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Posted on 01 November 2019

PIQC – Process Integrated Quality Control

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Multi-dimensional system for bond quality control without production

An increasing number of products are focusing on zero failure targets. This implies consideration of every wire bond connection. Apart from the technical challenges for such quality targets, the increasing costs for more sophisticated and/or additional quality measurements have to be considered.

By Dipl.-Ing. H. Stürmann, Hesse & Knipps

 

Several different methods of monitoring bond quality have been suggested over the past 20 years. The only methods being used in industrial production at present are nondestructive pull test methods, optical inspection methods and monitoring of transducer current and wire deformation.

Deformation measurements however are only partially suitable for quality monitoring or active control systems.

A significant influence is seen in the transducer current and the movement at the tip of the wedge tool. Deviations in the movement of the tip of the wedge tool can be proven when bonding on a surface contaminated with inorganic material.

Non-Destructive Integrated Pull- and Shear Test

Mechanical non-destructive pull tests can be integrated into wire bonders and are available from several machine suppliers. With respect to increasing the certainty of bond quality, these integrated pull tests raise questions about the right choice of force to apply while pulling. Which amount of force is necessary to judge bond quality and how high is the risk of weakening a bond by the act of pulling? It also needs to be considered that the integrated pull test can only detect selected failure modes. The time necessary for this test reduces the throughput, therefore this mechanical test is often only applied under statistical considerations. The loss of throughput can reach 20 to 30% .

In addition to the pull test a bondhead integrated shear test has been available since 2007. This new bondhead (HBK 06) by Hesse & Knipps GmbH leads to a significantly enhanced transparency in considering possible failure modes.

Bondhead HBK06 with integrated pull and shear test

In contrast to pull testing, the shear test measures especially the quality of the intermetallic connection. The original looping geometry remains untouched and the wire is not stressed in any location. The impact on process time falls into the same magnitude as with integrated pull testing.

Both of these integrated test methods can still not detect all of the possible failure modes.

Both methods add, beside the time for testing, some additional uncertainties which are to be eliminated by a new process integrated test method.

Process Integrated Quality Control

The goal of process integrated quality control is to monitor the most applicable and significant measures for judging bond quality.

These measures can include mechanical vibration, transducer current, wire deformation, resonance frequency and the scrub behavior during the welding process. An additional goal is to acquire this data in parallel to the bond process to avoid impact on machine throughput. This will allow a throughput neutral 100% quality control. The resulting quality control system does not rely on any statistical assumptions.

The PIQC Concept

A multidimensional control system for bond quality was designed based on patent EP 1 023 139 or US 6, 308, 881 B1. An additional sensor was added to the transducer in order to provide all relevant feedback data for calculating a bond quality value. This assures real-time feedback of the conditions at the tip of the wedge tool – especially wedge tool movement. The real-time feedback of these sensor signals through an additional interface in the DDS ultrasonic generator is realized by a new developed FPGA based circuitry.

The PIQC Concept

The multidimensional control system for bond quality includes the decision relevant oscillations at the tip of the wedge tool, the transducer current, the resonance frequency, the friction and to complete the assessment also the wire deformation.

All real-time acquired feedback signals are statistically analyzed in the PIQC box based on a newly developed mathematical decision model. The control algorithms are implemented in VHDL. The new quality control system can only be applied by using the new digital ultrasonic generator developed especially for PIQC. The PIQC box allows derivation of extensive quality statements.

PIQC Oscillation Sensor

Quality Statements by PIQC

The signal feedback and processing allows detailed analysis of the welding process and translation into a quasi optimized reference process. The PIQC box calculates a quality index for each bond based on the actual feedback data and the reference data. Raw data and quality index can be displayed graphically at any time.

After the learning phase, the PIQC box can recognize any deviations in real-time. These can be classified and interpreted by the user. It is possible to directly link certain signal deviations to a certain failure mode.

Figure 4 - Part 1

Extract from test results

The user is presented with a maximum amount of information. No other quality control system on the market offers such a potential in the bandwidth of acquired measurement signals as well as in the evaluation of these measurements. The most outstanding evolution lies in the real-time signal feedback of the wedge tip oscillation which is dampened by the friction during bonding.

Selected Results by PIQC

Under laboratory conditions 6000 bonds were analyzed in an experiment on contaminated and non-contaminated bond surfaces. The PIQC box was able to detect completely the pre-defined area with “good” and “bad” bonds.

Figure 5 shows exemplarily that the gained signals allow significant statements by analyzing the deviations. During the first 5 milliseconds of the welding process the signals don’t stay within the 3ó tolerance boundaries and can be easily recognized.

Processed signals of integrated sensor

This shows, that the sensitivity of the integrated sensor allows to generate sufficient differentiation to separate good bonds from critical results especially bonds on contaminated surface.

In contrast to the explained results the deformation signals or the transducer current signals couldn’t be used to identify the bonding quality differences.

Except “good - bad” bonds

The PIQC box is being tested intensively in a field test since January 2007. The production release will occur before the end of this year. The collected data not only shows process variations from changes in the surface conditions, clamping conditions of the substrate and the wedge tool can also be detected.

Critical Assessment

The process integrated quality control system PIQC is a thorough multidimensional control system for bond quality. For the first time all physically relevant measures are evaluated in realtime with an individual quality index for result.

This method allows a new definition of possibilities in quality control especially for companies with high and fast increasing quality demands.

It certainly becomes clear that this PIQC method is introducing a change in paradigm of ultrasonic bonding quality control. For the transition phase the bondhead HBK 06 allows the use of an additional mechanical non-destructive test depending on the detected failure mode. This test will confirm a direct correlation to the detected failure mode. Mid or long-term, the mechanical pull and shear tests will become obsolete.

 

 

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