Welcome back to “A Minute with Miranda.” This week we will be covering ESD Worksurface Mats.
SCS worksurface mats serve two purposes for an ESD worksurface in an EPA:
they provide a surface that does not generate a static charge and
they remove charges from all charged conductors, which include ESD susceptible devices and assemblies, that are placed on the surface.
SCS worksurface mats are constructed from either dissipative 2 layer rubber or 3 layer vinyl material and are available in both rolls and mat kits.
We can also quote custom sizes SCS worksurface mats meet ANSI/ESD STM4.1 ANSI/ESD S20.20 required limit of 1 x 106 to less than 1 x 109 ohms for Rtt and Rtg and the recommendations of ANSI/ESD S4.1
For more information on SCS Worksurface matting, check out our Selection Guide here.
Setting up an ESD-safe workstation is often
more challenging than it first appears. There are many methods of controlling
ElectroStatic Discharge (ESD), and typically, it requires a combination of
these to curb all static problems. Unfortunately, there is no single method
that will fill all requirements.
Wrist straps and work surface mats are probably the most familiar to everyone, draining charges from operators as well as from the product being worked on. But what if the static charge in question is on an insulator? Electronic products, by nature, will normally consist of conductors and insulators. Insulators at the workstation can be found on the product itself, tools being used, tapes for masking, even circuit boards. A static charge on an insulator cannot be drained by grounding, as you could with a conductive material.
Ionization
To effectively remove charges from insulators, we need to make the surrounding air more conductive. We have all seen a balloon cling to a wall because of a static charge, and we know that, after a period of time, it will drop. That is because the air is somewhat conductive and the charge eventually drains off. The problem with this concept is that it takes too long. The more conductive the air is, the faster the charge will be neutralized.
A balloon “stuck” on a wall by static charge.
The method most frequently used to increase
the conductivity of the air is ionization.
Ionizers are useful in preventing
electrostatic charge generation, ElectroStatic Discharge, ElectroStatic
Attraction, as well as preventing equipment latch-up. Per ANSI/ESD S20.20
section 6.2.3.1. Protected Areas Requirement states: “Ionization or other
charge mitigating techniques shall be used at the workstation to neutralize
electrostatic fields on all process essential insulators if the electrostatic
field is considered a threat.”
How
do Ionizers work?
Most ESD workstations will have some
insulators (e.g. product plastic housing) or isolated conductors (e.g. PCB
board components not in contact with ESD worksurface) that cannot be removed or
replaced. These should be controlled using ionization.
Ionizers create great numbers of positively
and negatively charged ions. Fans help the ions flow over the work area. If
there is a static charge present on an item in the work area, it will be
reduced and neutralized by attracting opposite polarity charges from the air.
Ionization can neutralize static charges on
an insulator in a matter of seconds, thereby reducing their potential to cause
ESD damage.
Electronic enclosures are process-essential insulators
The charged ions created by an ionizer
will:
neutralize charges on process
required insulators,
neutralize charges on non-
essential insulators,
neutralize isolated conductors
and
minimize triboelectric
charging.
How
does Ionization fit into an ESD Control Program?
Ionization is just one component of your
ESD Control Program. Before utilizing ionization, you should follow the
fundamental principles of ESD Control:
Ground all conductors
(including people) using conventional grounding methods (e.g. wrist straps or
footwear/flooring system).
Remove all insulators, e.g.
coffee cups, food wrappers etc.
“Air ionization is not a replacement for
grounding methods. It is one component of a complete static control program.
Ionizers are used when it is not possible to properly ground everything and as
backup to other static control methods. In clean rooms, air ionization may be
one of the few methods of static control available.” (ESD Handbook ESD TR20.20
Ionization, section 5.3.6.1 Introduction and Purpose / General Information)
Ionizers can be critical to
reduce induction charging caused by process necessary insulators
Ionizers can be critical in
eliminating charges on isolated conductors like devices on PCBs
Ionization can reduce
ElectroStatic Attraction (ESA) and charged particles clinging and contaminating
products.
It is recommended to use ionizers with feedback mechanisms, so you’re notified if the offset voltage is out of balance.
Ionizers should be pieces of equipment that
have serial numbers and are included in the company’s maintenance and
calibration schedules. This is particularly critical to ensure that the offset
voltage or balance is within acceptable limits. Otherwise, instead of
neutralizing charges the out of balance ionizer will charge insulators and
isolated conductors. The user, depending on the value and function of their
products, must determine the appropriate frequency of maintenance and
calibration.
Summary
The best way to keep electrostatic
sensitive devices (ESDs) from damage is to ground all conductive objects and
remove insulators. This is not always possible because some insulators are
“process-essential” and are necessary to build or assemble the finished product.
