Optimizing Contactors for High-performance Test Sockets
MOVING BEYOND POGO PINS
BY NICK LANGSTON, NICK LANGSTON JR. AND.HONGJUN YAO
A socket is a critical element in the semiconductor product development and test environment. Different classes of sockets provide specialized electrical and mechanical parameters needed for specific steps in the product testing process. Within high-performance sockets, the pogo pin has remained the contact method of choice for 90 percent of applications. For high-speed serial links and emerging radio-frequency (RF) applications, however, engineers must replace the familiar pogo pin with more exotic contactors to ensure signal integrity and reliable performance (Figure 1).
Sockets vary widely in electrical and mechanical performance needed to support diverse test requirements within the overall product development process (Table 1). For example, burn-in sockets tend to be used in applications where the test signals are rather low frequency, usually a few kHz. Burn-in test times are long and the temperatures are high, usually 125°C. The goal in this application is to detect device failures based on extended thermal stressing for 24 to 72 hours without removing the part. To generate a statistically good reliability model, large numbers of devices need to be screened and as many as 10,000 sockets typically are needed. Consequently, the burn-in socket has contacts that are stamped, for high volume economic reasons, and it is designed to capture the device into the socket without the use of a clam-shell-style lid.
Figure 1. Comparison of standard pogo pin with a high-frequency pogo pin and ultra-high-frequency elastomer contacts used in test sockets.
On the other end of the socket spectrum, test sockets are individually machined to deliver outstanding electrical and precise mechanical placement and guiding. A test socket's primary function is to transfer the signal between the device under test (DUT) and the system with the least amount of signal attenuation or degradation. To maintain good signal integrity, especially in automatic test equipment (ATE) environments, controlled impedance 50-Ω transmission allows the cleanest signal transfer and minimum of reflections, and 50-Ω contactors are commonly used for testing of gull wing package testing.
Table 1. Socket types.
High-frequency gull wing contactors have controlled impedance contacts developed in flex circuit or microstrip techniques. The disadvantage for high-speed digital devices is the trace length is longer and there may be too much trace inductance in the ground/power contacts causing excessive switching noise and ground bounce. Furthermore, with the introduction of fine-pitch area arrays, engineers could not get the 50-Ω socket contact into the inner array.
Pogo pins have been considered the best compromise for suitable electrical performance and mechanical reliability. For arrays, however, pogo pin approaches cannot achieve a 50-Ω contact in area array devices, because it is virtually impossible to make a pin that would have a characteristic impedance of 50 W and fit into the tight pitches currently used.
For area array devices such as BGA, QFN or LGA, it is impossible to fabricate 50-Ω coaxial contacts at the necessary pitches of 0.4 to 0.65 mm. The resolution to the problem is to make the pogo pin as short as possible. For example, a 2-mm pogo pin results in low parasitics and can support 20 GHz bandwidth applications. At these high frequencies, however, mutual parasitics (L+C) will introduce excessive mutual coupling between test contacts.
From a mechanical perspective, the socket contact length should be a short as possible while having 0.010 to .030" of Z compliance. For BGA sockets, it is common for socket makers to use a "floating guide plate," which is a suspended plate with holes that allows the balls to fall in and this acts as a positioner for the DUT, before the device is compressed against the contact. Today's automatic handlers are able to position devices precisely, so the floating plate is going the way of the 1.2-mm pitch device.
For the last several years, test socket manufacturers have been using double-ended spring pins for the bulk of testing of QFP, BGA and LGA packages. The pins can be used for any pitch of device from 0.15 mm for CSP and KGD to more standard 1-mm pitch for common BGA packages. Sockets with spring pins can be built easily for devices having pin counts in excess of 3,000 I/O.
To get maximum electrical performance and signal integrity, the contact length generally must be short. If the contact is too short, it sacrifices the necessary compliance to accommodate the non-planarity of the package of the DUT and also good oxide penetration of the package lead.
Pogo pin suppliers are now supplying pins with a single moving plunger in an effort to reduce the overall length of the pin and maintain compliance. With its sharp, crown-top plunger, a suitable pogo pin can penetrate the oxide with very little force (<20 g) and not cause damage to the package leads/pads or to the balls or leave unwanted "witness" marks on the DUT or the PC board. Force of 40 to 60 g on the lead can often leave a witness mark that may cause an early failure in the contact with the board when soldered in place.
Pogo Pin Performance
When considering the electrical performance of pogo pins, an evaluation should be conducted by an independent test lab in the exact configuration the pogo pins are planned to be used. The primary values to test for are bandwidth, rise time, self inductance, capacitance and the mutual inductance and capacitance. With this data, it is straightforward to calculate the effects of the pin on the rise times and signal attenuation.
