Component Design: Capacitor

Design of Chip Ceramic Capacitor

Ceramic Capacitor Basics

• A capacitor is an electrical device that stores energy in the   electric field between a pair of closely spaced plates
• Capacitors are used as energy-storage devices, and can also be used to differentiate between high-frequency and low-frequency signals. This makes them useful in electronic filters
• Capacitance Value: Measure of how much charge a capacitor can store at a certain voltage
• MLCC: Multilayer Ceramic Chip Capacitor L-ayers of ceramic and metal are alternated to make a multilayer chip

Process of Making Capacitor:

The process of making ceramic capacitors involves many steps.

Mixing: Ceramic powder is mixed with binder and solvents to create the slurry, this makes it easy to process the material.

Tape Casting: The slurry is poured onto conveyor belt inside a drying oven, resulting in the dry ceramic tape. This is then cut into square pieces called sheets. The thickness of the sheet determines the voltage rating of the capacitor.

Screen Printing and Stacking: The electrode ink is made from a metal powder that is mixed with solvents and ceramic material to make the electrode ink. The electrodes are now printed onto the ceramic sheets using a screen printing process. This is similar to a t-shirt printing process. After that the sheets are stacked to create a multilayer structure.

Lamination: Pressure is applied to the stack to fuse all the separate layers, this created a monolithic structure. This is called a bar.

Cutting: The bar is cut into all the separate capacitors. The parts are now in what is called a ‘green’ state. The smaller the size, the more parts there are in a bar.

Firing: The parts are fired in kilns with slow moving conveyor belts. The temperature profile is very important to the characteristics of the capacitors.

Termination: The termination provides the first layer of electrical and mechanical connection to the capacitor. Metal powder is mixed with solvents and glass frit to create the termination ink. Each terminal of the capacitor is then dipped in the ink and the parts are fired in kilns.

Plating: Using an electroplating process, the termination is plated with a layer of nickel and then a layer of tin. The nickel is a barrier layer between the termination and the tin plating. The tin is used to prevent the nickel from oxidizing.

Testing: The parts are tested and sorted to their correct capacitance tolerances.

At this point the capacitor manufacturing is complete. The parts could be packaged on tape and reel after this process or shipped as bulk.

Types of Material Systems Used to make capacitors

There are two material systems used today to make ceramic capacitors: Precious Metal Electrode and Base Metal Electrode. The precious metal system is the older technology and uses palladium silver electrodes, silver termination, then nickel and tin plating. Today this material system is mostly used on high voltage parts with a rating of 500V and higher. The base metal system is a newer technology and uses nickel electrodes, nickel or copper termination, and nickel and tin plating. This material system is typically used for parts with voltage ratings lower than 500VDC.

Precious Metal Vs Base Metal System

MLCC Basics

The capacitance value of a capacitor is determined by four factors. The number of layers in the part, the dielectric constant and the active area are all directly related to the capacitance value. The dielectric constant is determined by the ceramic material (NP0, X7R, X5R, or Y5V). The active area is just the overlap between two opposing electrodes.

The dielectric thickness is inversely related to the capacitance value, so the thicker the dielectric, the lower the capacitance value. This also determines the voltage rating of the part, with the thicker dielectric having a higher voltage rating that the thinner one. This is why the basic trade off in MLCCs is between voltage and capacitance.

Critical Specifications

Material	Dielectric Constant	% Capacitance Change	DF
NP0	15-100	<0.4% (-55 to 125C)	0.1%
X7R	2000-4000	+/-15% (-55 to 125C)	3.5%
Y5V	>16000	Up to 82% (-30 to 85C)	9.0%

• Dissipation factor: % of energy wasted as heat in the capacitor
• Dielectric Withstanding Voltage: Voltage above rating a capacitor can withstand for short periods of time
• Insulation resistance: Relates to leakage current of the part (aka DC resistance)

