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Case Study : Class 0 Manufacturing
(Class 0: See definition below)

Class 0 Definition | CDM, CBE & Class 0 | EOS Misdiagnosis

EPM Performance Benchmarking™ (EPM = ESD Program Management)
"You do not need to do this: survival is not mandatory!"

Dr. W. Edwards Deming

Background:

This case study took place in China at an MR Tape Head manufacturing site with Class 0 devices (See Class 0 definition below).   ESD failures rates had previously been documented to varying degrees and as high as 22%.  (See Figure 1)  Each data point on the trend lines is confirmed ESD damage and each colored line represents one product's performance. 

An HBM program had been in place for a number of years and concerted efforts to reduce the ESD yield losses had been unsuccessful.   Dangelmayer Associates (DA) was hired to not only to improve yields for current products but also to prepare for the next generation of heads that were expected to have sensitivities as low as 10 volts.   The yields are now over 99.6% as shown in figure 1, including the next generation.

a
Figure 1:  ESD Failure Trends and EPM Performance BenchmarkingT Tracking

Class 0 Definition for Manufacturing:

The term Class 0 has not been defined for manufacturing applications by any industry standard and that there is a growing need for one.  We have found that manufacturing failure rates escalate exponentially for devices with ESD withstand voltages below 250 volts for either HBM or CDM and then again at 125 volts and at 50 volts.   Thus, escalation of manufacturing counter measures becomes essential at each of these break points.   It is also vitally important for the manufacturing process to have well defined trigger points for risk assessments of these ultra-sensitive components. These risk assessments involve verification of manufacturing process capability as well as for any risks that may be passed on to customers.  In some instances, risk assessments have resulted in the redesign of components, circuit boards and/or assemblies to improve the ESD performance.  Manufacturing process capability tests become increasingly complex the deeper into the Class 0 region the devices are (see Class 0 case study).    Note: MM is intentionally omitted from this definition since it is largely redundant to HBM.

Thus, we define Class 0 for manufacturing as any component that falls into one of the three categories below.   Control methods escalate with each lower category.  

  • Class 0
    • Component Withstand Voltages Less than 250 volts HBM or CDM
       
  • Class 00
    • Component Withstand Voltages Less than 125 volts HBM or CDM
       
  • Class 000
    • Component Withstand Voltages Less than 50 volts HBM or CDM
       

Baseline Technical Assessment

The first step to resolving the yield losses was to conduct a baseline technical assessment.   A detailed analysis of each operation revealed numerous CDM compliance issues that ultimately proved to be the root cause of the high yield losses.  Advanced auditing techniques were applied that included a variety of ESD event detectors and high-bandwidth current probes.     More traditional measurement techniques such as voltages were helpful but insufficient.    Event detection and current probe measurements are essential tools for Class 0 applications. 

These advanced techniques enabled the systematic modification of each manufacturing operation to be either ESD discharge event free or to exhibit events no larger than 1/10th that of the current failure threshold of the MR Heads.  

Resolving CDM manufacturing issues requires a two pronged approach. (See Figure 2) Voltages on the product must be reduced with the customary strategies of removing static generators, minimizing triboelectrification of the product, and the strategic application of ionization, rather than global, use of ionization.   Targeted application of ionizers reduces both purchase and maintenance costs. MR Head competitors had installed ionizers at every work station at considerable expense and required maintenance.  

b

Figure 2: Two CDM Mitigation Techniques

The second of the CDM mitigation techniques is controlling surface resistances at the point of contact between the ESDS product and the various manufacturing tools and fixtures.    With the aid of the advanced auditing equipment, each operation was modified to ensure discharge currents were invariably below preset limits.  These modifications consisted largely of the substitution of static dissipative materials and special operating procedures.   This approach also resulted in a reduction of the number of ionizers utilized in the production area.  


Advanced CDM Training Workshop:

A multi-day workshop was conducted to elevate the customers understanding of CDM and the technically advanced measurement techniques.   Ultimately, the customer must   be able to fully understand CDM, both mitigation techniques and the measurement techniques.  ESD practioners often do not understand CDM and there are very few opportunities within the industry to be educated on the subject.    DA has the advantage of over 30 years experience with CDM in AT&T/Lucent manufacturing and Bell Labs R & D since it's development in 1974 by Tom Speakman, a good friend and AT&T colleague.  

EPM Performance BenchmarkingT and ESD Quality Metrics:

Throughout this process DA ESD Quality Metrics were used to track the improvements.( See figures 3; 4; 5; 6; & 7)   EPM Performance BenchmarkingT and meaningful quality metrics are an organizational manufacturing necessity.   This enables management to set measurable goals and objectives and to efficiently monitor progress.  

DA benchmarking is a tool that was developed over twenty five years ago and has been refined every year since.   It is now an accurate means of quantifying the performance of an ESD program and it correlates directly to personnel compliance with ESD procedures.  It has been successfully applied to hundreds of ESD programs and invariably producing robust long term program stability.    Even when conducted over the phone, the EPM Performance BenchmarkingT accurately (90%) reflects compliance to procedure and the corresponding yield improvements.

Each bar in Figure 3 represents the Benchmarking score of a single manufacturing location.  As noted, the MR Head company in this case study improved from a score of 36% to 94% in one year and realized a corresponding yield improvement.  CM (Contract Manufacturing) sites generally score in the lower end of the continuum but there are exceptions.   This data is also represented in Figure 4.   Note that a program can be optimized and, thus, avoid overkill with unnecessary and expensive solutions.

 

c
Figure 3:  Industry EPM Performance BenchmarkingT

d
Figure 4:  Industry EPM Performance BenchmarkingT

e
Figure 5:  EPM Performance MetricsT

A summary of the strengths and weaknesses of the ESD program are reflected in Figure 5 as well as the progress over one year.  These indexes are derived from the EPM Benchmarking survey and are indicators of the relative strengths of various elements of the ESD program.  The indexes shown indicate room for improvement to varying degrees.  For instance, there is room for considerable enhancements to the New Product Introduction process. 

Auditing is one of the more critical elements of program management and has become one of their strengths.  The data derived from sound auditing plans can be highly effective in the early identification and prioritization of process deviations.  This data can also be used to effectively leverage limited resources and time to achieve better Class 0 compliance.

Below are two EPM improvement roadmaps (figure 6 and 7) that highlight an effective tracking strategy for program enhancement.  Figure 6 is the EPM Performance BenchmarkingT and is a reflection of the ESD Quality System improvements.  The blue line is the projected roadmap and the red line is the actual performance that has been validated by DA.   Figure 7 is the closure timeline for the recommendations in the associated Technical Assessment.   

f

Figure 6:  EPM Performance MetricsT

g
Figure 7:  EPM Performance MetricsT

 

Conclusion:

The challenges presented by Class 0 ESD sensitivities are immense but manageable.  It is essential, however, to apply sound CDM mitigation techniques to control voltages on the product as well as with the surface resistance of materials that contact the conductive elements of the ESDS product.   It is essential for CDM and HBM counter measures to be executed flawlessly.   ESD Quality Metrics and ESD event detectors have become a critical element of ESD programs.   If you can not measure a manufacturing process you do not have a process!   With the process outlined here, you can successfully produce Class 0 products with very high yields.