Contactor Design for High-Volume RF Te s t i n g

Transcription

1 Contactor Design for High-Volume RF Te s t i n g by Scott Wa rt e n b e rg, Staff Engineer, RF Micro Devices Abstract Central to the design of a highvolume test fixture is the contactor. It acts as the final link connecting the RF test system to the RFIC package. The contactor s design impacts the product s cost, its manufacturing time and general market satisfaction. Designing a contactor requires a combination of several disciplines mechanical, electrical, thermal and functionality. Examined are choices in today s marketplace for contactors used in high-volume RF testing. A number of commercial choices are surveyed. Solutions are compared in terms of their mechanical construction and RF capabilities. I. Intro d u c t i o n February 2003 Figure 1: Basic element of a test head. In the past, high-volume contactors were used principally for testing digital ICs, optimizing their design to transmit pulses and minimize the ripple on the p u l s e s edges. With clock speeds of modern digital ICs exceeding 1 GHz, data rates are moving into the microwave range. Wide-band oscilloscopes are often used to follow a fast signal in the time domain. Yet a wide bandwidth also captures a large amount of noise power, making its dynamic range poor. Furthermore, the oscilloscope provides only scalar measurements. In such cases, frequency-domain methods are more convenient than working in the time-domain. A b e t t e r choice is a vector network analyzer (VNA). Because the V N A makes vector measurements, the sources of error such as fixture parasitics can be fully deembedded from the measurement. In such cases, the frequency domain provides the best data. Applications of contactors include high-volume test, characterization in the lab or burn-in test. IC manufacturers either design their contactors in-house or purchase them from commercial vendors. Cost, ease of use, maintenance and delivery are factors in deciding between contactors. Developing contactors inhouse ensures they will suit the need. They are designed for a specific test fixture application. However, universal contactors are virtually impossible because of the number of package sizes, styles and pad configurations available. Some IC industry trends impacting contactors are: 1 miniaturization reduced pitch increased pin count improved RF p e r f o r m a n c e

2 MICROWAVE PRODUCT DIGEST pg 2 multi-site testing integrated solution speed and flexibility The previous article in this series discussed fundamental issues to consider when designing high-volume test fixtures. This article surveys commercially available contactors for high-volume RF testing. II. The Contactor s Role in the Test System Figure 2: Removable contactor with replaceable contacts. Before discussing contactor design, the role of the contactor in an automated test system should be reviewed. Test systems are configured based on the type of component tested (such as digital, RF or memory ICs), the type of test (DC, RF or functional) as well as the number and speed of tests. Shown in Figure 1, the contactor sits atop a load board which itself is mounted to the side of the auto handler. Depending on the auto handler s design, the load board is mounted horizontally, vertically or at a 45-degree angle. The type of test system impacts the design of the contactor. The test system is outfitted according to the size and type of RFIC package. For instance, automated parts handlers (or autohandlers) used in manufacturing are commonly made by Ismeca, VTEK or Intelmatic, to name a few. The auto h a n d l e r s fittings include trays and boats inside which move the deviceu n d e r-test (DUT), a plunger to insert the DUT into the contactor and a load board to hold the contactor. Ideally, different contactors are compatible with the same load board (see F i g u re 2). The RF test specifications often decide which contactor style is most suitable. Sometimes the DUT calls for special requirements from the contactor. For instance, high-power RFICs need solid grounding. During actual use, it is soldered to a phone board. However during RF test, the individual contacts of the contactor add inductance to the signal path, similar to inductance added by bond wires. In the ground path, inductance can seriously distort the RF performance. With a power amplifier (PA), inductance changes the phase of the RF signal before it reaches ground. Because the ground plane is shared by the input of the PA, an out-of-phase output signal can couple back to the PA s input, resulting in oscillation. For filter testing, excess ground inductance degrades the filter s skirts. III. Contactor Design Criteria Factors that affect the contactor s design are: pitch of the DUT p a d s size of the pads body size molding marks on the DUT The last point refers to flash, mold parting lines or singulation tabs on a D U T s plastic body which leave abrasions and wear on the contactor, degrading its lifetime and performance. T h e contactor is designed to align to the pad layout of the DUT. As its size shrinks, the DUT becomes more difficult to handle and harder to move through the system. Smaller DUTs give less surface area to maintain planarity within the c o n t a c t o r, which makes it a challenge to properly locate them in the contactor, get them planar and apply even pressure across all contacts. The number of contacts in the contactor is important. For instance, a 2mm X 2mm package with four pads is easier to align than one with eight or ten pads. When the package has leads instead of pads, the contactor should take into account the compliance, shape and height of the leads. T h e tolerances on all dimensions should always be considered. In general, the tighter the package tolerances, the better the contactor can be designed to yield more accurate measurements and more demanding tests. Making a highly reliable contact is dependent on two factors: alignment and contact overtravel. This becomes more difficult as the packages become s m a l l e r, the number of pads increases and the pad pitch becomes narrower. The contactor s inner dimensions must allow the maximum dimensions

