WELLS Connected

Blog Title: “Sockets 101- Connecting to the board?”

Today, as test engineers struggle with the requirements and design details of a burn-in board, they must make a decision on how to socket the DUT (Device Under Test).  The last blog tried to define a socket and I shared my definition that a socket is:

A device which provides a temporary mechanical and electrical interface to allow the testing of a semiconductor package.

But as Ron Popeil, the famous late night TV salesman says, “but wait, there’s more” ….   The socket selection process includes all the commercial issues of:

• Supplier selection
• Approved vendors
• Availability
• Cost and delivery, etc.

 AND the technical issue/decisions of:

• How the socket interfaces with the board
• How the socket interfaces with the DUT

As part of “Sockets 101” and the discussion of the how’s and why’s of sockets, this blog will focus on how sockets are connected to the test board.  We will examine how the burn-in socket interfaces with the package or DUT at a later time.

There are 3 ways a socket is connected to a BiB, (burn-in board) or test board:

• Thru-hole mount
• Compression mount
• Surface mount

 I will describe each of these below and discuss the advantages and disadvantages of each.

Thru-hole mount

This is the conventional way electrical components are permanently attached to a printed circuit board.  The contacts are longer than the body of the socket and are visible on the bottom of the socket. These tails fit into plated thru-holes and a low melting point alloy is reflowed and joins the contact “tail” permanently attaching the socket to the BiB.

 

The advantages of this format include:

• A reliable and robust mechanical and electrical interconnect.
• A proven technology

Disadvantages of this format include:

• Difficult to repair
• High cost boards as the pitch decreases – One customer indicated that for thru hole burn-in boards at 0.4mm are 6X the cost of 0.5mm pitch 

Compression mount:

This method of attaching a socket to a BiB gained popularity as package I/O pitches were reduced to 0.5 and below.  At 0.4mm pitch this is the preferred socket attachment method for area array packages. 

The key to compression mount is that the contact is not permanently attached to the BiB.  Instead the contact is pressed against a pad on the BiB and the electrical interconnect is provided by the force between the contact and the pad.

This method is shown schematically below:

 

The advantages of this format are the ease of removal and replaceability of the socket should the contact be damaged.

The disadvantages of this format include:

• The socket provides the mechanical attach via screws which mount the socket to the BiB.
• Smaller contact area where the contact interfaces with the BiB.  This will result in higher contact resistance and can impact the thermal transfer of heat from the socket to the BiB.
• There is still some concern about the reliability of the interconnect to the board.
• To maintain good electrical contact with the BiB it is preferable that the contact is always being forced against the pad on the BiB – i.e. the contact is pre-loaded against the BiB.  This preload is typically 3 to 5 grams.
• For high I/O counts the preload of the contact to the BiB can cause bending of the contact assembly in the socket.
• Larger sockets with high I/O’s may require a “backing plate” because the contact force can bend the BiB. This backing plate can compromise the available area for mounting passives as close as possible to the contact for signal integrity.

Surface mount:

This method of attachment is commonly used by the printed circuit assembly houses to attach electronic packages to a board. The schematic below illustrates the basic elements which include the permanent attachment of a lead to the board by soldering to a pad on the surface of the board. 

 

The major advantage of this assembly technique is higher density and the ability to populate both sides of a PWB.  For the burn-in socket user, utilizing both sides of the BiB for sockets is not a factor.

Due to concerns of inspection and replaceability this attachment method is not used.

Summary:

While thru-hole sockets still represent the major method of attaching a socket to a BiB, compression mount is gaining favor. This is driven by the difficulty and cost of fine pitch thru-hole BiB’s and increased confidence users have in this format since they began using it for finer pitch packages.

I am not aware of any burn-in sockets which use a surface mount attach.

The next blog will discuss different types of sockets – open top and clam shell.

Portions of this Blog were taken from a presentation I co-authored with Prasanth Ambady, and Jason Cullen titled: “Contact Technology For 0.5 mm Pitch and Below”, presented at the BiTS Workshop 2003.  See: http://www.bitsworkshop.org/archive/archive2003/2003s7.pdf

For those not familiar with soldering I'd suggest doing a search of YouTube, there are many examples of soldering.  Here are a couple showing the basics of thru-hole and surface mount soldering:-

Lead Free Through Hole Soldering Tips.
http://www.youtube.com/watch?v=CGnt2vCAjfQ

Lead-Free Multi Lead Soldering: http://www.youtube.com/watch?v=vZ1qisX52rI

 

If you have any comments or thoughts please post a reply or send me a note at james.forster@wellscti.com

Sockets 101- What is a socket?

With the semiconductor industry improving, finding time to write this blog is certainly more difficult than I imagined.  With that said this is the first of a series on understanding burn-in sockets, essentially an introduction to the technology of burn-in sockets or perhaps Sockets 101 or, in the current vernacular, A Sockets 411.

So what is a socket?

The Oxford English Dictionary defines socket as...

1. a hollow in which something fits or revolves.
2. an electrical device receiving a plug or light bulb to make a connection.

Wikipedia has more than 10 different types of socket -

Certainly all the motorheads would think of a socket wrench where the sockets are the business end to help remove nuts and bolts.

Medical professionals would think of a the variety of sockets and joints in the body such as eye, hip, knee, elbow, foot or shoulder where a socket allows a variety of human movements.