The critical specifications of a capacitor are the dielectric constant, dissipation factor, dielectric withstanding voltage, and insulation resistance. Dielectric constant: this depends on the ceramic material used. The table shows different dielectrics and some of their specifications. As you can see NP0 has the lowest dielectric constant, followed by X7R which has a significantly higher constant, and Y5V which is higher still. This is why the capacitance values for X7R capacitors are much higher than NP0 capacitors, and Y5V has higher capacitance than X7R. The capacitance change vs temperature is very small for NP0 parts from -55C to 125C, and gets larger for X7R, then even larger for Y5V. So, the more capacitance a material provides, the lower the stability of capacitance over temperature. Dissipation Factor: this is the percentage of energy wasted as heat in the capacitor. As you can see, NP0 material is very efficient, followed by X7R, then Y5V which is the least efficient of the three materials. Dielectric withstanding voltage: this refers to the momentary over voltage the capacitor is capable of withstanding with no damage. Insulation resistance: this is the DC resistance of the capacitor; it is closely related to the leakage current.

Characteristics of Ceramic Capacitors

Low impedance, equivalent series resistance (ESR) and equivalent Series Inductance (ESL). As frequencies increase, ceramic has bigger advantage over electrolytic

The final part of this presentation will cover the characteristics of ceramic capacitors. MLCCs have low impedance when compared with tantalum and other electrolytic capacitors. This includes lower inductance and equivalent series resistance (ESR). This allows ceramic capacitors to be used at much higher frequencies than electrolytic capacitors.

Temperature Coefficient: Describes change of capacitance vs. temperature. Ceramic materials are defined by their temperature coefficient

Voltage Coefficient: Describes change of capacitance vs voltage applied. Capacitance loss can be as much as 80% at rated voltage. This is a property of ceramic materials and applies to all manufacturers

Voltage Coefficient of Capacitance: describes change of capacitance vs DC voltage applied. This is a property of ceramic materials and applies to all manufacturers. The graph shows typical voltage coefficient curves for 500VDC rated X7R and NP0 capacitors. Note that the capacitance of the NP0 remains stable with applied voltage, while the X7R material can have a capacitance loss of 80% at rated voltage.

Aging: X7R, X5R, and Y5V experience a decrease in capacitance over time caused by the relaxation or realignment of the electrical dipoles within the capacitor.

• For X7R and X5R the loss is 2.5% per decade hour and for Y5V it is 7% per decade hour, NP0 dielectric does not exhibit this phenomenon
• De-Aging: aging is reversible by heating the capacitors over the “Curie Point” (approx 125°C), the crystalline structure of the capacitor is returned to its original state and the capacitance value observed after manufacturing.

Aging: X7R, X5R, and Y5V experience a decrease in capacitance over time caused by the relaxation or realignment of the electrical dipoles within the capacitor. For X7R and X5R the loss is 2.5% per decade hour and for Y5V it is 7% per decade hour, NP0 dielectric does not exhibit any aging. Aging is reversible by heating the capacitors over the “Curie Point” (approx 125°C), the crystalline structure of the capacitor is returned to its original state and the capacitance value observed after manufacturing.

Johanson Part Number Breakdown

  

This slide is for reference and shows the Johanson Dielectrics part number breakdown.

Summary

• Manufacturing process and basic structure of ceramic capacitors
•  Material systems and basic specifications of ceramic capacitors

o   Precious Metal vs Base Metal

o   Critical Specifications of MLCCs

•        Characteristics of ceramic chip capacitors

o   Low impedance, temperature coefficient, voltage coefficient, aging

Engineering Research Paper: Capacitors

Production testing of capacitors

Author: D. S. GIRLING, C.Eng., F.I.E.E., F.I.E.R.E.

Introduction

The progress of automatic testing of capacitors is explained and an indication given of future trends.