3 MICROWAVE PRODUCT DIGEST pg 3 Figure 3: Induction coil embedded within the contactor ensures a discharge path for electrostatic buildup on the DUT. of the DUT s body while at the same time minimizing the amount of movement allowed once the part is inserted. With today s leadless packages, registration of the DUT to the contactor is critical. Hence, tight tolerances of the outer dimensions and pad dimensions are important 2. This includes the tolerance of the location of the pads as well as the pad sizes themselves. Combined, these make for a wide range of contact locations. JEDEC specifications of these tolerances are wide, making the job even more diff i- cult. Alignment of the outer dimensions is usually within +/ A loose system means that some of the contactor pins may not mate with the D U T pads. In such cases, an alignment plate on the contactor helps maneuver the part into position. T h e finer the motion of the alignment plate in adjusting the package fit, the more repeatable is the measurement. Packages that are smaller than nominal are a loose fit, making intermittent electrical continuity. When the package is larger than nominal, it can get stuck and jam in the contactor, damaging it. Pressure on the DUT is placed from overhead by an actuating arm or p l u n g e r. The plunger has a vacuum line in its center to hold the DUT. It is also spring-loaded to prevent undue force from being applied. W h e n pressing the DUT into the contactor, the plunger should have an alignment feature to make sure it is properly fitting the DUT into the contactor in terms of pressure and planarity. A n alignment plate often surrounds the contactor for the plunger to align to. The plunger may have a flat nail head to press against the plate. The compression of the contact depends on the force from the plunger while the compression of the contact determines its inductance. Electrical coupling from pin-to-pin depends on the proximity of the ground plane. As the applied voltage decreases, the more the part becomes susceptible to electrostatic d i s c h a rge (ESD). ESD can build up on a contactor by friction during continued contact with inserted parts (see F i g u re 3). Making sure the rubber pickup suction cup in the autohandler is instead made of a conductive material is one solution. Grounding moving parts inside the handler keeps c h a rge from building up. A c o i l embedded within the contactor can also dissipate charg e. Since the plunger is an integral part of the auto handler, the contactor must be designed to properly mate with it. To do so, not only must the contactor be well-designed but also the load board must dock well to the outside of the auto handler. To summarize, the contactor cannot be designed apart from the auto handler, but must be designed to meld in with it. I V. Style of Contacts A good quality contact will return to its original position and spring force even after 100,000s of DUT i n s e r- tions and removals. Contactor pins are ideally constructed to carry high current, low inductance, heat dissipation. The plating should be hard and present a low resistance. Nickel-plat-