The only way to control charges on these necessary non-conductive items is the
use of ionization systems. Applications include:
eliminating charges on process
essential insulators,
neutralizing workstations where
ESDS are handled,
removing charged particulates
to create a static free work area.
For more information and to select the
right ionizer for your application, check out our Ionizer
Selection Guide.
When the tip of a soldering iron comes into direct electrical contact with the pins of a sensitive component, there is a danger of voltage and/or current signal transfer between:
the grounded iron tip and the grounded PC board,
the ungrounded iron tip and the grounded PC board,
the grounded iron tip and the ungrounded PC board.
This can cause Electrical Overstress (EOS) and Electrostatic Discharge (ESD).
What is Electrical Overstress (EOS) and why is it important to detect?
EOS is the exposure of a component or PCB board to a current and/or voltage outside its operational range. This absolute maximum rating (AMR) differs from one device to the next and needs to be provided by the manufacturer of each component used during the soldering process. EOS can cause damage, malfunction or accelerated aging in sensitive devices.
ESD can be generated if a component and a board have different potentials and the voltage transfers from one to the other. When such an event happens, the component goes through EOS. ESD can influence EOS, but EOS can also be influenced by other signals.
Many people are familiar with Electrostatic Discharge (ESD) which is caused by the spontaneous discharge between two materials that are at different levels of ElectroStatic potential. Once electrostatic potential between the two materials is balanced, the ESD event will stop.
An EOS event on the other hand is created by voltage and/or current spikes when operating equipment; it can therefore last “as long as the originating signal exists”. [Source] The potentially never-ending stimulus of EOS is what makes it such a big concern in the electronics industry. Even though the voltage levels are generally much lower compared to an ESD event, applying this smaller voltage combined with a larger peak current over a long period of time will cause significant damage.
The high temperatures during an EOS event (created by the high current) can lead to visible EOS damage.
For more information on EOS and the differences to ESD, check-out this post.
Sources of EOS during the Soldering Process
When soldering components, it’s the tip of the soldering iron that comes into contact with the potentially sensitive device. Therefore, many people assume the soldering tip is the cause of ESD/EOS. However, the soldering iron and its tip are just some of the components used at a workbench. Other components on the bench like tweezers, wiring, test equipment, etc. can also be sources of ESD/EOS as they come into contact with the component or board.
There are many sources of EOS during the soldering process, which can include:
Loss of Ground The tip of an ungrounded soldering iron can accumulate a voltage of up to ½ of the iron’s supply voltage. It can be caused within the soldering iron itself or in power outlets.
Noise on Ground If a noise signal exists on ground, the tip of the solder iron will carry noise, too. These high-frequency signals, or electromagnetic interference (EMI), are disturbances that affect an electrical circuit, due to either electromagnetic induction or electromagnetic radiation emitted from an external source.
Noise on Power Lines Noise not only generates via ground but in power lines, too. Transformers and power supplies that convert voltages to 24V are the main culprit. They regularly carry high-frequency spikes which end up on the tip of the soldering iron.
Power Tools Although not technically related to the soldering process itself, it’s worth mentioning that the tips of power tools (e.g. electric screwdrivers) may not be properly grounded during rotation. This can result in high voltage on the tip itself.
Missing/Inadequate ESD Protection ESD can be a cause of EOS damage. Therefore, it is essential to have proper ESD Protection in place. A voltage on the operator or the PCB board can otherwise lead to an ESD Event and expose the components on the PCB to EOS.
Detecting EOS during the Soldering Process
EOS/ESD events can be detected, measured, and monitored during the soldering process using a variety of diagnostic tools.
Diagnostic Tools
SCS CTM051 Ground Pro Meter The SCS CTM051 Ground Pro Meter is a comprehensive instrument that measures ground impedance, AC and DC voltage on the ground as well as the presence of high-frequency noise or electromagnetic interference (EMI) voltage on the ground. It will alert if the soldering iron tip has lost its ground or has EMI voltage induced into the tip from an internal source on the soldering iron or from an EMI noisy ground or power lines.
SCS CTM048 EM Eye – ESD Event Meter The SCS CTM048 EM Eye – ESD Event Meter paired with the SCS CTC028 EM Field Sensor is a diagnostic tool for the detection and analysis of ESD events and electromagnetic fields and can identify sources of harmful ESD Events and electromagnetic interference (EMI).
The SCS CTM048 EM Eye – ESD Event Meter paired with the SCS CTC028 EM Field Sensor
EOS Continuous Monitors
SCS CTC331-WW Iron Man® Plus Workstation Monitor
The SCS CTC331-WW Iron Man® Plus Workstation Monitor is a single workstation continuous monitor which continuously monitors the path-to-ground integrity of an operator and conductive/dissipative worksurface and meets ANSI/ESD S20.20.The Iron Man® Plus Workstation Monitor is an essential tool when it comes to EOS detection. The unit is capable of detecting EOS on boards and alarms if an overvoltage (±5V or less) from a tool such as a soldering iron or electric screwdriver is applied to a circuit board under assembly.
