A layer of copper

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CHAPTER 1

INTRODUCTION

During the process of PCB (Printed Circuit Board) manufacturing, a layer of copper is coated on the PCB. This makes a blank PCB board. Then a layer of mask is applied onto the PCB, exposing the areas of PCB which has unwanted copper coatings. This layer of mask is called photo mask. The PCB along with the photo mask is dipped into a ferric chloride solution (FeCl3) to remove the unwanted copper. This process is called Chemical Etching Process. During this process the unwanted copper is etched away. The main problem arises now. During chemical etching process, the etching of wanted uncopper is not proper ie., either over-etching or under-etching takes place. The reason for over-etching or under-etching is mainly because; in order for correct etching to take place the solution should be maintained at optimum temperature. Theoretically, maintaining optimum temperature for the etching solution is possible but in practice it is not possible. This is the reason for over-etching or under-etching.

The aim of the project is to develop a sensor for monitoring in real time the chemical etching process of PCB, to ensure that the PCB is dipped into the solution just for the required amount of time and to ensure that unwanted copper is etched away correctly.

CHAPTER 2

DESCRIPTION OF PROJECT

2.1. PCB Fabrication

A Printed Circuit Board can be divided into 4 types [4]:-

  1. Rigid Board- This is the most widely used type of board.
  2. Flexible and rigid-flex boards
  3. Metal core boards
  4. Injection moulded boards

These boards can be further classified into:-

  1. Single sided- All the interconnections are made on the same side of the board.
  2. Double sided- The connections are made on both sides of the board. The advantage of double sided boards is that the interconnection wires could cross each other without the use of jumpers.

Mainstream manufacturing in PCBs is completed by bonding a layer of copper substrate either on one side or on both sides according to the type of board manufactured and then removing the unwanted copper by applying a temporary mask on the PCB. The process of removing the unwanted copper is through dry etching or wet etching. In order to remove unwanted copper a mask or substrate is applied. The process doing so is called photolithography. With advance in technology, computer aided methods are employed to check whether the mask is applied to the correct places or not. The software developed for this is called "Cameo". In this method, the process of photolithography is divided into 3 tasks:- firstly it is choosing the areas that needs to be exposed or which needs to be aligned, secondly choosing the type of resist that needs to be used and last decided the method of etching that will be used. The main advantage of this method is that, it reduces the time taken for the fabrication of PCBs [25]. In the process of photolithography, the patterns that were designed would not be exactly on the wafer. The reasons for this are mainly due to the restrictions of projection optics, there will be limitations in the mask fabrication process and also factors like focus and exposure of the wafer during the process. The process materials, the environment in which it is used and also the equipment used also has an effect on the design of mask being not exactly the same on wafer [27].The methods employed to remove the unwanted copper is of 2 types: - additive method and subtractive method.

There are 3 types of subtractive methods [10]:-

  1. Silk Screen Printing- The copper foil is protected by using etch-resistant inks. The etching process removes the unwanted copper.
  2. Photoengraving- A photo mask material is used on the board where copper need not be removed and chemical etching is done to remove the unwanted copper.
  3. PCB milling- The copper foil is grated away from the substrate by using a 2 or 3axis mechanical milling system.

In the additive method, the most common method is called "semi-additive" process. In this a reverse mask is applied. That is, in the semi-additive method, applying a reverse mask exposes those areas of the board that will become the traces on the board. In the other areas, copper is then coated and then it is etched away [11].

Drilling: - After the etching process, the holes are made on the PCB board. It is done through using tiny drill bits made of solid tungsten carbide [9].

The PCB is then coated with a conductive material to resist the removal of copper coated on it. The places where components are placed is plated, this is because copper oxidizes quickly and so soldering of components is tough. The areas that are not soldered are covered with a solder mask. This avoids short circuits on the PCB and also it provides protection for the PCB in harsh environments.

2.2. Non-Destructive Testing

Non-Destructive Testing (NDT) is one part of the function of quality control and is complementary to other long established methods [3]. By definition, Non-Destructive Testing is the testing of materials for surface or integral flaws or metallurgical condition, without interfering in any way with the integrity of material or its suitability for service [3].

The common methods of NDT used are as follows [3]:-

  1. Radiography- X Ray and Gamma
  2. Magnetic particle crack detection
  3. Dye penetrant testing
  4. Ultrasonic flaw detection
  5. Eddy current and Electro-magnetic testing

1. Radiography- X Ray and Gamma

This method is used mainly for detection internally in ferrous and non-ferrous materials [3].

Advantages of radiography [3]

  1. Pictorial representation of information.
  2. Method useful for thin sections
  3. It is suitable for any material
  4. A record of all the information is kept; this is helpful in viewing thee details at any time or place.