4 MICROWAVE PRODUCT DIGEST pg 4 ed gold contacts are common. Maintaining electrical and mechanical integrity during test is important. Low parasitic inductance is essential to high-frequency operation as well as low crosstalk between contacts. Mechanically, the contactor should maintain constant spring force on each pin regardless of the pin count. The force needed to make reliable electrical connection increases with the number of contacts. For instance, ball-grid arrays (BGA) can have a large number of balls on their underside. Unless carefully designed, extreme pressure on the contactor and the underlying test board will be needed. Rigidity of the test board is important, since a board that flexes can cause warpage or even cracks in the vias. Also, excessive pin force causes stress on the DUT, fatiguing the part and impairing its long-term reliability. The part will deform based on the force presented at discrete points by the contacts. The contact resistance R c is affected by the force F, where F is roughly proportional as 1/R c. By using separate contacts, any one can be replaced independently. T h i s reduces cost compared to replacing an entire contactor. When properly maintained, quality contacts are usable for more than 500,000 insertions and removals. While a number of styles are available, most contacts fall under four general catagories: Figure 4: Spring pin and accompanying load broad. - Spring pin - Elastomer - Cantilever - Microstrip lead frame These contacts are each explained in the following sections. A. Spring Pins The spring pin contact is a cylinder, small in diameter with a short spring on the inside or outside. The spring contact is composed of an outer barrel, a plunger and a spring (see F i g u re 4). Sometimes the spring pin has separate plungers at either end. The spring Figure 5: Surface of an elastomer interposer showing pin contacts resembling nail heads. inside forces the plunger against the inner surface of the barrel. In this way, the current flows on the outer surface of the contact and avoids the spring. Gold-plating the outer surface decreases surface resistance. Such a construction maintains a small inductance (<1nH) and resistance (<50mΩ).

5 MICROWAVE PRODUCT DIGEST pg 5 Holes are drilled in the contactor to hold the spring pins. As the spring compresses, the change in its physical length changes its phase length. Even if the plunger in the autohandler presses down with the same force each time, the tolerance of the package thickness means the spring will compress a little differently each time. When the contact area gets dirty, it further complicates the compression picture. The style of receptacle on the spring pin depends on the DUT. For instance, BGAs have solder balls rather than flat pads. A c u p - s h a p e d receptacle on the spring pin works best since it maximizes the amount of contact area on the ball. Pressure causes more area to contact between the ball and receptacle. A pointed tip would deform and puncture the center of the ball. A pointed, crown or serrated tip is better for flat DUT pads 3. In any event, the spring pin leaves an indentation on the DUT pad. On the other end, contact to the load board can be made with a point, crown or serrated tip. The pads on the load board should be rugged enough to accept the spring pin without losing p l a t i n g. In general, spring contacts are selected according to: Figure 6: Cantilever contacts used in leadless DUT testing. D U T pad pitch shape of DUT p a d s n u m b e r of DUT p a d s i n s e rtion forc e rigidity of test board Common spring contacts have <1nH of self-inductance with a length of < They also have low contact resistance (<50mΩ) and excellent r e p e a t a b i l i t y. Most springs compress about 20% of their length. Loose alignment of pins is a major cause of poor repeatability. In general, the looser the pins, the worse the repeatability. In the extreme, loose pins do not contact the pads underneath. They also increase the odds of jamming due to particles getting in-between. Mechanical crosstalk occurs in a Figure 7: Shown at top are solid-metal contacts inserted into a contactor. At bottom are the different designs. fashion similar to electrical crosstalk. When one spring pin physically deflects, it affects neighboring pins. Depressing one pin causes neighboring pins to deflect, too. The deflection is related to the size and placement variation of the spring pin on the load board. Mechanical crosstalk can cause

