ESD Susceptibility of Precision-film Resistors

Estimated reading time: 5 minutes

The electronics industry has some misconceptions regarding the electrostatic discharge (ESD) susceptibility of certain reeled resistors thought to be invulnerable to damage. This article reports that certain precision resistors can succumb to ESD damage - some with very moderate ESD zaps.

By Jim Colnar, Kenneth Ladd, and Roger Peirce

The electronics industry has some general misconceptions regarding the electrostatic discharge (ESD) susceptibility of certain reeled resistors commonly assumed invulnerable to ESD damage. Vendors currently deliver some varieties of precision-film resistors (specifically axial-lead components) to manufacturers on paper and cardboard reels with no ESD-protective packaging supplied. Consequently, it is common for electronic manufacturers to assume that ESD damage to these parts is impossible, and not use in-house controls.

This article documents findings that certain precision resistors could be damaged by even very moderate ESD zaps. The studies routinely showed evidence of total failures. Worse yet, shifts (not considered total failures) were observed in resistor values with some low human body model (HBM) ESD zaps less than 4 kV, perhaps even less than 2 kV. Many observed value shifts fell outside allowable tolerances, which can become insidious failure modes in final-product assembly. A range of slight to very substantial shifts were observed along with increasing ESD stress levels.

Charged device model (CDM) stress tests were also performed to evaluate potential risks created from the possible charging of reels. However, on devices tested, no damage from that potential risk was observed.

Four different thin-film resistor parts were used in this study:

Resistor 1 - 16.7 kΩ;Resistor 2 - 65.7 kΩ;Resistor 3 - fixed-axial precision resistor of 200.0 Ω; Resistor 4 - fixed-film precision resistor of 706.0 Ω.

Fifty samples of each part were collected for tests. All four parts had identical tolerance specifications of 0.10%. Baseline resistance measurements taken using a micro-ohmmeter and a multimeter. The samples were divided into two groups. Samples 1-25 were earmarked for HBM ESD stress testing, while samples 26-50 were earmarked for CDM ESD stress testing. Once baseline values of all samples were recorded, the appropriate samples were subjected to ESD Association Standards stress testing. HBM stress tests and CDM tests were performed using two different simulators. Following ESD stressing, samples were measured again for their resistance, to determine if changes in their values had occurred. Voltage intervals for samples 1-50 are shown in Table 1 for HBM and CDM tests.

Table 1ESD Stress LevelsSamples 1-5:HBM2 kVSamples 6-10:HBM4 kVSamples 11-15:HBM10 kVSamples 16-20:HBM16 kVSamples 21-25:HBM25 kVSamples 26-30:CDM2 kVSamples 31-35:CDM4 kVSamples 36-40:CDM10 kVSamples 41-45:CDM16 kVSamples 46-50:CDM25 kV

Tables 2 and 3 show test results, listing sample numbers for each part and the percent change in the value determined after the samples were retested following the stress tests. The data in red shows all subsequent samples that fell out of their required tolerance (>0.10%) after ESD stress testing.

HBM stress testing (Table 2) did result in failures that included total failures and shifts to values outside of their 0.10% allowable tolerances. All failed parts are highlighted in red in Table 2. Some failures were catastrophic; some were only slight shifts outside of allowable tolerances; many were between both extremes. Failures resulted in stresses of only 4 kV in three of the four part-types tested.

CDM stress testing (Table 3) did not result in failures, and the accuracy of the measurement technique and equipment is underscored in the vast number of samples yielding only a 0.00% to 0.02% change, an apparent experimental error. In the four parts selected, charging parts from their reels and packaging (even from nearby fields) and subsequent discharging of parts appeared to not be a risk.

It is also interesting to note that some samples shown in Table 2 received value shifts very near their 0.10% allowable tolerance (for example, Part 2/Sample 4 had a 0.08% shift), with only 2-kV ESD zaps. It would appear reasonable by the laws of probability that some samples, on occasion, could be forced past their 0.10% limit with even 2-kV stresses or lower.

Conclusion

The studies suggest that electronic manufacturers should take a closer look at the susceptibility of reeled resistors to determine if similar risks exist. This potential failure mode may be responsible for some yield loss and final test failures that can be difficult to identify. Care should be taken not to assume that all reeled resistors are invulnerable to ESD damage.

Jim Colnar, principal engineer and ESD program coordinator, General Dynamics AIS, may be contacted via e-mail: james.colnar@gd-ais.com. Kenneth Ladd, engineering technician, General Dynamics AIS, may be contacted via e-mail: kenneth.ladd@gd-ais.com. Roger Peirce, director of technical services, Simco Ionization for Electronic Manufacture, may be contacted via e-mail: rpeirce@esimco.com.

Humidity Indicator Card Standard for Moisture Protection

By Michelle Martin

The need for moisture protection for surface mount devices (SMDs) has been well publicized. When components are subjected to elevated temperatures during reflow soldering, moisture trapped inside plastic SMDs produces enough vapor pressure to damage or destroy the device.1 Higher liquidus temperatures needed for lead-free solders cause blistering or delamination due to rapid moisture egress. The key to eliminating these damaging effects is moisture control.2

The Joint Industry Standard, IPC/JEDEC J-STD-033B, for the “Handling, Packing, Shipping, and Use of Moisture/Reflow Sensitive Surface Mount Devices,” provides the requirements for dry-packing moisture-sensitive devices (MSDs), including the use of moisture-barrier bags with desiccant and a tested humidity indicator card (HIC). To standarize the quality and accuracy of color-change HICs, J-STD-033B requires that HICs adhere to a standard, minimum color-change quality level to ensure accuracy and readability between dry and humid package conditions. Compliant HICs must be subjected to the color meter test method, which quantitatively determines the accuracy of HICs (Figure 1).

Figure 1. J-STD-033B requires that HICs indicate humidity levels for MSL Level-2 parts, as well as MSL Levels 2a to 5a. HICs must feature color-change spots indicating 5%, 10%, and 60% relative humidity (RH).

REFERENCES

1 Rowland, Robert, “Moisture-sensitive Components,” SMT Magazine, October 2000.

2 McMullan, John J., “Minimizing Effects of Lead-free SMT Assembly on Connector Housing Resins, SMT Magazine, September 2004.

Michelle Martin, global topic manager, humidity indicators, Süd-Chemie Performance Packaging, may be contacted at (800) 966-1793; e-mail: mmartin@sud-chemieinc.com.