August 2017

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As reviewed previously, an Electrostatic Discharge is a rapid, spontaneous transfer of an electrostatic charge induced by a high electrostatic field through a spark between two bodies at different electrostatic potentials as they approach or are separated from one another.

The ESD Association characterizes three models of discharge, Human Body Model (HBM), Charged Device Model (CDM) and Machine Model (MM). Each model is intended to follow specific discharge properties such as the rise and fall times of the discharge current waveform.

Today, we will discuss HBM and CDM.

Human Body Model (HBM) simulates a person becoming charged and discharging from a bare finger to ground through the circuit under test. Humans are considered a primary source of ESD and HBM can be used to describe an ESD event due to the combination of the capacitance of a human body and resistance of skin touching a sensitive component. Typically, you need to pay better attention to personnel grounding to eliminate HBM.

Per ESD Handbook ESD TR20.20-2016 section 3.4.1 Human Body Model (HBM)

HBM has been in use for over 100 years. It was first defined to allow measurement and evaluation of explosion hazards for underground mining operations. There are a few different test standards describing the HBM for military and commercial applications, but the differences are in the application of the test, calibration of the system, and other ancillary items. The waveform, as defined by the human body resistance and capacitance, is virtually identical among all the test standards. The most widely used standard is ANSI/ESDA/JEDEC JS-001. The HBM test standard uses a stressing circuit which charges a 100 pF capacitor to a known voltage and discharges through a 1500-ohm resistor as shown in Figure 3. The simulators are verified by measuring various features of the current waveform, some of which are shown in Figure 4. Full details for tester qualification and waveform verification are described in ANSI/ESDA/JEDEC JS-001.

Charged Device Model (CDM) simulates an integrated circuit becoming charged and discharging to a grounded metal surface. CDM can be used to describe an ESD event due to an integrated circuit that is suspended on a vacuum pick and then placed on a metal surface during assembly.

Manual operation and handling is much less likely these days as operations have become more automated. CDM is the most pragmatic discharge model in automated production today. Anytime a sensitive device is lifted from a tray and transported it is most likely generating a charge.

Per ESD Handbook ESD TR20.20-2016 section 3.4.2 Charged Device Model (CDM)

In principle, there are two variations of CDM. The first considers the situation of a device that is charged (through tribocharging) on its package, lead frame, or other conductive paths followed by a rapid discharge to ground through one pin or connector. The second considers the situation of a device which is placed in an electric field due to the presence of a charged object near the device. The device’s electrostatic potential is increased by this field. This process is sometimes referred to as static induction. The device will discharge if it is grounded while still in the electric field. In both cases, the device will discharge, the failure mode will be the same, and the failure type and location will be the same. The most widely used CDM standards use the static induction approach. In CDM simulators, the device is grounded by a pogo pin contacting one pin or lead of the device. The current through the pogo pin can be measured and recorded which is particularly important as the discharge current determines the ESD threshold, a schematic of this is shown in Figure 5.

Experimental results show that the CDM discharge current is very fast, with rise-times measured often below 100 ps with a “pulse width” (full width half-maximum [FWHM]) of less than 500 ps to1 ns, an example waveform with the key parameters is shown in Figure 6. By comparison, the HBM discharge has a typical rise-time of 2 to 10 ns and durations of hundreds of ns. Until 2014, the most commonly used CDM standards were JEDEC JESD22-C101 or ANSI/ESD STM5.3.1. These have now been superseded by ANSI/ESDA/JEDEC JS-002.

So, why does it matter?
Different types of discharge can affect devices in different ways. HBM is a somewhat slow discharge and ranges from 10 to 30 nanoseconds. CDM is a very fast discharge which in turn means the energy has no time to dissipate. The CDM-type damage threshold is often 10 to 20 times lower than the one for an HBM-type discharge. If an HBM-type discharge causes damage at 2000V, it is not uncommon to have the same component damaged by a 100 to 150V CDM event.

Per ESD Handbook ESD TR20.20-2016 section 3.2.1     Threats in Electronic Production Lines
ESD threats in electronics manufacturing can be classified into three major categories:

  • Charged personnel – When one walks across a floor a static charge accumulates on the body. Simple contact of a finger to a device lead of a sensitive device or assembly which is on a different potential, e.g., grounded, allows the rapid transfer of charge to the device.
  • Charged (floating) conductor – If conductive elements of production equipment are not reliably connected to ground, these elements may be charged due to triboelectric charging or induction. Then these conductive elements may transfer charge to a device or assembly which is at a different potential.
  • Charged device/boards – During handling, devices or boards can acquire a static charge through triboelectric charging or can acquire an elevated electrostatic potential in the field of nearby charged objects. In these conditions, contact with ground or another conducting object at a different electrostatic potential will produce a very fast ESD transient.

