Push-fit connector Solderless connection (2023)

A common misconception about manufacturing (especially among the general public) is that there is no such thing as innovation. The fact is that new assembly technologies are constantly being developed and brought to the market. However, sometimes it is necessary to modify an innovation in order for it to become mainstream.

This is the case with the printed circuit board (PCB) assembly. From the early 1940s to the early 1990s, nearly all printed circuit board components were joined using one of three methods: point-to-point soldering, wave soldering, or reflow soldering.

Some components were assembled using press fit technology. Developed by the telecommunications industry in the late 1970s, this solderless method presses copper alloy pins (separately or preformed into plastic connectors) into through-holes to create a gas-tight electromechanical connection.

The process attracted little interest from other industries because the massive push pins used initially deformed the through-holes and caused microcracks in the circuit boards. That all changed in the late 1990s with the development of compatible press-fit pins, through-hole plating, and the introduction of servo-electric vises that offer precise control over the insertion process.

"Automotive suppliers of non-critical instrument clusters and switch assemblies began using press-fit technology more than 10 years ago," says Joe Lynch, director of Interplex Industries Inc. As these suppliers became more comfortable with reliability, they later used press-fit technology. technology for more critical functions, such as airbag sensors and crash-sensing control systems. Now the method is conventional because it also offers repeatability and ease of assembly.”

In recent years, a top-tier automotive company has used press-fit technology to assemble and test two types of PCB housings. One housing has a 16-pin molded connector with four power connections. The other case has two 16-pin connectors and four power pins.

Assembly and testing is done at a Schmidt Technology Corp. semi-automated workstation. A worker places each package (PCB on top) on a sliding table that transfers the package to a nesting fixture. The printed circuit board is positioned so that the connector pins line up correctly with the through-holes.

Once the cycle begins, a camera takes a picture of the case and circuit board, and a scanner reads the barcode on the circuit board. If the components are not compatible, a "not OK" message appears on an HMI and the device returns the casing to the sliding table. If the components are compatible, the case is loaded into a ServoPress 420 and its ram presses the circuit board onto the pins in 1.5 seconds.

After assembly, the camera performs a pin height check to verify accuracy to within 0.003 inches. The finished carcass is then transferred to a dump location. Total cycle time is 12 seconds. Barcode scanning allows full product traceability.

"Push-fit connections are at least 10 times more reliable than soldered connections according to the IEC1709 standard," says Glenn Nausley, president of Promess Inc. Sensitive components get hot, solder joints get cold, and jumpers of weld are cut.

Much better than soldering

Press fit technology offers many other advantages over solder paste and selective soldering. First, the solder is now lead-free and must reflow at high temperatures, which can damage the connectors and the board.

Press fit assembly can be easily automated, while secondary soldering processes are often manual, time consuming, expensive, and lack quality control. Also, no cleaning fumes, gases, or liquids are required for soldering, which often reduce contact reliability.

Lynch notes that compatible press-fit pins provide a direct contact connection with no metal fillers or voids in the connection. Pin compliance also offers some beneficial flex, including slight movement to compensate for high-vibration environments and temperature cycling.

some components

The press fit pins are made from a copper alloy and each pin carries from three to 50 amps. Originally, the pins were solid with a rectangular cross section, which deformed the through hole when inserted. To counter the problem, vendors have developed compatible pins that are several thousandths of an inch larger than the PCB hole diameter, but compress when inserted.

Once inserted, the pin expands and presses against the rigid sides of the hole to form a gas-tight connection. In addition, the local high pressure in the joint creates a cold welding effect that ensures mechanical and electrical integrity.

The most popular compatible pen design is the "eye of the needle", which has flexible bars on each side. Other compatible designs include the C-section and the action pin. Profile C Pins are crescent shaped and taper to expand or contract to touch the perimeter of the through hole. Action pens feature a split beam design.

The compatible pins not only prevent damage to the circuit board, but also provide spring force to hold and allow connector repair. They also require less insertion force than solid pins and allow for multiple press fit cycles. Because of these advantages, compatible pins are generally preferred.

In certain high temperature applications (above 125°C), press fit pins must be coated for durability. Tin plating or electroless nickel plating is usually sufficient. However, the tin should be kept as thin as possible (1 micron) so it doesn't get scratched when the pins are inserted.

“The most popular pin gauges are 0.64 and 0.81 millimeters to fit 1.016 and 1.486 millimeter holes, respectively,” Lynch explains. “Factors that determine the correct thickness include the spacing or density of the pins in a connector, as well as pin strength and current-carrying requirements. For example, applications that emphasize high mechanical resistance or performance, such as B. Hybrid cars, our macro pens (1 and 1.2 millimeters thick). But parts that require printed circuit boards to transfer data quickly are better off with our micro or mini pins (0.2 to 0.4 millimeters thick).”

Micropens are specifically designed to meet the needs of microwearable technology, including smartwatches, wrist-based monitors, and connected eyewear. Micro and mini pins are easily integrated into sensors, LEDs, and other specialized modules used in mobile medical monitoring devices. The Macro series is designed for industrial bus bars and other high current products with large heat sinks, multiple internal substrates, and complex control circuitry.

