Friday, 28 March 2014

Researching Damage mechanic models

I spent a while looking at damage mechanic models only to discover everyone who has written a paper says they have a model for what ever it is they are looking at, which is annoying. So I eventually searched for review of damage mechanic models yielding slightly more information but not descriptive enough. I got an email from my supervisor saying to model a basic solder model and look at "Cohesive Zone" and "Fracture Mechanics Method (Paris Law, etc)". Now I want to know what is "Cohesive Zone" and "Fracture Mechanics Method (Paris Law, etc)"

Cohesive Zone modelling

I wanted a description of Cohesive Zone Modelling, one of the first papers I found was written by a PhD student (not sure if the paper was accepted or not) the report went in to some detail. It says it was invented by Dugdale(1960), Barenblatt (1962), Rice (1968) amongst others. From what I can gather it regards a fracture as an gradual process where separation takes place between two adjacent surfaces across am extended crack tip (cohesive zone) and is resisted by the presence of cohesive forces.
Cohesive forces: it is the force of attraction between the particles of the same substance
Unfortunately I got confused after this, so I came across this paper

Saturday, 11 August 2012

Cohesive Zone basic Model

Cohesive zone model is part of Fracture Mechanics. It is used when a crack is present and when a load is applied. For an example I built a simple model in ANSYS. ANSYS has a short description about what is meant to go on in a cohesive zone model, however it's not that good. The main problem is some of the commands can only be done text based i.e. there is no submenu where you input these. I did a simple 2D model, here is the full text input

Show text

One thing I found out is that ANSYS will not show you cracks instead it will show you elements coloured in different colours and you have to use the scale to check to see if it has cracked

Friday, 15 June 2012

Three year plan

I have just completed my RDA1a form and so I now have a workplan:

Year 1 Tasks

  • Undertake literature review on the latest damage models generally and in the field of power electronics. This will include previous Univ Greenwich work.
  • Review and document damage models and material models for (i) Wire Bonds, (ii) SnAg solder joints and Iiii) Sintered Silver joints
  • Demonstrate damage mechanics modelling using simple model in ANSYS & PHYSICA.
  • Work closely with University of Nottingham team – obtain data for use in new model development
  • Develop damage models for wirebonds and SnAg solder
  • Investigate how to include microstructure effects in the models
  • Use ROMARA to develop reduced order models for above damage models
  • Work closely with Dr Hua Lu to embed above models into POWERLIFE software
  • Present research at department PhD presentations
  • Prepare paper for EuroSime 2013 conference
  • Consider writing a review paper for IEEE CPMT Transactions

Year 2 Tasks

  • Verify accuracy of models developed for wirebonds and SnAg silver
  • Work closely with Univ Nottingham to obtain data on Sintered Silver joints
  • Develop a damage model for Sintered Silver Joints including microstrure effects
  • Validate above model with data from Univ Nottingham and industrial partners
  • Write paper for EPTC conference (Singapore)
  • Disseminate work at school PhD seminars
  • Write journal paper for ASME Microelectronics Reliability
  • Work closely with Dr Hua Lu to embed Sintered Silver models into POWERLIFE
  • Quantify the accuracy of current models using data from Univ Nottingham and Industrial partners
  • Complete MPHIL/PHD transfer

Year 3 Tasks

  • Quantify process variations and model uncertainty for wire bond and die attach materials
  • Capture above variations into POWERLIFE software to provide a framework for design for Robustness
  • review physics-of-failure based prognostics models
  • Identify methodology to include developed damage models into a real-time prognostics and health monitoring framework
  • Use the schools NI PXI system to demonstrate the above.
  • Disseminate research at schools PhD seminar
  • prepare paper for ECTC conference in USA.
  • Write paper for Applied Mathematical Modelling Journal.
  • Prepare contents page for PhD thesis and agree with supervisors
  • Write PhD thesis
Should be fun... Travelling, writing papers and learning new software. Right I better get back to work.

