Posted by James Forster on Fri, Jul 23, 2010 @ 03:01 PM
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