Backplanes are the nerve centers of the information superhighway. Whether active or passive, these large circuit boards sport numerous connectors into which other circuit boards are plugged. It's not uncommon for a backplane to contain more than 20,000 pins for connecting wires, cables and daughter boards.
Few, if any, of those pin connections are soldered. Instead, the pins are pressed into the board. Because the pins aren't soldered, there's no risk of shorts caused by bridging, or weak joints caused by cold spots. There's no flux residue to reduce the reliability of the contacts, and there's no heat to harm surrounding components. The process is compatible with lead-free assembly, and it's relatively easy to remove and replace faulty connectors.
"The real beauty of the process is that you press the connector down, and it's done," says Dante A. Parenti, product manager with the automation group of Tyco Electronics Corp. (Willow Grove, PA). "You can do 800 or 900 pins in one shot."
Press-fit connections are durable, reliable and resistant to vibration and temperature fluctuations. Are they as reliable as soldered joints? "Let's put it this way," says Parenti. "Press-fits started in the telecommunications industry to connect backplanes and daughter boards, and those applications are expected to be error-free for 40 years."
Backplanes aren't the only products to use press-fit connectors, however. The technology is now used in a wide range of applications, including automotive, military and medical equipment. Many safety-critical devices, such as air bags and roll-over sensors for cars, rely on press-fit connections.
Press-fit connectors can be made from a variety of metals, including copper alloys, nickel alloys, phosphor bronze and brass. Numerous finishes are also available, including tin-lead, matte tin, bright tin, and gold over nickel.
The connectors can be posts or blades. Posts come in round, square or rectangular profiles, in various lengths and thicknesses. They can be straight or bent at a right angle, and the connection end is usually pointed to guide entry into a mating receptacle. Posts are available singly, or they can be grouped together in a plastic housing or strip. Often, the plastic strip can be designed so it can be cut to length, enabling assemblers to create custom connectors on demand.
Like posts, blades can be straight or bent at a right angle. They can be provided loose or on continuous strips or bandoliers. The connection end of the blade can be flat and tapered, or it can be notched for extra gripping strength.
Assemblers also have options for the other end of the connector-the pins that get inserted into the board. Originally, these were simply solid pins that relied on an interference fit. "The problem was, when you stuffed a solid pin into a plated through-hole, something had to give, and it was usually the board or the plating," Parenti explains. "If your tolerances were out of whack, you had a bad connection."
Pins on today's connectors are designed to be compliant, says Ken Krone, vice president of sales and marketing with Autosplice Inc. (San Diego). Each pin is several thousandths of an inch larger than its mating hole. As the pin is inserted into the hole, it compresses. Spring tension ensures that the pin maintains an electrical connection with the board. Friction and metal-to-metal bonding keep the pin from pulling out.
"Compliant pins give you a larger process window for drilling and plating the holes," says Krone. "The hole size doesn't have to be controlled very accurately."
Another advantage of compliant pins is that they don't significantly damage the hole during insertion. As a result, damaged pins can be removed and new ones installed without adversely affecting the electrical connection. A single insertion site can usually be reworked twice before it has to be scrapped.
Compliant pins are typically shaped like the eye of a needle, but other designs are available. Tyco's "action pin" consists of two spring members that are slightly offset. The beams are designed so that a plastic, as well as an elastic, deformation takes place during insertion. The spring members compress to different degrees to accommodate slight variations in hole size. Canoe-style pins have a C-shaped cross section. When inserted into the board, the cross section looks like an O. Because of its round shape, this design makes good contact with the hole and distributes the spring force evenly.
Press-fit connectors are installed with a press after the board has been fully populated and reflowed, says Daniel P. Baumann, president of Schmidt Technology Corp. (Cranberry Township, PA). Installation is usually a semiautomatic process in which an operator loads a board and one or more connectors into a fixture, and then activates the press. The ram can push the connectors onto the board or the board onto the connectors.
The press can be a manual arbor press, a pneumatic press, a hydropneumatic press or a servo-electric press. Which to choose depends on the requirements for cost, quality, force and volume.