The SCS CTC331-WW Iron Man® Plus Workstation Monitor
Data Acquisition
SCS Static Management Program SCS Static Management Program (SMP) continuously monitors the ESD parameters throughout all stages of manufacturing. It captures data from SCS workstation monitors, ground integrity monitors for equipment, ESD event and static voltage continuous monitors and provides real-time data of manufacturing processes.The SCS 770063 EM Aware Monitor, which is part of SMP, can help during the soldering process by monitoring ESD events and change of static voltage that may result in EOS. The EM Aware alarms (visual and audibly) locally and sends data to the database of the SMP system if any of the ESD parameters are detected to be higher than user-defined limits.
Once the source of ESD/EOS is known, there are many things that can be done to prevent it in the first place:
1. Managing Voltage on a PCB board
PCB boards contain isolated conductors and non-conductive (insulative) components. The only way to handle voltage on a PCB board is neutralizing potential static charges through ionization. An ionizer creates great numbers of positively and negatively charged ions. Fans help the generated ions flow over the work area to neutralize static charges (or voltage) on a PCB board in a matter of seconds.
For more information on ionization and how to choose the right type of ionizer for your application, please read these posts.
2. Managing Voltage on an Operator
Static voltage on an operator can be eliminated through proper grounding using a workstation monitor, e.g. WS Aware or Iron Man Plus Monitor, and proper grounding hardware. Sitting personnel are required to wear wrist straps. A wrist strap consists of a conductive wristband which provides an electrical connection to skin of an operator, and a coil cord, which is connected to a known ground point at a workbench, a tool or a continuous monitor. While a wrist strap does not prevent generation of voltages, its purpose is to dissipate these voltages to ground as quickly as possible.
Sitting personnel can also use continuous monitors – not only is the operator grounded through the continuous monitor, but they also provides a number of additional advantages:
Immediate feedback should a wrist strap fail
Monitoring of operators and work stations
Detection of split-second failures
Elimination of periodic testing
This post provides more details on continuous monitors.
Moving or standing personnel are grounded via a flooring/footwear system. ESD Footwear (e.g. foot grounders) are designed to reliably contact grounded ESD flooring and provide a continuous path-to-ground by removing electrostatic voltages from personnel.
3. Managing Current
One solution is the “re-routing of ground connection and separation of “noisy” ground from a clean one” as “connecting soldering iron and the workbench to the “quiet” ground often result in lower level of transient signals.“. [Source]
This will greatly reduce the high-frequency noise that could cause EOS damage.
If the noise on power lines and ground cannot be reduced manually, then the use of noise filters becomes necessary to reduce the risk of EOS exposure during the soldering process. Utilizing these filters suppresses the noise on power lines and will allow the solder iron to use “clean” power only.
In his papers, Vladimir Kraz, explains the set-up of a soldering station using a noise filter in more detail.
Soldering Iron with Power Line EMI Filter [Source]
Conclusion
During the soldering process, current and voltage spikes between the solder tip and PCB can cause ESD/EOS. Sources are varied and can include:
Loss of Ground
Noise on Ground
Noise on Power Lines
Power Tools
Missing/Inadequate ESD Protection
ESD/EOS can be identified and controlled using diagnostic tools. SCS offers a number of tools that can detect current, voltage and EMI – all potentially leading to ESD and EOS.
Once the source of ESD/EOS is known, the next step is eliminating the source:
Managing voltage on a PCB board using ionizers.
Managing voltage on an operator using workstation monitors or foot grounders.
Managing current using noise filters.
Managing voltage on materials at the work bench.
Managing ESD generation during specific processes.
Managing grounding.
For more information regarding this topic, please see below for additional references.
We have learned in a previous post that within an ESD Protected Area (EPA) all surfaces, objects, people and ESD Sensitive Devices (ESDs) are kept at the same electrical potential. We achieve this by using only ‘groundable’ materials.
But what do you do if an item in your EPA is essential to assembly and it cannot be grounded? Don’t sweat, not all hope is lost! Let us explain a couple of options which will allow you to use the non-groundable item in question.
Conductors and Insulators
In ESD Control, we differentiate items as conductors and insulators.
Materials that easily transfer electrons are called conductors. Examples of conductors are metals, carbon and the human body’s sweat layer.
A charged conductor can transfer electrons which allows it to be grounded
Insulators are materials that do not easily transfer electrons are non-conductors by definition. Some well-known insulators are common plastics, polystyrene foam, and glass.
Insulators like this plastic cup will hold the charge and cannot be grounded and “conduct” the charge away.