Disadvantages of radiography [3]

  1. Due to radiations there is a chance for health hazards.
  2. Method is not suitable for thick sections
  3. Method not possible for surface defects
  4. Need for film processing and viewing required

2. Magnetic particle crack detection

This method is used for detecting the surface and near surface discontinuities for magnetic materials like ferritic steel and iron [3].

Advantages of magnetic particle crack detection [3]

  1. Simple in its operation and application
  2. The method is very quantitative
  3. The method can be automated with developments in technology

Disadvantages of magnetic particle crack detection [3]

  1. Only ferromagnetic materials can be tested
  2. Tests on surfaces only can be carried out.
  3. Dye penetrant test

Mainly used for non-ferromagnetic materials in detecting surface breaking flaws.

Advantages of dye penetrant testing [3]

  1. Simple in operation
  2. Method is quantitative
  3. Method is best for surface breaking cracks in nonferrous metals

Disadvantages of dye penetrant testing [3]

  1. The method is used only for surface breaking cracks
  2. Sensitivity is less for this method
  3. The materials used is more
  4. Ultrasonic flaw detection

The tests are carried out for sound conducting materials to detect both internal and surface defects [3].

Advantages of ultrasonic flaw detection [3]

  1. Thickness and depths upto 30ft can be tested
  2. Details of the defect like position, type and size could be found out
  3. The device can be automated
  4. The device is portable
  5. It is highly sensitive

Disadvantages of ultrasonic flaw detection [3]

  1. Testing of thin materials is difficult
  2. Interpretations of the output required
  3. The operator will be able to decide whether the material is defective or not during the test
  4. Record of data output is not kept
  5. Eddy current and Electro-magnetic testing

The application of eddy current testing is mainly in surface or sub surface flaws detection, conductivity measurement, permeability and dimensions of a product [3]. The application in which eddy current method is used includes thickness measurement, quality inspection, coating and surface treatment [21].

In this method, an alternating current is passes through a coil and the coil is placed close to the material to which is to be tested. The alternating current through the coil generates a magnetic field in the coil. This magnetic field generated induces an eddy current into the material under test. This will cause a magnetic field to be developed in the test material. This induced magnetic field will cause the impedance of the coil to be changed. The output is monitored either by measuring the impedance change of the coil or by checking the magnetic field with a secondary coil. The properties of the test material like surface roughness, cracks, magnetic permeability, electrical resistance etc can be detected easily [22]. With advances in technology rigorous test methods are carried out using computer modelling. Although in certain tests, analog methods of operation are still followed [22]. This also helps in the automation of the system and also during such testing methods multiple parameters can be monitored by giving multi-frequency inputs [21].

Advantages of eddy current testing [3]

  1. Compact and portable
  2. Device can be fully automated
  3. According to the application, probes and frequencies required can be selected
  4. The range of applications is varied and can be used for a variety of metals

Disadvantages of eddy current testing [3]

  1. The tests is restricted to surface and to an extent sub surface defects
  2. In some applications required output won't be obtained, this may be due to the parameters that affect the eddy current characteristics. To avoid this, proper selection of probe and electronics is required.

After investigating through the above methods, all the methods have their own advantages and disadvantages. Out of the above 5 methods, Eddy current method of testing seems to have lesser number of limitations and also the usability is more.

2.3. Theory of eddy currents

Eddy currents were discovered by Jean Bernard Léon Foucault in 1851. Eddy currents are also called as Foucault currents [7]. Before going into the details of eddy currents a few terms are explained which will be delt with frequently.

Introduction to electromagnetism

Electric current

Electricity is a basic part of nature and is basically the flow of electrons due force acting on it. The force acting on the electrons can cause an imbalance in the movement of electrons. Electric current is measured in terms of Amperes (A).

Electromotive Force (EMF)

Electromotive force is the force with which electrons move in an electrical circuit [12]. Electromotive force is measured in terms of volts (V).

Ohm's Law

Ohm's Law states that, for a constant resistance in the circuit, the current flowing through it will be directly proportional to the voltage applied. The resistance in the circuit is inversely proportional to the electric current.

Induction

Induction or electromagnetic induction is the process by which an electric current is generated in a conductor when the conductor is placed in a magnetic field that changes. The name electromagnetic induction is because current is induced into the conductor. It was Faraday and Henry who discovered this phenomenon [14].

The amount of current or voltage that was induced was affected by the rate of change of magnetic field.

The unit for measuring inductance is Henries (H).

