by E. E. Bliley,
Lucent Technologies
When ESD wristband snaps disconnecting
from the coiled ground cords became a persistent problem for one manufacturing
line, a thorough investigation revealed the culprit—the stud snap on the
buckle. More specifically, a direct correlation was discovered between the
breakaway force and the unintentional disconnects for formed snaps vs. machined
snaps.
The solution resulted in an improved
ESD process as well as a reduction in the cost of wristband and coil-cord
products for Lucent Technologies in Columbus, OH. In this 2.2 million square
feet facility, approximately 5,200 employees are engaged in manufacturing
wireless telecommunications equipment deployed throughout the world. An
estimated 3,000 ESD-protected workstations are located in 39 separate
manufacturing areas.
All employees are trained in ESD
control per Lucent corporate requirements. Each employee is recertified at a
maximum of 24 months. Control of ESD is accomplished through wrist straps,
static-dissipative work surfaces, grounded workstations, and proper handling of
static-generating material.
The Problem Unfolds
In April 1999, a high-performance team
on the second shift asked the ESD coordinator to help solve a functionality
problem with wristbands. The team reported that the coiled ground cord was
easily disconnected from the wristband. Sometimes the accidental snap release
was discovered immediately, but more often it was several minutes later.
This problem puzzled the ESD
coordinator who had been in his position for nearly four years. Why had this
problem not surfaced earlier? After a couple of meetings with the team, the ESD
coordinator decided the concern was valid and started an investigation to
determine the magnitude of the problem.
Analysis
Production associates began keeping
records on the frequency of accidental snap releases over a given period of
time. Each of the 16 associates recorded the number of unintentional
disconnects over a 21-day period. During the study period, 241 unintentional
disconnects were experienced by the team members. This data yielded an average
of 0.72 releases per person per day and substantiated the magnitude of the
problem.
Since four employees had zero
disconnects, analysis of their work patterns was conducted to determine if
undue stretching of the cord and/or excessive mobility might be responsible for
the accidental releases. This study showed that employees seated at test sets
had the fewest releases.
A dimensional analysis was conducted
to determine if the stud on the wristband might be the problem. The shank,
minimum diameter, and the head, maximum diameter, were measured and the data
analyzed. In many cases, the analysis noted that the difference between the two
diameters was only a few thousandths of an inch. This finding convinced the ESD
coordinator that a real problem existed and further investigation was
warranted.
The supplier of the ground cords and
wristbands was contacted to discuss the breakaway force specification. The
supplier maintained that its breakaway force specification ranged between 0.5
and 5.0 lb.
This specification was compared to the
ESD Association ESD-S1.1 Standard for Protection of Electrostatic Discharge
Sensitive Items: Personnel Grounding Wrist Straps. It states that “at
least 1 pound but not more than 5 pounds of breakaway force applied to the
ground lead in the normal disconnect direction shall be required to cause
separation.”
The supplier was asked to provide
breakaway force data on a sample of wristbands and ground cords from recent
production.
Figure 1 is a line chart of that data.
Analysis of the data showed that
one-third or 15 of 45 band/cord combinations had a breakaway force of less than
1.0 lb. The average breakaway force was 1.52 lb, and the standard deviation of
the sample was 0.96 lb.
Another supplier suggested that an
alternate wristband be evaluated. The supplier hypothesized that a wristband
with a machined stud would provide the dimensional properties necessary not
only to meet ESD-S1.1, but also to alleviate a large portion of the
unintentional disconnects. The new supplier provided the breakaway force data
on sample bands with machined snaps, which appears in
Figure 2.
Analysis of the data for wristbands
with machined snaps showed an average of 3.22 lb breakaway force and a standard
deviation of 0.30 lb. Only one band/cord combination had a breakaway force of
less than that prescribed in ESD-S1.1.
Further analysis was conducted to
explain the difference in breakaway force data between formed snaps (the snaps
currently in use) and the machined snaps with an average approximately twice as
high.
Formed snaps are fabricated by a
stamping operation of the two pieces that eventually will become the assembled
snap. Machined snaps are exactly what the name implies. They are machined on
automated equipment from stainless steel rod stock. A sample of each snap was
mounted, cross-sectioned, and lapped for analysis.
Figure 3 (right) illustrates a
cross section of the two pieces of a formed snap after assembly onto a
wristband. The riveting operation produced significant distortion of the snap
head and body. This profile is responsible for the lower breakaway force
distribution shown in Figure 1.
Figure 4 (right) is the cross
section of a machined snap. Because of the solid construction and precise
machining, it appears completely different than the formed snap and explains
the corresponding difference in the breakaway force distributions.
The ESD coordinator was convinced that
the machined snaps were superior. He purchased new wristbands for each
second-shift team member and conducted another evaluation. Employees were
allowed to retain their standard ground cords. As before, each person was asked
to record all accidental snap releases for the study period.
Results
During the 28-day evaluation period, a
total of 31 unintentional disconnects was experienced by 15 participants. This
yielded an average of 0.07 releases per person per day and represented an order
of magnitude improvement; however, a complete solution to the problem had not
been found. Further investigation was necessary to determine if the disconnects
were the result of using the standard ground cord with the new wristbands.
Ground cords from the same
manufacturer that supplied the machine-snap wristbands were purchased, and a
third factory test was conducted for a period of 32 days. During this time,
using machine-snap wristbands and ground cords from the same supplier, there
were no accidental releases. The team was convinced that a complete solution to
the problem had been realized. Purchasing requirements were changed to specify
wristbands with machined snaps and the new high-reliability ground cords.
Conclusion
The ESD program manager learned that
employee input is crucial to continuously improve the process. He had nearly
dismissed their claims that unintentional disconnects were a problem. Together,
the team found the problem was much larger than just one department on second
shift.
As improvements were forthcoming, many
employees indicated that they had experienced the same problem. Some employees
said they had tied a loop in the ground cord around the wristband to provide a
strain relief.
Not only was the ESD process improved,
but the wristbands with machined snaps and the high-reliability ground cords
also were less expensive than the formed-snap products previously used.
Recommendation to ESD Coordinators
-
Listen honestly to your team members,
take their complaints seriously, and strive to create an atmosphere where
people feel comfortable bringing information forward.
-
Be willing to reevaluate previously
approved items and seek out best-in-class products.
-
Realize that ESD programs are
continuous improvement processes, not a one-time event that is established and
forgotten.
-
Partner with suppliers and share your
concerns and problems with them. They can help identify the reality of problems
and suggest possible solutions.
Acknowledgements
The author expresses his appreciation
to each team member who participated in this project. He also thanks his
engineering and supplier colleagues for their support and contributions.
About the Author
Gene Bliley manages the ESD program at
the Lucent Technologies facility
in Columbus. He also is a member of Lucent’s Global Leadership Team and
the ESD Task Group and serves as co-chairman of the corporate ESD Control
Subcommittee. Lucent Technologies, 6200 E. Broad St., Columbus, OH 43213,
614-860-5873, e-mail: bliley@lucent.com.
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