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5 (additional) common mistakes in ESD Control & how to avoid them

vermason
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A little while ago we published a post listing some of the most common issues we see when visiting EPA areas and how to fix them. We had a lot of positive feedback on this post so thought we’d create a follow-up post with another 5 mistakes that are creeping up on a regular basis.

Introduction

You may remember how we talked in the previous post about companies wasting a lot of money by misusing their ESD products? No? Catch-up here.

The bottom line is: the job doesn’t end with purchasing ESD control items. Operators need to be trained on how to use their ESD products and ESD products need to be checked on a regular basis. If this doesn’t happen, you might as well throw the money you invested in your ESD Control Programme out the window…

Remember: ElectroStatic Discharge (ESD) is silent, quick and potentially lethal to electronic parts. When electronic parts are not properly handled during manufacturing, assembly, storage or shipping, damage from ESD can reach into the millions of dollars each year.

5 (additional) common Mistakes in ESD Control

 

1. Using insulators at the workstation

Non-essential insulators at an ESD protective workstation might include regular packaging, document holders, binders and tape. In addition, workers like to personalise their work areas so they might have high charging plastics in the form of radios, picture frames, purses, drinking cups etc. on the bench. None of these are essential to get your job done and all of them pose a risk to your sensitive components.

CheckInsulators can be controlled by doing the following within an EPA:
• Keep insulators a minimum of 30cm from ESDS items at all times; or
• Replace regular insulative items with an ESD protective version; or
• Periodically apply a coat of topical antistat

 

Standard“All non-essential insulators and items (plastics and paper), such as coffee cups, food wrappers and personal items shall be removed from the workstation or any operation where unprotected ESDS are handled.The ESD threat associated with process essential insulators or electrostatic field sources shall be evaluated to ensure that:
• the electrostatic field at the position where the ESDS are handled shall not exceed 5 000 V/m;or
• if the electrostatic potential measured at the surface of the process required insulator exceeds 2 000 V, the item shall be kept a minimum of 30 cm from the ESDS; and
• if the electrostatic potential measured at the surface of the process required insulator exceeds 125 V, the item shall be kept a minimum of 2,5 cm from the ESDS.”
[IEC 61340-5-1:2016 clause 5.3.4.2 Insulators]

If you want to learn more about controlling insulators, have a look at this post.

2. Using open shielding bags or containers

So, you may have heard of a Faraday Cage but do you know what role it plays in ESD Protection? We see a lot of companies that have a state-of-the-art EPA but when it comes to shipping sensitive components, everything falls apart. They may use component shippers but without a lid or they use shielding bags that are stapled together. None of these practices will do your sensitive components any good.

CheckIn ESD Protection, the Faraday Cage effect causes charges to be conducted around the outside surface of the conductor. Since similar charges repel, charges will rest on the exterior and ESD sensitive items on the inside will be ‘safe’. However, to complete the enclosure, make sure to place lids on boxes/containers and seal shielding bags using a label or tape. This is the only way to ensure ESD sensitive devices placed inside the shielding bag are protected.

Standard“Transportation of sensitive products outside of an EPA shall require packaging that provides both: – dissipative or conductive materials for intimate contact; – a structure that provides electrostatic discharge shielding."
[EN 61340-5-3 Clause 5.3 Outside an EPA]

 


3. Ungrounded ESD Work Surface

ESD mats and laminate work surfaces cost a lot more than their regular insulative counterparts. The ESD dissipative characteristics are added so when charged conductors (conductive or dissipative) items are placed on the surface, a controlled discharge occurs and electrostatic charges are removed go ground. However, this only occurs if the ESD work surface is actually connected to ground.


Check

Best industry practice is that ESD ground connections should be firm fitting connecting devices such as metallic crimps, snaps and banana plugs that shall be connected to designated ground points. The use of alligator clips is not recommended. The companies’ Compliance Verification Plan should include periodic checks of worksurfaces measuring Resistance-to-Ground from the work surface centre or the most worn area to ground. Many companies also use a daily checklist, which requires the operator to verify that ground cords are firmly connected.

Standard“Periodic testing of work surfaces is necessary to ensure that they continue to meet specifications. Resistance to ground measurements are typically used to verify that the path to ground is intact. In cases where the resistance to ground measurements exceeds the established resistance limits, the following steps can be taken to identify the cause of the high resistance readings:

  • Verify visually that the work surface is connected to the ground reference.
  • Clean the work surface. Sometimes a dirty surface can cause the resistance to exceed acceptable limits. Once the surface has been cleaned (note: clean the bottom of the resistance measuring electrode as well) repeat the resistance to ground measurement. If the second measurement is within specification this might lead to a further investigation concerning the cleaning practices used by the organization.
  • Disconnect the grounding wire and measure the resistance from the top surface of the work surface to the work surface groundable point. This measurement shall show whether or not the work surface is functioning as designed and it will verify that there is a good connection between the groudable point and the work surface.
  • Using an ohmmeter, measure the resistance of the wire used to ground the work surface. The measurement is made from the point where the wire is connected to the work surface's groundable point to the ground reference.

