Potential ESD Hazards When Using Adhesive Tapes

In printed circuit board (PCB) assembly operations, some static-sensitive component failures are caused by pressure-sensitive adhesive tapes. These tapes are used for masking purposes in wave solder, conformal coating and painting operations, to name just a few.

Many different ESD failure modes can occur when working with adhesive tapes, and even the best tapes cannot guard against all failure modes at all times. Some simple static checks can ensure that all the potential failure modes are addressed and eliminated before finalizing a particular adhesive-tape operation.

ESD Failure Modes

There are three basic risks when using adhesive tapes in the masking and demasking of PCBs.

Charge on Tape After Removal From Reel

During tape application, a high static charge can be present on the tape when it is removed from the reel. The highly-charged material can come into close proximity to electronic circuitry (setting up field-induced damage possibilities) during application. Regular insulating tapes typically generate between 10,000 and 20,000 V during removal from their reels.

Charge Generated Into Electronic Circuitry During Tape Removal

When tapes are removed from electronic component circuit leads (this most commonly occurs when the tape is lifted off the PCB lands or leads), triboelectric charging can occur. This causes voltage spikes (typically 1,500 V) and subsequent damage to the circuitry.

Charge Remaining on PCBs After Tape Removal

Although not commonly known throughout the electronic manufacturing community, substantial charges can be created on the bare areas of the PCB. Since these materials are insulative (plastic, epoxy and ceramic), the charges can remain for many hours on the board.

This charge couples into the circuit elements (inductively) and damage can occur if the circuitry discharges subsequently by contacting a person or machine. Regular insulating tapes can leave charges as great as 25,000 V on the bare areas of the PCB when removed. The amount of charge that remains is extremely dependent on the PCB material.

Only very recently (1994-1995) have some tape manufacturers improved the performance of ESD-safe tapes to eliminate the dangerous 1,500-V spikes during tape removal from the PCBs. Bare areas of the PCB charging during tape removal is still an issue even with the best tapes currently available.

Case Study

I worked with four manufacturers where true-yield loss problems were traced to PCB charging (from tape removal) that would inductively couple into the electronic circuitry on the boards. The circuitry was subsequently damaged when contacted by a conductor. In these cases, ionizers were effective in correcting the problem.

Based on these findings, my recommendations are:

Use an ESD-safe tape (instead of regular insulating tapes) that generates low voltage when removed from the reel, has low voltage when removed from metals and does not leave charges on the PCB material when removed. All three of these requirements are important.

Use an air ionizer to remove residual charges on the PCB, because all commercially available tapes can leave a charge on certain insulative surfaces when removed.

Most ESD-safe tapes in the marketplace are quite good at not generating charge when removed from their reels. Only a few, however, are designed to also be effective in preventing charging on metal surfaces during removal. This is a critical feature for true static protection.

Summary of Test Results

Table 1 shows the results of some tests conducted on several different types of adhesive tapes. Samples C, D, E and F exhibit excellent properties when removed from their reels, as well as excellent low charging properties when removed from metal surfaces (Table 1). Also, Samples C, D, E and F performed noticeably better than Samples A and B when removed from the 10 different bare PCB materials selected for the test. Note that the charge remaining on the various PCB materials was very dependent on the interaction of the tapes under test. Consequently, be sure to test the proposed tapes on

your particular PCB material.

Enough charge can be generated by any tape (on insulative PCBs) to warrant using an air ionizer. I would urge you to perform these basic tests when selecting an ESD-safe tape for your facilities.

About the Author

Roger Peirce is the president of ESD Technical Services, an independent ESD consulting organization. He worked as a research engineer at Bell Telephone Laboratories from 1970 to 1983, co-founded Voyager Technologies in 1983 and founded ESD Technical Services in 1986. ESD Technical Services, 31 Bridgetown Pike, Langhorne, PA 19053, (215) 364-1050.

Jay Shah is the technical director at Finite Industries. He has a master’s degree in physical organic chemistry and an MBA in marketing. Finite Industries, 746 Gotham Parkway, Carlstadt, NJ 07072, (201) 939-0585.

Table 1 — one large table with 3 sections

Charge on Tape After Removal From Reel

Removed from roll, 12″/s, 3M ETM 59134 Test Method

Tape Sample

Charge @ 10% RH

@ 50% RH

A—Regular Insulative Tape 1

~20,000 V

~15,000 V

B—Regular Insulative Tape 2

~20,000 V

~15,000 V

C—Antistatic Tape 1

D—Low Static Tape

E—Antistatic Tape 2

F—Antistatic Tape 3

Residual Charge on Metal After Tape Removal

Residual charge on substrate, removal from stainless steel, 12″/s, 3M ETM 59135 Test Method

Tape Sample

Charge @ 10% RH

@ 50% RH

A—Regular Insulative Tape 1

1,500 V

1,200 V

B—Regular Insulative Tape 2

1,300 V

1,200 V

C—Antistatic Tape 1

D—Low Static Tape

E—Antistatic Tape 2

F—Antistatic Tape 3

Residual Charge on Bare PCB After Tape Removal

Residual charges observed on 10 different PCB materials, 12″/s, static fieldmeter used to observe charging. (RH = 12%) Values in volts)

Tape Sample	PCB #1	PCB #2	PCB #3	PCB #4	PCB #5	PCB #6	PCB #7	PCB #8	PCB #9	PCB #10
A	7,000	7,000	8,100	6,300	1,900	2,800	3,800	7,800	2,500	1,600
B	6,000	8,100	7,200	5,600	2,300	2,000	4,500	7,500	2,000	1,800
C	4,400	2,600	3,100	5,100	700	300	1,300	7,800	2,100	200
D	4,500	4,300	5,100	6,000	5,500	1,500	1,100	4,200	1,000	800
E	1,900	3,200	800	3,600	4,200	1,300	900	4,300	7,000	300
F	1,300	4,700	1,400	2,000	3,200	1,300	500	2,800	1,000	300