This categorization is useful in that each category implies a set of ESD controls to be applied in the workplace. ESD threats from personnel are minimized by grounding personnel through the use of wrist straps and/or footwear/flooring systems. Discharges from conductive objects are avoided by assuring that all conductive parts that might contact devices are adequately and reliably grounded. The occurrence of ESD involving charged devices or boards is minimized by a) preventing charge generation (low-charging materials, ionization) or b) by providing low-current “soft landings” using dissipative materials.

Since these preventive measures are seldom perfectly deployed, the overall threat of ESD failure remains and the risk ultimately depends on how well the controls are maintained and the relative sensitivities of the devices being handled.

Taking Action
SCS recommends reviewing your manufacturing process and determining what model is the most relevant for your facility. Are your components handled directly by hand or by a hand tool such as tweezers or a vacuum pick?

Finding the root cause of ESD events is crucial to solving the problem. SCS technology can identify events in areas like SMT line, soldering, printer and repair stations. SCS has instrumentation to identify component sensitivity and measure ESD events as well as ensure compliance within your facility.

The SCS CTM082 ESD Pro Event Indicator has a special CDM filter switch to filter and reject EMI signals that are not caused by CDM discharges. Make sure to set requirements for static voltage and discharge strength within your production environment based on the most sensitive component in production.

The SCS CTM048-21 EM Eye ESD Event Meter will calculate the event magnitude for HBM and CDM. It also logs the events to a microSD card so they can be downloaded to a PC. Solving ESD problems requires data; a before-and-after analysis of data may now be measured and used to tailor your ESD control program.

The SCS 770063 EM Aware Monitor is ideal for automated equipment and will provide an approximate voltage for the ESD event based on HBM and CDM models. The EM Aware Monitor has Ethernet network connectivity and communicates with our Static Management Program (SMP). All activity is stored into a database for on-going quality control purposes. SMP allows you to pinpoint areas of concern and prevent ESD events. Quantifiable data allows you to see trends, become more proactive and prove the efficiency of your ESD process control system.

There is a lot of confusion out there as to what the difference is between resistivity and resistance. We get asked questions on a regular basis so hopefully this post will put an end to any misunderstanding – we’ll explain the difference between the two and will point out the measurements you really need to worry about when it comes to your ESD Control Program.

The difference between Resistivity and Resistance
“Resistance or resistivity measurements help define the material’s ability to provide electrostatic shielding or charge dissipation.” [Source]
However, resistance and resistivity values are not interchangeable. Let’s get a bit technical here to illustrate the difference between the two:

  1. The resistance expresses the ability of a material to conduct electricity. It is therefore related to current and voltage. In fact, the surface resistance of a material is the ratio of the voltage and current that’s flowing between two pre-defined electrodes.
    With a pure resistive material, where:
    – R is the resistance (expressed in Ohm W),
    – U is the voltage (expressed in Volt) and
    – I is the current (expressed in Amp).The unit of measure for surface resistance is ohms (W). It is important to remember that the surface resistance of a material is dependent on the electrodes used (shape as well as distance). If your company implements an ESD Control Program compliant to the ESD Standard ANSI/ESD S20.20, it is therefore vital to carry out surface resistance measurements as described in the Standard itself.
  2. The surface resistivity of a material describes a general physical property. It is not influenced by the shape of the electrodes used or the distance between them. “Surface resistivity, ρ,  can  be  defined  for electric current flowing across a surface as the ratio of DC voltage drop per unit length to the surface current per unit width.” [Dr. Jaakko Paasi, VTT Industrial Systems: “Surface resistance or surface resistivity?”]
    As Dr. Jaakko Paasi describes in his research note, surface resistivity can be expressed by using a concentric ring probe as
    where:
    – k is the geometrical coefficient of the electrode assembly,
    – rcentre is the outside radius of the centre electrode and
    – router is the inside radius of the outer electrode.For the electrodes recommended by ESD TR53 (Compliance Verification of ESD Protective Equipment and Materials), the coefficient k = 10.The unit of measure for surface resistivity is ohms (W) but in practice you will often see ohms/square (W/square) (which technically is not a physical unit).
    As previously explained, the surface resistivity does not depend on shape or distance of the electrodes used when performing the test. You can compare results freely – no matter what type of electrode was used to get the measurements in the first place.