6 MICROWAVE PRODUCT DIGEST pg 6 false opens in the socket during test. Variation in the flatness of the bottom of the DUT or load board contributes to the problem. B. Interposers An interposer is a series of metal whiskers suspended in a non-conductive, rubbery sheet or elastomer 4. It makes two-sided friction contact, one side contacting the pads of the DUT and the other contacting the pads on the load board. The contacts within the interposer flex vertically. A g o o d interposer material allows for unevenness between neighboring pads or balls. For packages with balls, a hard polymide stopper is mounted on top of the interposer to align the balls to the contacts. The stopper also keeps the balls from severely deforming due to excessive force by the plunger. A n interposer offers finer-pitch contacts compared to other contactor solutions. At the heart of the interposer is a thin gold whisker slightly bent into an S-shape. Initially formed on a flat pad, the whisker and pad are joined and thickened by overcoating with a gold-plated alloy. The choice of alloy determines its durability and spring constant K. Once plating is completed, the contacts are etched free from one another. The size of the whisker (its height and length) determines its electrical performance and wipe motion. A long S-shaped whisker has excessive inductance. Capacitance exists between loops in the S. L a rger metal contacts can be used in place of plated whiskers, overmolded into a single piece with the pad. A similar approach uses miniature springs or columns of conductive particles suspended in the interposer (see F i g u re 5, pg 4). C. Solid-Metal Contacts Solid-metal contacts can be either stamped or formed for a low-cost solution. The purpose of an independent metal contact is to reach up and electrically connect to the DUT s pad. The contacts are secured in the socket Figure 8: Metal lead frame contacts. by solder or another board on back. Some have a cantilever style (see F i g u re 6, pg 5). The design of the cantilever must include a means of overtravel stop. The amount of overtravel should be consistent from one D U T insertion to another. The deflection of the contact must be well-controlled, too. How the contact deforms can be modeled with mechanical software simulator tools. Carefully considering its design will maintain a reliable electrical interface over thousands of insertions and removals. Shown in F i g u re 7 (pg 5) are gullwing-shaped contacts developed by Gryphics/Molex and an alternate style by Plastronics 5. V. Wiping Action of Contacts When metal contacts wipe across the D U T s pads, the wiping action serves to break through oxide film built up on solder pads 6 and to self-clean the contacts. With solder pads, not enough wipe results in a higher contact resistance and overall poor connection. Penetrating the oxide depends on the shape of the contact and the wiping action. In particular, a 45-degree contact pin grabs more solder than the other styles of pins 7. Metal lead frame contacts are one means of wiping across DUT c o n t a c t s (see F i g u re 8). When wiping across s o l d e r-tinned pads, solder can transfer from the pad to the contact. Once this occurs, wiping becomes less e ffective 8. As the oxide clings to the s o l d e r-covered contact, the contact resistance R c increases. R c will continue to increase with insertions as the solder builds up. In such instances, socket cleaning and maintenance become especially important. Easy and effective cleaning is imperative. Contactors are often qualified by how often cleaning is required and how easy it is to replace individual c o n t a c t s. VI. Thermal Considerations Another special area are fixtures which require cooling. Extremely high-power RFICs, such as SiC or GaN amplifiers used in base stations, can dissipate 50W or more. RF power amplifiers (particularly ones on GaAs substrates) must dissipate heat. A method to bring cooling close to the contact area is necessary. Otherwise thermal failure can occur. As the pin count increases and the pad pitch decreases, the heat becomes more concentrated within the contact o r. One solution is to pulse the power

7 MICROWAVE PRODUCT DIGEST pg 7 to the part 9. Pulsing the bias and RF to the D U T is one way to avoid heat buildup. A low duty cycle lowers the average temperature of the DUT. Heat sinks reduce the temperature by pulling heat from the DUT and transferring it to cooling fins. Made of a conductive metal such as copper or aluminum, the heat sink makes good thermal contact to the DUT. To do so, the heat sink is mounted in the center of the contactor with fins pointing to the bottom. A i r flow across the fins (measured in feet per minute) keeps them cool. A f a n under the contactor blows air. Sometimes the contactor has a hole in its side to act as an air intake. Because the DUT is not perfectly flat, the heat sink often does not make continuous contact across the entire bottom surface. Rough surfaces are particularly poor at transferring heat. When physical contact is not made, heat is transferred through convection. Thermal grease helps to fill in the voids in this case (see F i g u re 9). Recall that conduction is the transfer of heat across two interfaces in contact while convection is heat transfer between a physical material and a fluid or air. In addition to the thermal conductivity of the materials involved, other factors affect the thermal resistance, such as the compliance of the surfaces, the materials t h i c k n e s s e s, surface wear and contact pressure. When using a thermocouple, knowing the thermal resistances of all the materials involved is crucial to determining the thermal gradient to the DUT. VII. Strip Te s t i n g RF test can be a bottleneck when producing a large number of components. A wafer produces thousands of die at once, the die are packaged in strips of a few dozen packages and final test is usually done one DUT at a time. Strip testing is one solution. It allows packaged DUTs to be tested while the packages are attached in a single leadframe or strip. Depending on the test setup, more than one unit can be tested. Testing can be done simultaneously on a number of units at once. Anumber of contactors are Figure 9: Making thermal contact. The top figure shows the thermal path between rough surfaces, which includes both conduction and convection. In the bottom figure, applying thermal grease provides an improved path. laid out in parallel to create a strip test interface (see F i g u re 10, pg 8). For testing multiple DUTs, a number of contactors are attached to a single load board. To accommodate, the load board can be a dual site, quad site or more. Strip testing requires less handling than single-unit handlers. Testing multiple units at once increases throughput, reducing the amount of work in progress. The drawback is that contactor alignment and planarity becomes orders of magnitude more difficult, since several contactors must be aligned to the DUTs at once. Alignment error will affect all of the units tested on the strip. Also, the contact force is increased substantially to contact all DUTs. For multi-site testing, an interposer can be formed on a silicon wafer to match the DUT pads on the wafer 1 0. In this way, whole wafers can be contacted and tested in a single motion. The disadvantage is the force required to clamp down on the wafer, not to mention registration to the pads. VIII. Summary F u n d a m e n t a l l y, the contactor serves as the transition from the test system s RF ports to the pads of the RFIC. Well-designed fixtures minimize impedance discontinuities to the