The flow of electricity in a circuit is affected when inductance occurs. Inductance is further classified into 2 types

  1. Self- Inductance- In this, if there is a change in current in the circuit there will be a change in voltage in the same circuit.
  2. From fig 2, it is visible that the number of turns of the coil will have an effect on the voltage that is induced in the coil. That is, when either the number of turns of the coil is increased or when the rate of change of magnetic flux is increased the voltage induced in the coil is increased. This is given by the equation below, Where: VL= induced voltage in volts

    N = number of turns in the coil dø/dt = rate of change of magnetic flux in webers/second [15]

  3. Mutual Inductance- This is the main basis for eddy current inspection. The magnetic flux in one circuit can be linked to the current in that circuit and also the current in nearby circuits. This will depend on the distance at which the second circuit is placed from the first. The constant for mutual inductance is M.

Impedance

Electrical impedance is denoted as Z. It is the opposition showed by a circuit against the alternating current. Impedance is measured in ohms. Impedance of a circuit includes the resistance (R), inductive reactance (XL) and the capacitive reactance (Xc) [17].

Z=X2+R2 ; Tan Ø=XR Where Z=impedance R=Resistance X= Inductive or capacitive reactance

From figure 5, it is understood that when the resistive component is replaced by an inductive component, a lag will be there between voltage and current. The lag is 90degrees. In this current lags voltage by 90 degrees.

From figure 6, it is understood that when the inductor is replaced by a capacitor, a phase lag occurs. In this current leads voltage by 90 degrees.

Impedance measurement can be done by using a digital signal processing instrument. In this measurement can be done is 2 techniques. In the first technique, time varying impedance can be measured and in the second the accuracy of measurement is maintained. These two techniques are quite general. They are not only used to measure resistance but inductance and capacitance can also be measured [23].

From figure 7, when the eddy current circuit is balanced in air, and then it is placed on a piece of material like aluminium or copper, the resistance of the circuit increases while the inductive impedance keeps decreasing. This is because the magnetic field created by the eddy currents opposes the magnetic field of the coil. Suppose there is defect on the test material the opposite happens, that is the inductive reactance increases and resistance decreases. This is because the only less eddy currents are created [24].

In order to increase the accuracy of measurement of instruments, a numerical model is created. The numerical simulation results are then compared with the results obtained from instruments or from the data obtained from experiments. This will help in giving the behaviour of equipment and also help in the correction of any systematic errors that occur [26].

Lenz's Law

The theory behind Lenz's Law is that, the flux that is produced by the current induced will be in a direction that opposes the flux that induced the current into the circuit [2].

Skin depth and Skin effect

Skin effect is the decaying of alternating electric current with distance from the surface of the object [2]. Skin depth is given by the following equation [1][19]

Skin depth, ?=1?=1/v(p*f*µ*s) Where f=frequency µ=permeability of material s=Conductivity of the material

For copper skin depth is found out to be 0.066f [19].

Skin depth varies with frequency. At higher frequencies skin depth is less that is penetration of the magnetic flux into the material is less compared to at lower frequencies. The standard depth of penetration is when the value of eddy current becomes 1/e [1].

Lift-off

Lift-off is a major factor affecting the sensitivity of eddy current measurements. When the probe is closer to the surface the sensitivity of measurement is higher. This can cause undesirable effects in the output obtained from the sensor.

2.4. Block diagram to find the thickness of PCB

Various methods have been used for finding out the thickness of copper on PCB. The block diagram of the measurement system is shown below

Input- The input to the instrument can be a signal from a signal generator or a high frequency AC current.

Sensor and the material that is tested- In this different methods can be employed to find the thickness. The sensor used depends on the kind of method that is used.

Output- The displaying of the thickness of the copper on the PCB during etching process is also done in different ways. The output can be displayed in analog format by using a voltmeter or ammeter. If the system needs to be automated then the output can be displayed onto a computer by interfacing it with a microprocessor [20].

CHAPTER 3

CONCLUSION

Printed Circuit Board (PCB) has now become an integral part of any electrical or electronic circuit. According to the application different types of PCBs are used, like single sided, double sided and surface mount. The size of PCBs have now been reduced comparatively with the integration of components and have made the manufacturing of PCBS a lot more difficult, to precisely design and making it cost efficient. The usage of PCBs is varied, from carrying out experiments in lab to the design of circuits for industry level. During the manufacture of PCB, there are chances of defects that occur during the manufacturing process.

In my project, I am intending to design a sensor for checking the thickness of copper during the chemical etching process. Because some of the defects that a PCB has, are due to the incorrect etching process that results in copper that is masked onto the PCB being etched away inappropriately or copper not being etched proportionally. I would like a develop a sensor which will monitor the etching process in real time and provide information about the thickness of copper being etched during the chemical etching process.

