ribbon wires. In addition to microwave packages, ribbon wire is becoming more common in power devices, and small signal devices such as wireless device components.

Ribbon wire offers some advantages over round wire. These advantages include lower current densities allowing higher current capability, lower parasitics at high frequencies, and low, stable loop heights. The low loop heights allow packaging engineers to reduce the overall thickness of packages.

Procedure:
A Royce Instruments 580 Universal Bond Tester was used to perform a destructive pull test on the ribbon wires to find their break load. Both the break load values and the failure modes were collected for each pull test. The pull tests were performed at the mid-span of the wire loop. MIL-STD-883 Method 2011.7 (Bond Strength - Destructive Bond Pull Test) Condition D was referenced for the analysis. This standard does not list criteria specifically for ribbon wire, but does include a provision to use the round wire diameter specification which gives an equivalent cross-sectional area as the ribbon wire being tested. A 4.0 mil diameter round wire provides an equivalent cross-sectional area as 1.3 mil x 10.0 mil ribbon wire. As a result, the post seal specifications for a 4.0 mil round wire were used for the analysis (the package under test was unsealed to conduct the evaluation).

To measure the bond pad thicknesses, several random bond pads were first mounted in resin and cross-sectioned to approximately half their pad length. An Amray Model 1830I Scanning Electron Microscope (SEM) was then used to measure the bond pad thickness. Measurements were taken along the entire width of the bond pad and an average bond pad thickness was obtained.

Results/Conclusions:
Ribbon wire pulls were conducted on 25 wires. Per MIL-STD-883 Method 2011.7, the minimum bond pull test post seal specification for the equivalent 4.0 mil Au round wire is 19 grams force. The average pull test of the wires was 75.5 grams force with a minimum pull of 24.6 grams. All bond pull tests exceeded the minimum specification force.

All but one of the ribbon bonds failed at the neck of the first bond. This is the typical failure mode for ribbon wires since the area behind first bond is deformed and weakened during the welding process. This makes the neck area the weakest point of the ribbon wire. One ribbon wire pull resulted in a lifted first bond. However, the pull value was 65.9 grams, well above the minimum allowed pull force. The lifted ribbon wire was not considered a failure. The EMPF conducted a failure analysis of the lifted wire pull, but was not able to determine the root cause. A typical cause of a lifted wire is a poor weld or contamination either on the wire or pad surface.

A sampling of nine random bond pads were measured across three devices to determine if the bond pad’s thicknesses were within the customer’s specification. To obtain a value of a bond pad thickness, an average value was taken of several measurements of one pad. The bond pad includes a copper base layer and a top layer of thin gold. The average thickness of all bond pads measured was 27.3µm with a minimum thickness of 24.9µm and a maximum thickness of 29.2µm. This met the customer thickness specification of 27µm +/-2.5µm.

In addition to wire bond pull and shear testing, the EMPF’s Power Packaging Lab has capabilities for bonding fine round wire (Al and Au wire - 0.7 mil to 2.4 mil diameter), ribbon wire (Al and Au ribbon 0.25 mil x 1.4 mil up to 1.0 x 10.0 mil) and heavy wire (Al wire 10.0 mil up to 30.0 mil diameters). The EMPF also has die attach and flip chip capabilities.