Converting from Resistivity to Resistance
Values of surface resistance and surface resistivity become comparable if the measured surface resistance value is multiplied by the geometrical coefficient of the used electrode fixture.” [Dr. Jaakko Paasi, VTT Industrial Systems: “Surface resistance or surface resistivity?”]
If you measure surface resistance according to ESD TR53, then the corresponding surface resistivity can be calculated by multiplying the resistance value by the geometrical coefficient factor k = 10. Likewise, surface resistivities can be converted to surface resistances by dividing the surface resistivity value by 10.

Care is needed in interpreting results when measuring non-homogeneous materials such as multilayer mats or conductive-backed synthetic fiber carpeting containing a small amount of conductive fiber. Buried conductive layers can provide shunt paths. Be clear when stating what you have measured!
A few notes in regards to measuring surface resistance and resistivity:

  • On large surfaces, such as worksurface mats, readings will sometimes vary with increasing time of measurement. This is due to the ‘electrification’ of the mat beyond the area measured. It is therefore important to measure properly and to keep the duration of measurement constant. Fifteen seconds is an arbitrary but practical duration for measurement time.
  • Moreover, the materials needing to be checked in an EPA are most of the time, non-conductive polymers that have been made conductive or antistatic by addition of conductive particles or by special treatments during manufacture. The resistivity of such materials may vary from one point to another or they may be direction dependent (anisotropic).
  • ESD TR53 goes some way to specifying the procedures to be followed and test probes to be used, so that the results can be compared, at least roughly.
  • Also, the resistance of some materials may vary with humidity level and temperature. It is therefore good practice to take a note of these two parameters when measuring.

So now that we’ve identified what the difference is between surface resistance and resistivity, there is one more thing we want to cover in today’s post: the different types of surface resistances you will come across when dealing with ESD and how to measure them:

1. Resistance to Ground (Rg)
Resistance to Ground is a measurement that indicates the capability of an item to conduct an electrical charge (current flow) to an attached ground connection. The higher the resistance in the path, the more slowly the charge will move though that defined path.” [Source]
The Resistance to Ground is measured to ensure that surfaces in an EPA are correctly grounded. This is certainly one of the most useful measurements in an EPA.

Resistance-To-Ground
Performing a Resistance to Ground Test

To perform the test:

  • One 5lb cylindrical probe is required for this measurement.
  • Connect the probe to a megohmmeter and place it on the surface to test.
  • Connect the other ohmmeter lead to earth or to an ESD ground point.
  • Measure the resistance at 10V for conductive items and 100V for dissipative items.

2. Resistance Point-To-Point (Rp-p)
A point-to-point measurement used during the qualification process evaluates floor and worksurface materials, garments, chair elements, some packaging items, and many other static-control materials.“ [Source]
Resistance Point-To-Point is used to assess the performance of an item used in an EPA.

To perform the test:

  • Two 5lb cylindrical probes are required for this measurement
  • Connect the probes to a megohmmeter.
  • Place the material to be tested on an insulative surface such as clean glass and place the probes on the material.
  • Measure the resistance at 10V for conductive items and 100V for dissipative items.
  • Move the probes to measure in a cross direction and repeat the test.

Point-to-point measurements are important during the qualification process for proper evaluation of flooring and worksurface materials. After installation, the resistance-to-ground measurement is more applicable since it emulates how the material really behaves in practice.” [Source]

3. Volume Resistance (RV)
Although this is one of the less common measurements when it comes to ESD, it’s still worth to mention the volume resistance here. You would measure the volume resistance when a non-grounded item such as a container is to be placed on a grounded item, such as a mat. The volume resistance will indicate whether the item can be used in the desired manner.

Volume-Resistance
Performing a Volume Resistance Test

To perform the test:

  • Two 5lb cylindrical probes are required for this measurement
  • Connect the probes to a megohm meter.
  • Put the first probe upside down and ‘sandwich’ the test sample between it and the second probe placed on top.
  • Measure the resistance.

 

So hopefully we have put an end to any confusion in regards to surface resistivity and resistance and answered all your questions. If there is anything else you’d like to know, let us know in the comments.

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