The most promising method of testing is to make contact to a large number simultaneously and to carry out a sequence of tests by automatic switching under computer control. The best place for this testing is while the components are orientated in manufacturing jigs. At the present time the speed of testing is limited by that of the transfer system

Tests Required

Tests on capacitors are required in three distinct areas:

(i) Production sorting

The production of capacitors, as with most components,is associated with a 'yield'. This normally varies between 80 to 95% overall and is basically due to the fact that the spreads of the parameters exceed those that are acceptable to the customer. It is, therefore, normal practice to carry out 100% testing using  automatic equipment by production operators.

(ii) Acceptance sampling

This consists of the formation of production lots and the taking of samples to determine acceptability. This sampling  is carried out in accordance with BS 9000. Since the manufacturer is required to allow for experimental error in his testing, it pays to use the most accurate methods available.

(iii) Quality assurance

A further requirement of BS 9000 is for lot-by-lot tests  periodic tests  every three months, and  every 3 years. These tests are of environmental nature and are destructive. All of these tests require the results to be recorded both initially and finally. The cost of this testing is an important factor in determining the acceptability of the BS 9000 scheme.

The main measurements or tests required on capacitors are as follows:

(i) voltage proof

(ii) capacitance

(iii) tan 5

(iv) leakage current or insulation resistance (see

Appendix)

(v) impedance (l.f. or h.f.)

  

Testing Methods

·         General Principles

Measurements may be made either on an attributes or a variables basis. The first of these is sometimes referred to as a go/no-go test, although this term strictly applies to dimensional gauging. In the field of variables measurements the problem is more complex. Let us consider the measurement on a manual bridge. This consists of the following operations:

(i) load,

(ii) balance,

(iii) read result,

(iv) decide if in limits (some calculation may be required)

(v) write down (if necessary)

(vi) unload.

·         Manual Equipment

This needs little or no explanation, but it is surprising how much of this equipment is still in use and even sold commercially.

Generally speaking, it is any equipment which requires manual adjustment of controls, visual reading of results, and manual recording if necessary.

·         Comparators

These are widely used for production operations, particularly where some adjustment is required. It consists of a standard of the value required and some means of indicating an out-of-balance between that and the work under adjustment. A typical accuracy is about 1 % but it is possible to expand the scale to give greater accuracy over a limited range provided that the zero can be maintained.

·         Automatic Test Set

One of the most well known of these consists of a wheel into which the capacitors are loaded in contact jaws. They are then carried round so that contact is made in turn to each of a number of test equipments. The results of each test is recorded in a memory so that when the capacitors reach the eject position they are ejected into the appropriate drawer.

·         Measurement of Leakage Current or Insulation Resistance

One of the simplest, and yet the most time consuming, is the measurement of leakage current. This is the conduction current flowing through the capacitor with a direct (either the rated or an arbitrary fixed value)

voltage applied. A schematic diagram of the method of measuring leakage current is shown in Fig.

Fig. Measurement of Leakage Current or Insulation Resistance

The resistance of the circuit combined with the capacitance of the capacitor under test result in a time-constant. When the voltage is applied to the capacitor it is necessary to wait for the charging current to decay before the leakage current can be measured.

Conclusions

(i) Investment in automatic test equipment for large scale component testing can be justified if advantage

is taken of their high inherent speeds.

 (ii) Automatic test methods permit the high accuracy testing of capacitors at speeds up to 3000 per hour for electrolytics and about 10000 per hour for non-electrolytics. The difference is mainly due to the different measuring frequency.

(iii) The main problem is the electrification time before the measurement of leakage current. By the use of

low impedance charging circuits early decisions may be made as follows:

(a) The use of reduced time periods and corrected limits to take accept/reject decisions.

(b) To accept as soon as the limit value has been passed. In all cases it is necessary to charge a number of

capacitors in parallel.

