Using Brushes in ESD Protected Areas
We recently received a question regarding brushes in ESD protected areas (EPAs) so we thought this would be a great opportunity to share the information with all of you guys.
Problems with using regular brushes in an EPA
When a regular brush is being held by an operator and they wipe it upon a product or assembly, tribocharging occurs due to the contact and separation of the brush bristles on the product. Per the ESD Handbook ESD TR20.20 section 2.4 Sources of Static Electricity includes “Brushes (camel/pig hair and synthetic bristles).” Even if the operator is grounded, the electrostatic charge will remain on the brush fibres and/or handle. It is therefore a threat to discharge possibly damaging ESD sensitive products. All portions of the ESD brush are to be conductive or dissipative.”
Most man-made synthetic materials are high charging and generate a substantial electrostatic charge. Per the ESD Handbook ESD TR20.20 section 2.4 “It should be understood that any object, item, material or person could be a source of static electricity in the work environment. Removal of unnecessary nonconductors, replacing nonconductive materials with dissipative or conductive materials and grounding all conductors are the principle methods of controlling static electricity in the workplace, regardless of the activity.” Brushes whose job is intimate contact with products may be critical and it is recommended to change from high charging insulative materials to conductive and/or dissipative materials.
Often in an EPA, products are cleaned off by brushing or by blowing using a compressed gas. For any hand tool used in the EPA, the handle of the tool cannot be made from an insulative material as that will block the path-to-ground to the hand of the grounded operator. Charges on the tool can discharge and possibly caused damage to ESD sensitive products. There have been numerous examples in the industry where brushing a circuit board after soldering with insulative nylon brushes have caused product failures.
Types of brushes to use in an EPA
ESD control advises removing all insulators (non-conductors) and grounding all conductors in an ESD protected area (EPA). This can be accomplished by switching from regular brushes to ESD protective brushes.
Examples of Conductive Brushes
In an ESD protected area (EPA) an operator should use dissipative or conductive brushes when working on ESD sensitive products or assemblies. All portions of the brush need to be conductive or dissipative – so that includes both handle and bristles.
For ESD control purposes, conductive is less than 1 x 104 ohms (10,000 ohms) and static dissipative is 1 x 104 to less than 1 x 1011 ohms (10,000 to less than 100,000,000,000 ohms). This resistance range is able to remove electrostatic charges to ground when grounded.
Dissipative bristles (instead of conductive bristles) should be selected if the product or assembly may be holding a charge and Charged Device Model (CDM) failures are a concern.
Examples of Dissipative Brushes
An ESD brush is grounded by being held by the grounded operator. Also, the ESD brush will be grounded when in contact with a grounded working surface or bench mat.
Summing-up the most important points in regards to using brushes in your ESD protected area (EPA):
- Use dissipative or conductive brushes in an EPA.
- All portions of the brush (handle and bristles) need to be conductive/dissipative.
- Operators need to be grounded during use.
- Choose dissipative bristles if your product/assembly may be holding a charge and Charged Device Model (CDM) failures are a concern.
ESD brushes come in many sizes, shapes and hardness – all serving different applications. For more information, check our website.