Pogo Pin Limitations
In applications such as 802.11b devices, even pogo pins as short as 2 mm can result in radio-frequency (RF) adjacent-channel interference. Using a pogo pin for these devices, the 2.8 GHz RF signal radiates to adjacent pins. RF devices are shrinking in size to fit into the wireless portable applications. A popular package for these RF devices is the QFN/MLF package with body sizes of 3- to 5-mm square and with pads of 0.5-mm pitch. In a socket it is difficult to maintain isolation between the device pads that are so close when you use spring pins. At 2 mm in length, there is too much radiation and interference between pins. The most effective way to transmit the RF signals through the MLF-type socket, without interference or leakage, is to use the super conducting polymer contact that is only 0.3 mm in overall length before compression. The superconducting polymer contact is compliant and can be compressed too much, so the contact sheet is designed with built-in overtravel protection.
Digital circuits are sensitive to crosstalk generated by fast signal edge rates that are common in the high-speed serial links like XAUI, PCI Express II and OC-192. Using short pins of 2 mm or smaller can still result in reflections caused by the impedance mismatch with the board and the DUT, but they will generally eliminate the more serious cross talk issues. A problem might be cost, where the pin prices will jump from $2 for a 5-mm long pin to $4 or $5 for a 2-mm pin. In RF, however, engineers face problems due to signals radiating from pins. For RF applications, reducing contact length is often the only method to reduce interference between pins. A pogo pin of 2-mm length may be too long. For the system to operate correctly, the contact length has to be less than 1 mm.
The issue of pin length is not a difficult obstacle for typical digital applications. However, the highest speed serial interfaces are pushing to go above 10 Gbps, with rise times of less than 20 ps. This performance dictates bandpass in excess of 20 GHz (BW= 0.35/Tr). Vendors are developing new methods of contact to replace pogo pins to provide this kind of needed performance.
Figure 2 shows a new type of contactor comprising six different springs interwound at different angles. The resulting contactor is only 30 mil in length, but can be compressed 15 mils. This 50 pecent compliance makes it useful for any BGA, MLF or board-to-board application. Other all-wire contact methods continue to extend the application of spring contacts for high-performance applications.
For RF applications, conventional contactor methods reach the edge of the required performance envelope (Figure 3). RF signals radiate from small QFN/MLF packages and induce RF coupling in other channels. For high-precision test applications, individual channels must be isolated, which requires shrinking the size of the contacts. Currently, the most effective isolation method is the use of superconductive polymers.
To improve the signal integrity, several socket vendors have developed elastomeric contacting schemes (Table 2). Several of the oldest technologies are the Shinetsu MT Matrix (SMM) that has been used in sockets by Liberty Research for more than 10 years and is continuing to be used by several socket makers as a low inductance contact. Some others include: Fuji Poly; Tyco Electronics MTI silver particles; and Paricon Technologies interposer called "Pariposer," originally developed at Bell Labs.
Table 2. Elastomer approaches.
Fuji polymer uses C-shaped wires, which can take many insertions. The steel wires, however, can be problematic for RF applications because of their higher resistance. The wires themselves are placed vertically within the substrate, so that after a number of insertions the wires tend to squeeze out — limiting their use in production. Shinetsu MT Matrix elastomer uses hundreds of small wires at a 60-degree angle. BGA balls contact the wires, which in turn contact the DUT board below. Because the wires are made out of brass and angled in the elastomer, this approach provides an improved lifecycle, but the offset introduced by the angled wires introduces a further complication.
The superconductive polymer contact sheet approach has proven to be the most production-worthy of the compliant non-metal contacts, providing 100,000+ insertions. The lifetime can be extended to 300,000 insertions with a protective top hat. But this addition extends the length to about 35 mils, which can be a problem for RF applications.
Although pogo pins are preferred for most applications, leading-edge applications bring test requirements beyond the capabilities of conventional pogo pin solutions. By moving to elastomer and novel contact methods such as highly conductive polymers and new solid wire forms, high-performance test sockets can support the performance requirements of emerging test applications.
NICK LANGSTON, director of business development; NICK LANGSTON JR., CAD engineer; and HONGJUN YAO, technical staff member, may be contacted at Dimensions Consulting Inc., 3350 Scott Blvd., Building 58, Santa Clara, CA 95054; (408) 988-6800; e-mail: firstname.lastname@example.org; email@example.com; firstname.lastname@example.org.