According to Frederick W. Grabau, vice president, Autosplice Inc. press-fit pins are available in two thicknesses (0.64 and 0.81 millimeters) and eight widths (0.64, 1, 1.5, 1.8 , 2.8, 6, 6.3 and 9.5 mm) available. global business development for autoempalmes. The insertion and retention forces for the 0.64-millimeter pin are 59.6 and 46.7 Newtons. For the 0.81 pin it is 132.1 and 88.1 Newtons.

All pins operating temperature is -40 to 125°C and can withstand 125°C for 1008 hours. Right angle pins (single and double row) are also available for connecting vertical circuit boards. All products are offered with RoHS lead free coating.

Connectors are molded of durable plastic with a wide range of pins. They can hold as few as three pens and up to 256 pens.

The trend is for increased density and more stringent requirements when used in harsh environment applications. As data rate requirements increase, the differential signal, power supply, and ESD grounding must be more firmly integrated into the connector.

According to Lynch, Interplex recently developed a unique connector for a car supplier. The connector is 1.2 inches wide with 120 pins, each bent at 90 degrees, and 1.6 inches long.

insert and check

Pressing a connector onto a PCB is a simple process that can be done manually, semi-automatically or fully automatically. After connecting the circuit board and placing and aligning the connector on it (or vice versa), the press is activated. Any type of press can be used (toggle, hydraulic, pneumatic and servoelectric) as long as it has the appropriate tools and its ram has the required force.

Pin insertion (one or more) is also easy and can be done manually, semi-automatically or fully automatically. Manual and semi-automatic insertion requires that the pins be placed in the correct holes and secured with a support piece (usually made of UHMW polyethylene) that will not damage the circuit board. The pins can also be placed in a spring-loaded top tool that releases them during insertion.

Fully automatic insertion is done with a sewing machine such as Lazpiur's LZ-Insert-2M, which presses each pin into a through-hole at high volume (up to six per second). An indicator under the circuit board verifies the correct insertion force for each pin, making sure it protrudes slightly from the bottom of the through-hole. Proper board design leaves room for bumps so performance isn't compromised.

At its Dayton, OH facility, Würth Elektronik ICS Inc. relies on various pneumatic and toggle presses from BalTec Corp. to insert push pins and connectors. On presses up to 56 kilonewtons, workers assemble printed circuit boards of all shapes and sizes for farm machinery, fire trucks, golf carts and buses.

According to Chuck Rupprecht, BalTec's vice president and general manager, the company's VK and L-VK series are commonly used for low-volume PCB assembly. Toggle presses offer force capacities from 5 to 12 kilonewtons. They also feature a square pusher to prevent rotation.

Connector vendors often specify top and bottom tooling for each of their plug or connector types. Most connectors, including right angle models, can be inserted with a flat top tool. The lower die supports the PCB during pressing.

Some vendors make connectors with holes in the top that expose the tips of the pins. Tool protrusions fit into these top holes to lock the connector and ensure that the same insertion force is applied to each pin during crimping.

To avoid deformation of the board, a distance of 0.5-1 millimeter should be maintained between the board and the bottom of the connector when pressing. The plastic casing of the connector must also be strong enough to withstand the force of pressure.

"Always make sure the press can generate enough force for the job," recommends Rupprecht. “Let's assume that each pin only requires a few pounds of insertion force. But if a connector has more than 100 pins, you probably need around 500 pounds of force. And if the application requires multiple connectors to mate at once, you may need a 1,000 or 2,000 pound force press.”

Just as important, Nausley says, is making sure the connector and PCB fit together correctly the first time. Otherwise a good insertion is impossible.

“Many things can prevent proper engagement, including a slightly bent pin, using the wrong connector, imperfectly formed holes, and a machine malfunction that causes misalignment,” says Nausley. "If tampering occurs, the press typically damages the connector and circuit board to the point where they are unusable."

To prevent this problem, Promess has developed the Digital Signal Conditioner (DSC) for its electromechanical presses. The DSC has a built-in linearization algorithm that accurately measures the initial engagement force.
According to Nausley, the algorithm together with closed-loop position control ensures that the press stops mid-cycle if it detects a problem.

Most PCB manufacturers also rely on some form of press fit monitoring to ensure quality assembly. Simple load cells can be connected to a manual press to measure and record the pressing force. Semi-automatic and fully automatic devices are capable of documenting multiple variables for the board and connector and displaying them as a force displacement diagram.

“Beyond monitoring the press force, a secondary inspection is required,” says Dave Zabrosky, Schmidt's sales manager. "This inspection can be initiated with a simple pogo switch built into the top or bottom tool."

According to Zabrosky, the ServoPress and ElectricPress series are programmable and capable of checking for the presence of pins before and after crimping, the height of all pins relative to each other, the flatness of the printed circuit board, and the height of the board in relation to the printed circuit board case.