The frequency of periodic testing is normally specified in corporate operating procedures. However, a common guide would be to conduct these measurements at least quarterly.”
[CLC TR 61340-5-2 User guide Work surface clause 4.7.1.4.3 Periodic tests]

“The most important functional consideration for work surfaces is the resistance from the top of the surface to the groundable point. This establishes the resistance of the primary path to ground for items placed on the surface. IEC 61340-5-1 has set a resistance to ground range for work surfaces of less than 1,0 x 109 Ω.”
[CLC TR 61340-5-2 User guide Work surface clause 4.7.1.2.5 Electrical considerations]


For more information how to ground and look after your ESD work surface, have a look at this post.

4. Dissipative ESD Floor is measuring high

Electrostatic dissipative materials have a resistance to ground of greater than 1 x 105 ohm but less than 1 x 1012 ohm. EN 61340-5-1 requires the Resistance-to-Ground of ESD flooring to be less than 1 x 109 ohms. So, if you install new dissipative flooring and it measures 1 x 106 ohms, you’re all good. The problem with flooring is that when it gets dirty (and trust us, it will get dirty!), the resistance increases which potentially results in out-of-spec flooring.

CheckA regular maintenance schedule needs to be followed and floor resistance measurements need to be taken as outlined in the companies’ Compliance Verification Plan. A dissipative floor finish can be used to reduce floor resistance. Periodic verification will identify how often the floor finish needs to be applied. As the layer(s) of dissipative floor finish wear, the resistance measurements will increase. So, after some amount of data collection, a cost-effective maintenance schedule can be established.

Standard“For standing operations, personnel can be grounded via a wrist strap system or by a footwear-flooring system. When a footwear-flooring system is used, personnel shall wear ESD footwear on both feet and the two following conditions shall be met:

  • the total resistance of the system (from the person, through the footwear and flooring to ground) shall be less than 1,0 × 109Ω;
  • the maximum body voltage generation shall be less than 100 V.”

[IEC 61340-5-1 Clause 5.3.3 Personnel grounding]


5. Poorly fitting Wrist Straps

As discharges from people handling sensitive items cause significant ESD damage, the wrist strap is considered the first line of ESD control. However, there are a number of issues we see repeatedly when it comes to wrist straps:

  • Operators feel restricted by the wrist strap and stop wearing it altogether.
  • Operators leave their workstation and forget to re-connect their wrist strap when returning to their workstation.
  • Operators don’t pay attention when fitting their wrist straps resulting in an incorrect fit.
  • Operators use ripped wristbands or patched-up coiled cords.

Remember: if your wrist strap is worn incorrectly (or not at all), charges on your body will not dissipate to ground resulting in dangerous ESD exposure to sensitive ESD components.

CheckThe wrist strap should be effectively tested while worn on the person and records should be kept. Wiggling the resistor strain relief portion of the coiled cord during the test will help identify failures sooner. Analysis and corrective action should take place when a wrist strap tester indicates a failure.An even better solution is the use of continuous monitors that will alarm if the person is not properly grounded. Some monitors will beep if a discharge occurs or when a certain voltage level of electrostatic charge is on the person.

Standard“Because wrist straps do not last forever, they should be tested periodically. A good testing program not only tests the wrist strap itself, but also indicates the quality of the skin contact when performing a system test. Wrist strap bands that are soiled, incorrectly sized or improperly worn will show resistance higher than acceptable.”
[CLC TR 61340-5-2 User guide Wrist strap clause 4.7.2.4 Wrist strap testing]

“Proper testing of the wrist strap includes the resistance of the groundable point on the end of the cord, the cord itself, the current-limiting resistor, the cord-to-band snap connector, the resistance of the interface of the cuff, the cuff/wrist interface and the resistance of the person between the wrist and the hand that contacts the test electrode. The maximum acceptable resistance for wrist strap grounding is less than 3,5 x107 Ω.” [CLC TR 61340-5-2 User guide Wrist strap clause 4.7.2.4.2 Additional user wrist strap testing]

Do you have anything to add? Let us know in the comments.

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vermason
Desco Europe is the newest brand in the Desco Industries family, consolidating our two previous UK-based brands, Charleswater and Vermason. Desco Europe sells the full range of DII products, manufactured in our facilities in the USA and UK, servicing the European market via trained distributors who will sell the Desco Europe value package and complete ESD solution to all ESD users in their territory, leading with hi-end solutions that mark us out from the competition.
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