Concepts of Electrostatic Discharge in Surface Mount Technology

Concepts of Electrostatic Discharge in Surface Mount Technology 2015

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Concepts of Electrostatic Discharge in Surface Mount

Technology

*Akinde Paul, **Abdullateef Saheed, *** Adeniyi Matthew

*[email protected],[email protected] **[email protected],[email protected] ***[email protected],[email protected]

Surface Mount Technology (SMT) Department Momas Electricity Manufacturing Company

Limited Ogun State

NIGERIA

www.memmcol.com

Abstract: The intent of this paper is to present an analysis of electrostatic discharge and

electrical overstress in surface mount technology. It recognises the recent development

in modern electronics and emergence of solid state device in small and miniaturised

forms as its building blocks which are electrostatic sensitive by nature. It began with an

overview of electrostatic discharge and electrical stress, classification of ESD sensitivity,

its effects on solid state devices and the various sources of ESD. It explains the

preventive/ precautionary procedures and the need to improve on activities in a

workstation through the conditioning of various tools/equipment for best smt practices

as well as the increasing requirement for product quality and security. It also discusses

the handling and storage procedures for electrostatic sensitive components and an

effective operative and maintenance program for a surface mount technology

workstation. This investigates electrostatic discharge and its effects on modern

electronics to an optimal level in order to prevent damages and ensure best smt practices.

Keywords: Electrostatic Discharge, Electrical Overstress, Surface Mount Technology

I. INTRODUCTION

In modern electronics otherwise known as digital electronics, large electronic

components that form the building blocks of electronic/electrical systems are

being miniaturised hereby setting the evolution of small electronic component

called Surface Mount Devices (SMD). These devices are formed or built from solid

state compounds or semiconductors such as silicon and germanium.

Recent technologies have embraced this new innovation and have absorbed the

use of these devices in their production line hereby replacing the large

components for these reasons:

mailto:*[email protected],[email protected]

mailto:**[email protected],[email protected]

mailto:***[email protected],[email protected]

http://www.memmcol.com/


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Reduced overall cost of production. The cost of making SMDs is cheaper, so

for a large production line; the rate at which you buy large components in

bulk is in multiples of the rate you purchase SMD so you are saving more

and the overall cost of producing a single product is lessened.

Reduced weight/Portability. Due to the small size of the SMD which can

perform the same way or more than the large components, the overall

weight of a single product is reduced making the product handy/portable.

In the past where the large sizes of component add up to the overall weight,

SMD have evolved hereby shedding the net weight.

However, this technology called SMD is very sensitive and easily susceptible to

an electronic epidemic called ESD and EOS. Thus, SMD are referred to as ESDS,

Electrostatic Discharge Sensitive Components.

Electrostatic Discharge (ESD): is the rapid transfer of a static electric charge

from one object to another of a different potential that was created from

electrostatic sources. It can damage a sensitive component when in contact.

Electrical Overstress (EOS): is the internal result of an unwanted application of

electrical energy that results in damaged components.

1.1 CLASSIFICATION OF ESD SENSITIVITY

Electrostatic discharge sensitive (ESDS) parts are commonly characterized to

three defined models:

1.1.1 Human Body Model (HBM)

The most common model is the HBM. This model simulates when a

discharge occurs between a human (hand/finger) to a conductor (metal

rail).


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Fig. 0a Fig. 0b

1.1.2 Machine Model (MM)

Machine model discharges occur when charged; conductive surfaces come

into contact with ESD sensitive devices. To minimize Machine Model

discharges, ensure that all metal surfaces that come into contact with ESD

sensitive devices are grounded. Measurements should be made to ensure

that moving parts remain grounded throughout the process.

Fig. 0c


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1.1.3 Charged Device Model (CDM)

CDM damage from an electrostatic field occurs when a charged item is

brought into close proximity to an ESD sensitive device and the device is

then grounded while in the presence of the field. Effective ESD control

programs ensure that process required insulators will not induce damaging

voltage levels onto the devices being handled.

1.2 EFFECTS OF ESD/EOS

The sensitivity of components to ESD is determined by its construction and

materials. So as the components structure becomes smaller, it works faster; and

the sensitivity increases. The effects of ESD/EOS on electronic components can be

immediate or latent.

Immediate effect is usually caused by rework practices, additional testing

or scrap.

Latent effect is the most serious; the product may fail after delivery as a

result of ESD potentials or electric spikes from tools.