   

If we think about our homes and our electrical applicances such as toasters, coffee makers etc we have all used electrical wall sockets and are familiar with the plugs which provide a temporary connection to the houses electrical supply . 

The images here show a variety of different sockets and plugs from around the world.

        

Unfortunately there is no international standard so travelers can be frustrated as they try to use their computers, hair dryers or other electrical appliances.  An international "adapter socket" has even been created which allows these devices to be "plugged" into the sockets in the foreign country, even if the voltage and current may not be compatible

We are also familiar with the different "sockets" on a computer which provide a place to connect a variety  of different "peripherals" such as printers, monitors etc so that they can communicate and talk to the computer. USB is now the dominant "plug" but even within that there are different formats sizes.

In all of these cases the socket holds the plug and provides an electrical connection to allow the passage of an electrical power or a signal.  This is very different than soldering which is a method to permanently attach the two.

When testing a semiconductor packages it is necessary to hold the device and provide a temporary electrical connection.  So for a burn-in socket my definition would be:-

A burn-in socket is a device which provides a temporary mechanical and electrical interconnection to allow the testing of a semiconductor package.

The key words in this definition are temporary and electrical interconnection. 

The socket must hold the package or device under test (DUT) while it is being tested.  Once the test is complete the package must be undamaged and easily removed so that another device can be inserted.

The next blog will discuss how the socket is connected to the test board.

The picture below shows some of our burn-in sockets

 

 

If you have any comments or thoughts please post a reply or send me a note at james.forster@wellscti.com

BiTS – Our Annual Workshop

It's January and the east coast of the USA is experiencing the normal cold, wet and sometimes snow.  But wait, March is approaching and the e-mails titled ... "BiTS - are you going?" have already started. 

So what are you doing March 7-10?  Check out the advance program at:  http://www.bitsworkshop.org/advprog/advprog.htm Phoenix in March is warm and for people from the colder climates, you can remember what the sun feels like.

As usual Fred Taber and his team of volunteers are working hard to make sure the workshop is a success. Previous attendees will recognize the names of some of the stalwarts of the workshop who are presenting the latest on their work.

This year I did not submit any abstracts so am not working late at night finishing a presentation or promising myself ... "I will never submit an abstract until all the results are complete and I actually understand all the data" - although a friend has asked me about a couple of pictures from a prior presentation he would like to use; I'll get to that tomorrow.

Last year BiTS celebrated its 10^th annual session.  The attendance was a down a little but the people I spoke to enjoyed themselves and comments were generally positive about the program and the workshop.  Certainly there were kudos to Jon Diller for his tutorial titled Sockets 101.  The 36 slides on the BiTS Workshop archive (http://www.bitsworkshop.org/advprog/advprog.htm ) are dry and cold and unfortunately do not convey the enthusiasm and style of Jon's presentation.  Those that attended had the opportunity to learn as Jon shared his knowledge and experience in his own unique way - and that's what the workshop is all about, sharing and learning.  I hope to see you there this year.

So to all the invisible volunteers who help make the BiTS workshop a success - thanks.  I looked for a suitable quote about volunteering and found this which seems apropos, although I do not know who Elizabeth Andrews is....

Volunteers do not necessarily have the time; they just have the heart. 

- Elizabeth Andrew

If you have any comments or thoughts please post a reply or send me a note at james.forster@wellscti.com

“If you build it he will come” …. The First WELLS-CTI Blog

It's 2010 - Happy New Year - and this is my first "WELLS Connected" blog.  The people helping me to understand this new media tell me .... write the blog and people will be interested - It's a sort of "write it and they will read it" type of analogy - thanks to the  movie "Field Of Dreams".

This blog will discuss issues and topics associated with burn-in sockets and thermal management.  Our hope is to provide another avenue to communicate with customers, friends and others interested in sockets and interconnection.

While we will focus our discussions on interesting technologies, where our industry is headed, contacting technologies, etc., we will also discuss other topics which are peripheral to our industry and which I think are interesting and stimulating.

Cell phones, PDA's, notebooks and other mobile devices are a significant market segment for our products.  These devices not only allow us to communicate but are at the forefront of the communication revolution and every day brings new and exciting products.   Now for a shocker - that movie, "Field of Dreams" was released in 1989 - 20 years ago!  Today we could not imagine our lives without a cell phone - in 1989 there were about 3 million cell phone subscribers and the technology was about to change the way we live.

Phones were analog and the first ads for cell phones were appearing. Take a look at the following links to see how far the industry has advanced.

http://www.tecfre.com/video-first-cell-phone-commercial-in-1989/

General Electric Cell Phone 1989

http://www.youtube.com/watch?v=nAeBlL1zuko

Cell phones were "bag phones" in those days and the costs were in the hundreds of dollars.  It would only be a few years though before cell phone service companies developed the idea of giving away the equipment, the phone, for a commitment to a service contact lasting 2 years.

Handheld phones were not commonplace - but 1989 saw the introduction of the Motorola MicroTac whose design, it is said, was based on the "communicator" used by Captain Kirk and others in the 1966 TV series - Star Trek ....

The cost of the MicroTac - about $3,000..... and those were 1989 dollars. 

In closing here is an interesting quote about our technology from Sergey Brin, co-founder of Google ...... 

"As we go forward, I hope we're going to continue to use technology to make really big differences in how people live and work."

So welcome and let's blog.  If you have any comments or thoughts please post a reply or send me a note at james.forster@wellscti.com