Both, conductors and insulators, may become charged with static electricity and discharge.
Electrostatic charges can effectively be removed from conductive or dissipative conductors by grounding them. A non-conductive insulator will hold the electron charge and cannot be grounded and “conduct” the charge away.
Conductors and Insulators in an EPA
The first two fundamental principles of ESD Control are:
Ground all conductors (including people).
Remove all insulators.
To ground all conductors per the first ESD Control principal, all surfaces, products and people are electrically bonded to ground. Bonding means linking or connecting, usually through a resistance of between 1 and 10 megohms.
Wrist straps and worksurface mats are some of the most common devices used to remove static charges:
Wrist straps drain charges from operators and a properly grounded mat will provide path-to-ground for exposed ESD susceptible devices.
Movable items (such as containers and tools) are bonded by standing on a bonded surface or being held by a bonded person.
If the static charge in question is on something that cannot be grounded, i.e. an insulator, then #2 of our ESD Control principles will kick in and insulators must be removed. Per the ESD Standard ANSI/ESD S20.20, “All nonessential insulators such as coffee cups, food wrappers and personal items shall be removed from the EPA.” [ANSI/ESD S20.20 clause 8.3.1 Insulators]
The ESD Standard differentiates between these two options:
If the field measured on the insulator is greater than 2000 volts/inch, keep it at a minimum distance of 12 inches from the ESDs or
If the field measured on the insulator is greater than 125 volts/inch, keep it at a minimum distance of 1 inch from the ESDs.
Aim to keep insulators away from ESDs
“Process-Essential” Insulators
Well, nothing in life is black and white. It would be easy if we were always able to follow the above ESD Control ‘rules’ but there are situations where said insulator is an item used at the workstation, e.g. hand tools. They are “process-essential” insulators – you cannot remove them from the EPA or the job won’t get done.
How do you ‘remove’ these vital insulators without actually ‘removing’ them from your EPA?
Here are four ways to reduce the ESD risk of these insulators:
Keep all insulators a minimum of 1 inch or 12 inches from ESDs at all times per recommendation of the ESD Standard.
This reduces the chance of insulators coming in contact with ESDs during workstation processes and assembly.
Replace regular insulative items with an ESD protective version.
There are numerous tools and accessories available that are ESD safe – from document handling to cups & dispensers, soldering tools, brushes and waste bins. They are either conductive or dissipative and replace the standard insulative varieties that are generally used at a workbench.
Periodically apply Topical Antistat on non-ESD surfaces. After Topical Antistat has been applied and the surface dries, an antistatic and protective static dissipative coating is left behind. The static dissipative coating will allow charges to drain off when grounded. The antistatic properties will reduce triboelectric voltage to under 200 volts. It therefore gives non-ESD surfaces electrical properties until the hard coat is worn away.
Neutralization with Ionization If these three options are not feasible for your application, the insulator is termed “process-essential” and therefore neutralization using an ionizer becomes a necessary part of your ESD control program. This allows for control of charged particles that can cause ESD events which we will cover next.
Neutralization
Most ESD workstations will have some insulators or isolated conductors that cannot be removed or replaced. These should be addressed with ionization.
Examples of some common process essential insulators are a PC board substrate, insulative test fixtures and product plastic housings.
Electronic enclosures are process-essential insulators
An example of isolated conductors are conductive traces or components loaded on a PC board that is not in contact with the ESD worksurface.
An ionizer creates great numbers of positively and negatively charged ions. Fans help the ions flow over the work area. Ionization can neutralize static charges on an insulator in a matter of seconds, thereby reducing their potential to cause ESD damage.
The charged ions created by an ionizer will:
neutralize charges on process required insulators,
neutralize charges on non- essential insulators,
neutralize isolated conductors and
minimize triboelectric charging.
Insulators and isolated conductors are common in ESDs – Ionizers can help
For more information on ionizers and how to choose the right type of ionizer for your application, read this post.
Summary
The best way to keep electrostatic sensitive devices (ESDs) from damage is to ground all conductive objects and remove insulators. This is not always possible because some insulators are “process-essential” and are necessary to build or assemble the ESDs.
Insulators, by definition, are non-conductors and therefore cannot be grounded, but they can be controlled to minimize potential ESD damage.
Insulators can be controlled by doing the following within an EPA:
Keep insulators a minimum distance from ESDS at all times (1 or 12 inch minimum distance depending on field voltage measurements of the insulator per ESD Standard recommendation)
Replace regular insulative items with ESD protective versions
Periodically apply a coat of Topical Antistat
Neutralize charges for “process-essential” insulators with ionization
With these steps added to your ESD control process, all surfaces, objects, people and ESD Sensitive Devices (ESDs) are kept at the same electrical potential in an ESD Protected Area (EPA) to reduce the risk of ESD events and ESD damage.