8 MICROWAVE PRODUCT DIGEST pg 8 R F I C s pads. Any of the contact choices described herein allow customizable geometry. The socket itself is made of a hard elastomer that can withstand hundreds of thousands of insertions. Key parameters for a contactor are: - R F p e r f o r m a n c e - Durability of construction (long life) - Mechanical variations with wear - Load board reuse/modularity of c o n t a c t o r - Use manually or with automated t e s t e r s - Easily serviced - C o s t Miniaturization means the package has a smaller footprint where the dimensions are smaller and the pad pitch tighter. Developing the contactor becomes even more challenging. New IC processes may extend the horizons of RF performance, yet the contactor can be the limiting factor in what they can offer. Scott Wartenberg is also the author of the book, RF Measurements of Die and Packages, by Artech House, R e f e re n c e s 1 Iraj Barabi and Mehdi Attaran, I n t e rconnect Solutions for ATE, IEEE Burn-In and Test Socket Workshop, pp , David Pfaff and Marc A b e l a n e t, Package Tolerance of VFQON: Effect on Socket Design, IEEE Burn-In and Test Socket Workshop, pp. 4-19, Figure 10: A dual-row, 3x2 strip tester. 3 Susumu Kasukabe et al., Contact P ro p e rties of the Spring Probe for Probing on a Solder Bump, IEEE Holm Conference on Electrical Contacts, pp , 1992, 4 D a - Yuan Shih et al., New Ball Grid A rray Module Test Sockets, IEEE Electronic Components and Technology Conference, pp , James Rathburn, Lowering the Cost of High Performance Test and Burn-In, IEEE Burn-In and Test Socket Workshop, pp , Jochen Kuhmann et al., Oxidation and Reduction Kinetics of Eutectic SnPb, InSn, and AuSn: A Knowledge Base for Fluxless Solder Bonding Applications, IEEE Transactions on Components, Packaging, and Manufacturing Technology - Part C, Vol. 21, No. 2, pp , Francois Billaut, Geary Chew and Ken K l a r k i n, Characterization of High Performance Contactors for P roduction RDRAM Chip-Scale Package Te s t, IEEE Burn-In and Test Sockets Workshop, pp , Robert Malucci, The Effects of Wipe on Contact Resistance of Aged S u r f a c e s, IEEE 40th Holm Conference on Electrical Contacts, pp , Sebastian Nuttick et al., Study of Self-Heating Effects, Te m p e r a t u re - Dependent Modeling, and Pulsed Load- Pull Measurements on GaN HEMTs, IEEE Transactions on Microwave Theory and Techniques, Vol. 49, No. 12, pp , Dec John Novitsky, Using Microspring Contacts as Second Level Interc o n n e c t, IEEE Burn-in & Test Socket Workshop, 2000, pp Thorndike Road Greensboro, NC Tel. (336) Fax (336) TM, & 2002, RF Micro Devices, Inc. 1,000 02/03