"If you're installing a single, 50-cent connector in a simple circuit board for an inexpensive product, you can use a pneumatic press or an arbor press," says Parenti. "If you're installing a high-density connector in a motherboard for a telecommunications application, you want a servo-electric press with PC control and force feedback. By the time it's completed, that motherboard could cost $10,000. In that case, you really want to make sure that the connector is installed correctly. You want to monitor the pressing process from beginning to end, so if something goes wrong, you can stop the process and you don't have to scrap the board."
Insertion force varies with the materials, finish, size, design and number of pins in the connector. The force needed to insert a pin 0.64 millimeter in diameter is typically 7 to 13 pounds. The force needed to insert a pin 0.81 millimeter in diameter ranges from 13 to 29 pounds.
That's for one pin. To determine the force needed to install a multipin connector, multiply the force for one pin by the total number of pins. For example, if it takes 20 pounds of force to insert one pin, and the connector has 20 pins, the press will need to produce 400 pounds of force.
"It's very common to see connectors that require a 3-ton press, and it's not unusual to see connectors that need a 5- or 6-ton press," says Parenti.
To control these potentially destructive forces, equipment manufacturers have created a number of different methods for monitoring the quality of the press-fit connection before, during and after installation.
For example, Schmidt has developed proprietary tooling that verifies the quality of the connector before it's installed in the board. When a connector is inserted into the tooling, sensors determine whether all the pins are present and if they're the right length. If any pins are missing, the press won't cycle. After insertion, the fixture confirms that all the pins have been pushed completely through the holes.
Other presses are equipped with vision systems. Before the press installs a connector, the vision system ensures that all the pins are present, straight and within established tolerances.
During insertion, assemblers can also use a load cell to determine whether the insertion force is falling between an established minimum and maximum. This is important, because if the insertion force is below the minimum, it could mean that the holes are too large and are not applying sufficient force against the compliant section of the pin. As a result, the electrical connection won't be electrically or mechanically sound.
If the insertion force exceeds the maximum, the connector could be off-center, or the holes on the board could be too small. In either case, if the press completes its stroke, the pins could buckle or the connector housing could crack, possibly ruining the board. With a control setup, a hydropneumatic or servo-electric press can be programmed to back off as soon as the insertion force exceeds a set point. For example, if 200 pounds of force is required to install a connector, and the actual insertion force is between, say, 198 and 202 pounds, a good connection has been made. If it's off by 30 or 50 pounds, assemblers know something has gone wrong.
"With an electrically controlled hydropneumatic press, the ram can come down with a very light force. Then, when it reaches a certain force, it stops and verifies that all the pins are partially engaged," explains Baumann. "If a pin is missing, the ram backs off. If all the pins are present, the ram presses the board to its final position."
How accurately insertion force needs to be measured depends on the application, but the process window is typically large, says Krone. "You don't have to be super accurate," he says. "You're looking for anomalies and extremes."
An additional benefit of monitoring the insertion force is that it often obviates the need to inspect the connectors after they're installed.
Presses aren't the only way to install press-fit connectors. Individual pins and blades can be "stitched" into boards with a high-speed insertion machine. For example, the VersaSert VIP-3 from Autosplice can insert 20,000 cph. It can be configured with one, two or three insertion heads, and it can operate as an in-line or stand-alone machine. The insertion area measures 18 by 18 inches, and an optional programmable stage can rotate a board 90 degrees in less than 0.8 second. An optional vision system automatically corrects errors in board positioning.
Like presses, high-speed insertion machines can be equipped with sensors to monitor insertion force, says Krone. In this case, the sensors are mounted in posts that rise up from beneath the board to support it while the connectors are installed.
Handle With Care
After assembly, circuit boards with press-fit connectors should be handled with care. To prevent damage, assemblers should follow these guidelines:
- Assemblies should be handled slowly and carefully. They should never be dragged.
- Personnel who handle assemblies should not wear loose clothing, which could catch on pointed pins.
- Assemblies should be handled by their edges and placed in plastic trays. They should not be placed on a cushioned surface, such as foam.
- Assemblies should not be stacked.
- Pins should only be touched during repair.