1.3 SOURCES OF ESD/EOS

Insulators; they concentrate energy

Rubber Paper Glass


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Styrofoam

Plastic solder removers

Sheet protectors


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Plastic or paper notebook folders

Employees personal items

Electric spikes from tools; small components react faster to small

electrical charge or wider frequency ranges

Improper grounding; static charges needs to be eliminated in order not to

harm electronic components so a faulty grounding can allow easy damage

on ESDS components.

2. PREVENTION/PRECAUTION PROCEDURE

2.1. ESD WORKSTATION

ESDS items must be removed, packaged, and worked on only at a static safe

workstation. The degree of static energy generated is relative to the characteristics

of the source. Relative motion is required to generate energy and harm is done on

the ESD material by contacting, separating or rubbing of the material. To avoid

these, the following are to be done;


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Path to Ground; it is very essential to create a path to ground to neutralise

static charges that might otherwise discharge to a device or assembly.

Static dissipative or antistatic work mat; the work mat has to be laid on

the workbench in a workstation. However, the table mat has an resistance

of 1000megaohms to ground to create a good ground path (check IPC-A-

610D)

Wrist strap; it is usually worn on the wrist of the technician to discharge

static energy that can cause harm on the electronic component from the

body either on the clothes or skin but it must be connected to ground. The

recommended resistance to ground is 100megaohms(check IPC-A-610D)

Floor; the workstation floor has to be ESD compliant to be able to

accommodate electronic component in case the component drops on the

floor. The maximum resistance to ground is 1000megaohms.


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Figure 1a:

Figure 1b:

1. Personal wrist strap

2. EOS protective trays

and shunts.

3. EOS protective table

top.

4. EOS protective floor

or mat.

5. Building floor

6. Common ground

point

7. Ground

1. Personal wrist strap

2. EOS protective trays

and shunts.

3. EOS protective table

top.

4. EOS protective floor

or mat.

5. Building floor

6. Common ground

point

7. Ground


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Figure 2: Full ESD Protection Scheme

2.2 EQUIPMENT AND TOOL

Before processing sensitive components, it is important to ensure that tools

and equipment will not generate damaging energy. However, voltage spikes less

than 0.5V is acceptable and for some other components, it is greater or equal to

0.3V.

Periodic testing and maintenance programs for tools and equipment will

prevent EOS damage on electronic components.


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2.3 PRINTED CIRCUIT BOARD (PCB)

PCB absorbs and releases moisture at different rates. During soldering, heat

causes expansion of the moisture that can damage the ability of the materials not

to perform according to design requirement. Damages such as: crack, pop corning,

and internal declamation.

PCB should be packed inside a static shielding box when not in use

Handle PCB at edges as shown in Fig. 3 below

Avoid touching solder-able parts/spaces prior to soldering

PCB should be baked to reduce the internal moisture content. However, the

baking temperature selection and duration should be controlled to prevent

reduction of solder ability through metallic growth, surface oxidation or

other internal component damage.

For further reading on PCB handling and moisture content, check IPC 1601

and IPC/JEDEC J-STD-033

Figure 3: How to handle a PCB


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2.4 ELECTRONIC COMPONENTS

The safety of electronic components is paramount in modern day design

because it forms the building block of any electronic/electrical system. It must be

guarded against contamination to avoid failure.

Standard packaging; ESDS components must be packaged very well to

avoid the intruding of static charges caused by relative motion. The use of

static shielding package, which prevents an electrostatic discharge from

passing through the package and antistatic bag, which doesn’t generate

static when in motion is advisable to be used for packing when the

component is not in use.

Avoid static generating materials near the workstation

Eliminate contaminants such as body salts, body oils, and unauthorised

hand creams by washing of hands and use of protective handling devices.

Use of pallets and carriers with full ESD protection is recommended for

stacking components.

3. COMPONENT HANDLING/STORAGE

As earlier discussed about Electrostatic discharge and its effects on SMD devices

and equipment, it is essential to put into consideration the storing conditions of

these components and how to handle them for best SMT practices.

3.1 STORAGE CONDITIONS

SMD devices are usually moisture/reflow sensitive. Moisture from

atmospheric humidity enters permeable packaging materials by diffusion.

Assembly processed used to solder SMD packages to PCBs expose the entire

package body to temperature between 160° - 260°. During solder reflow, rapid

moisture expansion can result in package cracking, delamination of critical

interfaces within the package, or damaged gold wire.


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All SMT components shelf life depends on storage conditions and

packaging type, time for storage and shelf life for SMT passive components

is two years (24 months).

The main concerns with the storage of SMT are humidity content,

temperature and Tape & Reel degradation which could ultimately affect

solderability.