CHAPTER 4

REFERENCES

  1. Electromagnetic concepts and applications, third edition. By Stanley V Marshall, Gabriel G Skitek
  2. Electromagnetism for engineers, third edition. By Hammond
  3. http://www.insight-ndt.com/papers/technical/t001.pdf
  4. http://www.scribd.com/doc/18644392/PCB-EMC-SI-
  5. http://www.asnt.org/publications/materialseval/solution/feb98solution/feb98sol.htm
  6. N. Harfield and J.R.Bowler, "Theory of thin-skin eddy-current interaction with surface cracks", United Kingdom, 14 july 1997
  7. http://www.newadvent.org/cathen/06156c.htm
  8. http://www.ndt-ed.org/EducationResources/CommunityCollege/EddyCurrents/Introduction/historyofET.htm
  9. http://books.google.co.uk/books?id=Fh5d6y2mligC&pg=PA16&lpg=PA16&dq=pcb+fabrication+notes&source=bl&ots=5mSZMMDDZR&sig=FVbNAYFAyEb1NgCk68WKFk2my24&hl=en&ei=cZ0aS5XcN9St4Qb2i93kAg&sa=X&oi=book_result&ct=result&resnum=6&ved=0CB8Q6AEwBTgU#v=onepage&q=pcb%20fabrication%20notes&f=false
  10. http://www.freepatentsonline.com/3615951.pdf
  11. http://www.freepatentsonline.com/4782007.pdf
  12. http://www.ndt-ed.org/EducationResources/CommunityCollege/EddyCurrents/Physics/propertiesofelectricity.htm
  13. http://www.ndt-ed.org/EducationResources/CommunityCollege/EddyCurrents/Physics/currentflow.htm
  14. http://www.ndt-ed.org/EducationResources/CommunityCollege/EddyCurrents/Physics/inductance.htm
  15. http://www.ndt-ed.org/EducationResources/CommunityCollege/EddyCurrents/Physics/selfinductance.htm
  16. http://www.ndt-ed.org/EducationResources/CommunityCollege/EddyCurrents/Physics/mutualinductance.htm
  17. http://www.ndt-ed.org/EducationResources/CommunityCollege/EddyCurrents/Physics/impedance.htm
  18. http://www.ndt-ed.org/EducationResources/CommunityCollege/EddyCurrents/Physics/circuitsphase.htm
  19. http://www.nessengr.com/techdata/skin/skindepth.html
  20. H.Geirinhas Ramos, A. Lopes Ribeiro, P.Jezdik and J. Neskudla, "Eddy current testing of conductive materials", IEEE International Instrumentation and Measurement Technology Conference Canada, May 12-15 2008.
  21. W Yin, S J Dickinson and A J Peyton, "A multi-frequency impedance analysing instrument for eddy current testing", pg 393-402, 19 January 2006.
  22. C C Holt, "Configurable instrumentation for eddy current testing" , Harwell Laboratory, 1 march 1988 http://ieeexplore.ieee.org/xpl/freeabs_all.jsp?tp=&arnumber=208861&isnumber=5360
  23. L. Angrisani, A. Baccigalupi, and A. Pietrosanto, "A digital-signal processing instrument for impedance measurement", lEEE TRANSACTIONS ON INSTRUMENTATION AND MEASUREMENT, VOL. 45, NO 6, pg 930-934, DECEMBER 1996
  24. http://www.ndt-ed.org/EducationResources/CommunityCollege/EddyCurrents/Instrumentation/impedanceplane.htm
  25. Michael F Klein, "A Structured Methodology for IC Photolithography synthesis in Semiconductor Manufacturing", IEEE TRANSACTlONS ON SEMICONDUCTOR MANUFACTURING. VOL. I, NO I., pg 28-35, FEBRUARY 1988
  26. Henrique Jorge Quaresma, Student Member, IEEE, António Pedro Silva, and António M. Cruz Serra, Senior Member, IEEE, "Error Correction Technique for Dynamic Impedance Measurement", IEEE TRANSACTIONS ON INSTRUMENTATION AND MEASUREMENT, VOL. 54, NO. 3, pg 1249-1253, JUNE 2005
  27. W. L. Pearn, H. Y. Kang, A. H.-I. Lee and M. Y. Liao, "Photolithography Control in Wafer Fabrication Based on Process Capability Indices With Multiple Characteristics", IEEE TRANSACTIONS ON SEMICONDUCTOR MANUFACTURING, VOL. 22, NO. 3, pg 351-356, AUGUST 2009

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