Friday, 11 May 2012

Playing with Electronics Circuits

When I started this postition, one thing people asked me was to learn how electronics circuits work especially power electronics circuits. Most of it is quite confusing, so the best way to learn is normally to play with circuits, since it can be quite dangerous, I was looking for software to play with circuits which are preferably freeware. I have been introduced to PLECS which can be used in MATLAB or stand alone application unfortunatley this is a paid application. So the freeware application I have seen so far are:
  1. Gecko This has a free Java applet but the full one is not free.
  2. Circuit Simulation Applet Not sure this has the full range of componets ie it doesn't have a IGBT, still it looks good
  3. 5Spice is a graphic design of circuits that you will have to buy after 30 days, I think However it is related to spice
  4. Spice this is a text based way of doing circuits, there is many application who have added graphical displays to run spice, normally this isn't for free.
If you do manage to find any electronic/electric circuit websites/applications please post them in the comments.

Friday, 20 April 2012

MPhil 2 Months in

These months I have been reading up on power electronics, how it works, how to change from a AC current to a DC current and visa versa. There is a heck of a lot of information, I did start reading books on it, which is boring so what has the web have to offer I found out about PEEB It is also on power electronics, It has the same information as a book but there is in some chapters, experiments, haven't tried them but it is a nice idea.

The project I'm working on which is on wire bonds is in collabration with University of Nottingham, so we all the people who are involved with project RODENT meet up for a preliminary on what the proposal is for the project.

After I had a look at power elecronics, I thought I'd have a look at what the proposal meant I had to do. I did a bit of research on some papers and got some information.

Damage mechanics-When searching on wikipedia it redirect to Tribology. Which has something to do with the exposed suface of a solid interacting with materials or environment may result in loss of material from the surface. Tribology come from greek tribo-"I rub".

Wire bonding is the primary method of making interconnections between an Intergrated Circuit (IC) and a printed circuit board. Bond wires ususally consist of one of the following materials:

  1. Aluminium (Al)
  2. Copper (Cu)
  3. Gold (Au)
There are 2 main classes of wire bonding:
  1. Ball Bonding
  2. Wedge Bonding
Ball Bonding is usually restricted to gold or copper wire and usually requires heat. Wedge bonding can use either gold or aluminium wire, only gold requires heat. Both types the wire is attached using a combination of heat, pressure and ultrasonic energy to make a weld.

MOSFET stands for Metal Oxide Semiconductor Field effect transistor. This is an IC. MOSFET are fast switches which are turned on and off by applying a voltage to a gate terminal. They require a continuous gate source voltage in order to keep them in the on state. The switching is around 100 nano seconds which depends on a gate resistor.

IGBT stands for Insulated Gate Bioploar transistor, this is also an IC. IGBT are fast switches with high voltage and current capabilities. Commonly used in most of high power converters when fast switching is required. Off and ON process similar to MOSFET with high gate impedance which requires a small amount of energy to switch the device. Turn on speed of IGBT can be controlled by a gate resistor. IGBTs have turn on and off times in the order of microseconds. If an IGBT stucture Contains a parasitic thyristor, it should not be turned on or else the gate will lose the ability to turn off the device.

After this, I was given a set of papers to look at and see what they were doing. There is a lot of potential relevant stuff.

The next aims I have got is:

  1. Add more information to RDA1 form
  2. Do a literature review and write it up
  3. After attending Power electronics workshop, write up a summary of what I have leant and if any of it is relevant

Thursday, 23 February 2012

Introduction report

I officially started this MPhil 8th February and so far I have met my supervisors and I have been told this is a partnership with Nottingham and some companies as well. Just to start off with I had to do a report and presentation within 4 weeks on:
  1. What is power electronics?
  2. How is it different to Microelectronics?
  3. How do we predict reliability of power electronics modules?(classify each technique)
  4. How can finite element analysis be used to predict reliability?
  5. What is meant by the term "Prognostic" and "Health Management"
  6. How are power electronic modules used in:
    1. Wind Power
    2. Solar Power
    3. Traction and drive
    4. Electric veichles
    5. Airbus 380
So far this is what I have got:

What is power electronics?

Power electronics deals with the control and flow of electricity so it is involved in converting from AC to DC current. Also involved in converting the voltage up and down. It can also be used in delivering power gradually. . “Power electronics uses semiconductor technology to convert and control electrical power.” From Hua Lu, Chris Bailey, Chunyan Yin on the paper “Design for reliability of power electronics modules”

How is it different to Microelectronics?