The terminations of all components will typically oxidize over time

degrading solderability and affecting the wetting characteristics of the capacitor if

not stored properly. Contact with Sulphur Dioxide or Chlorine gas will accelerate

an oxidation layer forming on the terminations. Tape and Reel products should be

protected from direct sunlight and used on a “first-in, first-out” basis (FIFO).

This is especially the case for the C-series ceramic capacitors that utilize class II

dielectric materials where ageing characteristics will continue to drop capacitance

over time.

Note: Typical ageing rates can be found in the product data sheet or specifications.

All components and their corresponding packaging should not be opened until the

components are ready for use and should be re-sealed and properly stored as soon

as possible

Components are to be stored indoors and in their original packaging where

environmental conditions can be monitored and controlled.

Dry box storage is recommended as soon as the aluminium bag has been

opened to prevent moisture absorption.

The following conditions should be observed, if dry boxes are not available:

Storage temperature 10 °C to 30 °C

Storage humidity ≤60 % RH max.

After more than 72hrs under these conditions moisture content will be too

high for reflow soldering.


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In case of moisture absorption, the devices will recover to the former

condition by drying under the following condition:

: 192hrs at 40 °C + 5 °C / - 0 °C and < 5 % RH (dry air/nitrogen)

: 96hrs at 60 °C + 5 °C and < 5 % RH for all device containers

: 24hrs at 125 °C + 5 °C not suitable for reel or tubes.

3.2 MATERIALS FOR COMPONENTS’ STORAGE

1. SMT components automatic storage

2. Moisture Barrier Bags

3. Cabinet

4. Boxes

Figure 4: Antistatic bag


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3.3 STORAGE TYPE

Manual Storage devices

This modern, adjustable trolley is designed for the safe transport and storage of

SMD reels. One trolley combination offers storage for up to 360 SMD spools.

Figure 5: Component Carrier

Automatic storage devices

This storage device is designed to meet the requirements of a production

supply depot for modern SMD production. The storage system can mount SMD

reels and trays. It can monitor the useful life of moisture sensitive devices (MSDs),

and place them in controlled storage according to IPC/JEDEC J-STD-033C.1.

The packing units can also remain in the protected dry pack or outer

packaging. Any materials such as PCBs, small parts, or remaining quantities can be


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stored for production using the Smart Carrier boxes. The Smart Carrier boxes each

offer a maximum storage area of 390 mm x 390 mm x 77 mm (W/D/H).

Figure 6: Automatic storage


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Figure 7: Packing units

4. IMPLEMENTATION OF EFFECTIVE ESD PROGRAM

Design in protection: by designing products and assemblies to be as robust

as reasonable from the effects of ESD.

Define the level of control needed in your environment.

Identify and define the electrostatic protected areas (EPAs), the areas in

which you will be handling ESD sensitive parts (ESDS).

Reduce Electrostatic charge generation by reducing and eliminating

static generating processes, keeping processes and materials at the same

electrostatic potential, and by providing appropriate ground paths to

reduce charge generation and accumulation.

Dissipate and neutralize by grounding, ionization, and the use of

conductive and dissipative static control materials.

Protect products from ESD with proper grounding or shunting and the use

of static control packaging and material handling products.

Wearing a ESD wrist strap is IMPORTANT at any level of work in an SMT

environment


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Figure 8: A typical Workstation

5. MAINTENANCE

ESD protective floor and table coverings must be properly maintained. Do

not wax them. Cleaners must not degrade their electrical properties.

Vacuum to remove loose particles, followed by a wet mop with a solution of

mild detergent and hot water.


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5.1 MATERIALS NEEDED FOR ESD

a. Antistatic table mat

b. Grounded wrist-strap

c. Antistatic floor tiles

d. Antistatic foot wear

e. Static control test station

f. Wrist strap Tester

g. ESD hot gloves

h. Antistatic hand lotion

i. ESD Trash Can

j. - Static Safe Wiper


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I. Conclusion

In this paper, the electrostatic discharge as a menace against the evolving modern

electronics has been analysed and several ways by which it effects on electronic

components can be prevented also itemised. However, due to the dynamic nature

of modern technology, smaller components/ devices are emerging which are even

more sensitive to electrostatic discharge. Therefore, the future challenge is how to

efficiently and accurately solve the problem of its effects on these electronic

components in order to enjoy crisis free surface mount technology practice.


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APPENDIX 1


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APPENDIX 2


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APPENDIX 3


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References

1. IPC STANDARDS for printed circuit board

2. www.youtube.com/ESD videos

3. www.esd.org

4. www.google.com/search

http://www.esd.org/

http://www.google.com/search

Concepts of Electrostatic Discharge in Surface Mount Technology

Design

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