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Reasearch Paper: IE Concept Ergonomics

Tips for Computer Vision Syndrome Relief and Prevention

Authors: J. Tribleya, S.McClaina, A.Karbasia and J. Kaldenberga 

Source: Work 2011, Vol. 39 Issue 1

 

 

Introduction  

The Paper presents tips for the relief and prevention of computer vision syndrome. The diagnosis and treatment of symptoms including eyestrain, visual discomfort, and visual fatigue are caused by visual display terminals (VDT), can lead to the improvement of performance of the user and lower the risk of ocular problems. When spending time in front of the computer, the article suggests following the 20/20/20 rule to improve work efficiency and prevent eye strain.

•  Keep an eye out for eye fatigue symptoms

Computer-related vision symptoms such as eye fatigue have some common causes that should not be ignored. Visual display Terminals (VDTs) cause symptoms such as eyestrain, visual discomfort, and visual fatigue,light sensitivity, neck, back, and shoulder pain, and double vision . Detecting the likely diagnosis and treating the cause can result in improving performance of the user and lowering the risk of ocular problems .

• Limit the amount of time you spend in front of the computer if possible

Limiting the amount of time spent in front of the computer will have a dramatic impact on symptoms associated with computer vision syndrome.It is suggested to follow the 20/20/20 rule in that the computer user after working on a computer for 20 minutes, the user must look away at 20 ft for at least 20 seconds. This can improve the work efficiency and prevent eye strain.

• Get an eye exam aimed at computer vision

The National Institute of Occupational Safety and Health (NIOSH) suggests that computer users have an eye exam before working on a computer and once a year thereafter. Individuals should be sure to tell their optometrist how often and where they use a computer as well as the distance they normally sit from their computer.

• Set up your computer or laptop properly to minimize and relieve symptoms

Height and inclination of monitor has influence on the visual discomfort for both men and women. In a study by Rechichi, and Scullica (1990), it was shown that visual discomfort has a high correlation with height and inclination of computer monitor, therefore it is highly suggested the use of an ergonomic position of the computer monitor and chair, found that smaller angle of gaze leads to more CVS symptoms than large angle (14 degrees or more).

• Use Proper Lighting and Minimize Glare

When using a computer the lighting should be half that of normal room illumination. This can be done with dimmer switches, closing blinds or shades, use of 3 way bulbs, or use of low intensity bulbs. Glare and reflections on computer screens can also cause eye strain.  The best way to minimize glare is to use an anti-glare cover over the screen and use of flat screens when possible.

• Adjust and Clean Your Computer Screen

Adjust the brightness so it is about the same as your surroundings and adjust contrast as high as possible to eliminate discomfort Dust may also impair one’s vision when viewing a computer screen by
affecting the glare, so be sure all monitors or screens are clean and free of dust.

• Make Sure You Are Blinking

The environment, including poor lighting, focusing on the VDT, and blinking helps to fend of dryness and irritation in your eyes and studies show that people blink less frequently when using computers .  Dry eye is another cause of eye strain and its associated symptoms.  Although screen glare and character size don’t affect blink rate, studies have shown that a visual demanding computer work can decrease the blink rate from 24 to 5 blinks per minute .

• Take Breaks

To reduce your risk for computer vision syndrome as well as for neck, back, and shoulder pain, take frequent breaks. Make sure to stand up and move as well as look away from the computer. Take frequent work break (at least once per hour) in order to prevent the eye strain associated by prolong eye work . It also helps the muscles of the eye to relax, decreasing the eye fatigue and headache.

• Consider Computer Eyewear

To maximize comfort when at your computer talk to your optometrist about a customized prescription made especially for computer working distance. This is especially important if you wear contacts, which can become dry and uncomfortable during computer work, or when using bifocals or progressives, which may not be optimal for computer distances.

• Check How Far You or Your Child is Sitting From the Computer Screen

It is important for parents and teachers to monitor how far the children sits from the computer screen. There is a difference between retinal images when eyes are shifted from distance to near. In order to adapt to this change, eyes accommodate which means they change the thickness of the lens . According to Guidelines for work with VDUs the safe distance from the computer screen is between 45 cm and 70 cm .