IC Test Socket Contamination

Knowledge Base Article # Q200240

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Summary This article explain the effects of IC test socket contamination on contact resistance with different lead platings
  
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Introduction

One of the most important, if not the most important connection between the TS-900 Semiconductor tester and the DUT is the test socket on the load board.

Because of the nature of this connection, the constant insertion and removal of devices in the socket, it will deteriorate over time and compromise the connection which is critical to the overall integrity of the test. The damage occurs when devices are inserted and removed from the test socket and is caused by metallic residue buildup on the test socket contacts from small particles from the lead plating of the DUT.

These particles are transferred and get embedded on the surface of the test socket contact; these particles become resistive and eventually degrade the connection between the test socket pin and the DUT lead.

Another factor is the wear out of the plating on the test socket contacts themselves. This wear out mechanism happens because of the extreme hardness of the plating on some DUT leads. Repetitive insertions and removal of the DUT will erode the test socket contact plating requiring more and more downward pressure be applied to the DUT to make contact with the test socket, eventually the connection will become intermittent or fail altogether.
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The most common plating processes for IC leads, after the ROHS directive became effective in 2006, are matte tin (Sn) and nickel-palladium-gold (NiPdAu) for leaded and pad packages and tin-silver-copper (SnAgCu) for balls on ball grid array packages.

Matte tin plating is the most widely used lead plating for leaded and pad packages because of its low cost and availability; extensive research has been conducted by the packaging industry, semiconductor manufacturers and test socket manufacturers and the processes for using this type of platting are well understood.

Because matte tin plating develops an oxide layer almost immediately after exposure to air, it is necessary to mechanically break through this oxide to make a good, reliable contact. In older test socket designs, those used to test pre-ROHS tin-lead (SnPb) plated leads, the contacts just presses against DUT lead with little to no mechanical scrubbing action. When this type of socket is new the contacts will penetrate the oxide layer on the tin plated leads; however as the number of parts inserted in the socket increases very small particles from the lead plating will buildup on the surface of the contact (these particles will immediately begin to oxidize and become resistive), and after a number of insertions this residue build up will cause a resistive contact. This resistance is now in series between the DUT and the TS-900 test equipment and will cause intermittent to full contact failure. On devices that draw significant amounts of current, the voltage drop developed across this resistance can cause damage or destruction of the test socket, the load board and the DUT.

Effects of Contamination on Contact Resistance

Device Plating 2mm

Effects of Pb-Free Device Plating –Matte Tin (Courtesy Johnstech)


Device Plating 4mm
Effects of Pb-Free Device Plating –Matte Tin (Courtesy Johnstech)


On devices with NiPdAu lead plating, because of the hardness of this plating, the contact degrading mechanism is the wearing out of the plating on the test socket contact surface over time. This exposes the base contact metal and a resistive contact develops because of oxidation. Balls in BGAs will also develop an oxide layer because of the tin in the SnAgCu plating. The pogo pin contacts on BGA sockets will get contaminated with these metallic oxide particles and the contact will also become resistive.

Contact Resistance
Effects of Pb-Free Device Plating –NiPdAu (Courtesy Johnstech)


New Socket Technologies

New socket technology designs addresses this oxide layer issue by creating contacts that mechanically wipes across the surface of the lead. The socket contacts are plated with a hard metal alloy which will displace the oxide as it slides on the surface of the lead. Some test sockets are manufactured with pins made of solid hard alloy metal; these pins will last longer because as the contact metal wears out, its surface metal stays the same.

New Socket Technology
New socket technology with movable contacts and force elastometers  (Courtesy Johnstech)


Effects of Contact Insertions

1 insertion10 insertion50 insertion  
Device I/O Surface Oxide Penetration and Removal: SnPb 1, 10 and 50 insertions (Courtesy Johnstech)


Test socket manufacturers recommend that test sockets be cleaned after a certain number of insertions. Each manufacturer has its own cleaning procedure and most sockets can be rebuilt and contacts can be replaced if the they become unusable. Test sockets used in the production test floor should be placed under a preventive maintenance schedule to minimize contact deterioration due to tin oxide buildup and wear. This will maximize your first-time-pass yields and minimizes the amount of product re-test. Taking care of the test sockets will guarantee that they will perform consistently and reliably for a long time.

Test Socket manufacturers and suppliers

Johnstech
Loranger International
JF Technology Berhad
Aries Electronics, Inc
Ironwood Electronics
Emulation Technology, Inc.
Advanced Interconnections
3M Textool
Custom Interconnects

Copyright

All Graphs and Pictures copyright and courtesy of Johnstec
Article Date 4/25/2013 , 6/1/2021
Keywords Production IC Test Socket, IC Test socket problems, IC Test socket


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