Microelectronics relates to the study and manufacture of microelectronic designs and components. Components found in normal electric design are found in microelectronics e.g. transistors, capacitors, inductors, resistors, diodes and of course insulators and conductors. Another definition is Microelectronics carries communication and data.

How do we predict reliability of power electronics modules?

Reliability is to do with the lifetime prediction of the product. There are several predictions methods available many can be found from Reli Soft or you could look at their lists:

MIL-HDBK-217 Predictive Method

This consists of two parts:
  1. One part is called “parts count” where it assumes typical operating conditions like temperature, electrical stresses etc. These are called Reference conditions. The failure rate for a part under the reference conditions is calculated as (see diagram). Yref is the failure rate under reference conditions and i is the number of parts.
  2. The other part is called “parts stress”. Since the real operating conditions will be different to the parts count part stress needs the specific part complexity.

How can finite element analysis be used to predict reliability?

Finite element analysis is used to solve partial differentiable equations (PDEs). Finite element analysis is needed to perform a stress analysis of the component that will be used to predict reliability. We need to know what components of a model need to be accelerated in order to work out to test it’s lifespan.

What is meant by the term "Prognostic" and "Health Management"

Prognostic is the discipline in predicting the time in which a component can no longer perform its function. Health management has a couple of meanings, one is it the field of administrating, leadership and management of hospitals. The other definition is concerning itself with the different effects on health like physical, social, nutrition, housing etc.
The definition of "Integrated systems health management" include fault detection, fault diagnosis (fault isolation) and fault prognosis.

How are power electronic modules used in:

Wind Power

Power electronics enables the variable speed operation in variable speed wind turbines. Whereas fixed speed wind turbines can’t follow optimal aerodynamic efficiency point. (source). I know I'm not meant to talk about it but some bright spark may have eliminated power electronics from wind power. Artemis hydraulic system eliminates the gearbox and power electronics from wind turbines, making them lighter, cheaper and more robust. (source)

Solar Power

Power electronics is used in 2 different ways:
To interconnect the individual solar panels – two solar panels cannot be identical hence a dc-dc converter interfacing the two will help maintain the required current and voltage, and with regulation improve the overall efficiency. Several non-isolated dc-dc converters have been employed for this purpose. Buck, buck-boost, boost, and Cuk topologies with suitable modifications can be employed for this purpose.
To interface the dc output of the photovoltaic system to the grid or the load - This includes the previously discussed topics of dc-dc-ac and dc-ac-ac conversion. The topologies considered for fuel-cell system grid interconnection correlates to the grid interconnection of photovoltaic based system as well including the usage of the Z-source inverter. Source

Traction and drive

From Wikipedia I believe power electronics helps trains/trams to stop from slipping and a smooth acceleration. “As the DC motor starts to turn, the magnetic fields inside start to join together. They create an internal voltage. This electromagnetic force (EMF) works against the voltage sent to the motor. The EMF controls the current flow in the motor. As the motor speeds up, the EMF falls. Less current flows into the motor, and it makes less torque. The motor will stop increasing its speed when the torque matches (is the same as) the drag on the train. To accelerate the train, more voltage must be sent to the motor. One or more resistors are removed to increase the voltage. This will increase the current. The torque will increase, and so will the speed of the train. When no resistors are left in the circuit, full line voltage is applied directly to the motor.”

Electric vehicles

From Tesla website it talks about the power electronic module converting AC to DC when charging and converts DC to AC when driving. When in drive mode it gathers information about how much pressure on the gas pedal, motor speed sensor, ABS speed sensors and other sensors. It can detect if the car is slipping, then produce torque by directing current from the batteries to the motor terminals. Where it can go from 0 to 60 mph in 3.7 seconds.

Airbus 380

"The Airbus A380 first introduced electrical control actuators in civil service to back up the primary hydraulic systems" from p26 of Power electronics: A strategy for success. Areas in which power electronics can be used for A380 are:
  • Motor drives for flight control actuators
  • Electrical starter generator
  • Environmental cooling system
  • Power conversion (e.g. DC/DC, DC/AC converter)