By Derek Schultz
 Figure 1. A flexible circuit with contact pads. Each little dot is a pad. |
A connection method using palladium dendrite plating technology offers benefits including low resistance, inductance and capacitance.
Flexible printed circuits are not new to the electronics and packaging industry. The requirements and applications for flexible circuits, however, have increased dramatically in recent years. Leading-edge product designers are constantly looking for ways to make things faster, lighter, smaller and with more functionality. Flexible circuitry is becoming an integral part of electronic products design because of its dynamic flex capabilities and its superior means of interconnection within electronic systems.
When considering interconnection between printed circuit boards (PCBs), display panels, sensors and other components, the choice and method of termination is a critical factor in providing system integrity and performance. There is an array of termination methods available including solder, mechanical crimping and conductive adhesives.
Palladium Dendrites
In addition to these traditional choices of termination, there is another, relatively unexplored, solderless connection method that uses palladium dendrites plated on circuit or contact pads. This Velcro-like method of termination provides high-density connections with improved electrical performance.
Palladium dendrites grown on copper contact pads can provide a Z-axis, direct-contact connection to a mating circuit pad plated with gold or tin/lead solder with as little as 30 g of pressure. A connection of this type can provide contact characteristics such as resistance as low as 3 to 5 mW, inductance as low as 0.3 nH at 2.03 GHz and capacitance of less than 0.0065 pF. Connect/disconnect testing to 5,000 cycles has shown no wear or destruction to the dendrites and negligible wear on the mating gold surface. In fact, laboratory testing to 16,000 cycles did not affect the integrity of the connection or the electrical requirements.
Exactly what are dendrites? In conventional plating processes, dendrites are "uncontrolled" growth and normally considered a yield detractor. The growth can occur when the plating bath chemistry is out of specification or when the parts are in the plating bath for an incorrect amount of time. It is an uneven buildup of plating that can bridge across circuit lines creating shorts and can also lead to nodules, which are an uneven surface. However, DendriPlate technology, originally developed by IBM Corp., makes use of the uncontrolled growth. IBM originally developed the DendriPlate connector to meet the need for a connector exhibiting superior electrical performance, including the elimination of noise and resistance in the electrical connection.
The flexible circuit (see Figure 1) is designed and manufactured using a roll-to-roll process and excised into panels for the final process steps. While still in panel form, a mask is applied with openings on the pads where the palladium dendrites are required. First, a base of 30 to 40 min nickel is plated over the exposed copper. Next, a smooth layer of palladium, typically 30 to 40 min, is plated over the nickel. Following this, palladium is electrodeposited onto the exposed pads. This is accomplished by concentrating electrical current on the exposed pad area, letting the needle-like dendrites grow 20 to 40 mm, yielding an overall dendrite height of approximately 100 mm above the original copper surface.
The process is meticulously controlled by the stringent specifications of the plating bath and the length of time the parts spend in that bath. Palladium was chosen because it facilitates the growth of dendrites and is a hard metal. Thus, the dendrites are hard and can penetrate the softer gold pad they mate with for a good connection.
The dendrites on contact pad vary in size with a maximum spike height at 50 mm. There are approximately 100 dendrites per 0.64 mm-diameter pad. Figure 2 shows random dendrite growth patterns.
Contact Methods
The unique shape and randomness of the dendrites allows for contact with relatively low pressure applied to the backside of the contact pads. As a pressure or pressfit connection, there are two methods to make contact between mating surfaces — dendrites on dendrites or dendrites on a flat pad.
 Figure 2. Cross-section of an individual contact pad. The needle-like palladium dendrite spikes can be seen. |
Contact resistance with dendrites on dendrites is slightly higher than dendrites on a pad. The dendrites on dendrites method makes contact by sliding the upright, elongated shapes against each other side by side, making contact as well as pressing the longer points into the valleys between the dendrites on the mating pad.
The dendrites on a flat pad method makes contact by dendrites being pressed against a flat gold or tin/lead-plated surface. With a gold mating surface, the electrical connection is made by point contact.
Constant contact with the gold surface is achieved by applying pressure as low as 30 g, with a thin, high-compliancy elastomer on the opposite side of the dendrite-plated pad. The dendrite tips press tightly against the surface of the gold, making slight indentations at the contact points.
Contact to a plated tin/lead solder finish actually penetrates the surface, sinking the dendrites into the solder. By penetrating the surface finish, contact is made on the tips and sides of the dendrites, which can provide a gastight connection across the pad.
Another attribute assuring a highly reliable electrical connection is the self-wiping action of the dendrite-plated surface. When the needle-like dendrite spikes mate with the gold pad, the spikes actually pierce the gold, making a good electrical connection. This mating action self-wipes, or cleans off, debris and dust that in traditional pin connections can affect the integrity of the connection. In a flat-surface to flat-surface connection, tarnish or dust and fiber particles can cause a dielectric between mating surfaces. As a noble metal, palladium is resistant to oxidation and corrosion and the dendrites actually penetrate the contamination in a Z-axis motion. The random high and low points also overcome light dust/fiber contamination. Testing shows the contact pads to have a fiber and dust particulate tolerance up to 0.5 mg per sq in.
Applications
The DendriPlate connection method has been tested and used in many different applications including integrated circuit (IC) test sockets, IC burn-in sockets, and board-to-board, module-to-board and card-to-card interconnects. With high-cycle, field-separable connection capability, low-pressure test connections can be designed for manufacturing or field conditions. Z-axis pressure connections can eliminate flat-blade or pin-type plugs where insertion cycles are limited by surface finish wear or spring tension loss because of metal fatigue.
 Figure 3. CPOP connectors. |
Direct, low-pressure contact connections to flat panel displays can eliminate the need for isotropic adhesives. Consider a display frame where the glass can be set in place and safely clamped down, eliminating the need to open a unit and make connection by zero-insertion force (ZIF) or some other plugging method. A drop-in display could eliminate the cost of assembly of a flexible circuit to glass. Furthermore, packaging or shipping trays can be reduced in size because there will be no pre-attached flexible circuit.
Another application is test probes. Dendrites on contact pads used for test points on a flexible printed circuit provide a slip-free surface to ensure solid contact.
Compression pad-on-pad (CPOP) connectors are another use (see Figure 3). These high-performance, 90°, direct-pressure connectors are offered with 300 and 96 inputs/outputs (I/Os) and may be coupled for additional I/O capability in backplane assemblies to connect motherboards and daughtercards.
The DendriPlate connection method provides low resistance, inductance and capacitance, while affording a high cycle of self-wiping connect/disconnects.
For several years, this Z-axis flexible interconnect has proven successful in the high-end super computer environment.
Now, it may be just the clever technology to take flexible printed circuits to the next level.
DEREK SCHULTZ is West Coast Sales Manager, International Flex Technologies Inc., 1093 Clark St., Endicott, NY 13760; (607) 658-5000; Fax: (607) 658-5001; Web site: www.internationalflex.com.
SPEC SHEET
End Applications:
IC test sockets, IC burn-in sockets, and board-to-board, module-to-board and card-to-card interconnects
Related Products:
Flexible printed circuits, PCBs, sensors
Main Point:
A connection method for flexible printed circuits using palladium dendrite plating technology offers benefits including low resistance, inductance and capacitance, while affording a high cycle of self-wiping connects/disconnects. The Velcro-like method of termination provides high-density connections with improved electrical performance. This Z-axis flexible interconnect may be just the technology to take flexible printed circuits to the next level.
