Faster LED Connections
By Lukas Muth and Andrew Bogaczyk
Combicon Product Marketing, Phoenix Contact

Thanks to their extremely high energy efficiency and other advantages, High Brightness LEDs (HB LEDs) are becoming increasingly desirable in the marketplace. However, they also can present design and assembly challenges.

LEDs begin to lose their efficiency as temperature rises, so HB LEDs are mounted on metal core printed circuit boards (MCPCB). This metal core helps heat generated by the LEDs to dissipate quickly, but it also prevents the use of conventional through-hole connectors. If designers want to ensure the speedy wiring that through-hole connectors provide, they need to find a new solution. PCB terminal blocks that use push-in spring technology can overcome this problem. The terminals need to be capable of quick wiring and suitable for surface mounting technology (SMT).

SMT Processes
In SMT processes, components are automatically fed to the pick-and-place position. The machine mounts all components onto the PCB, and the module is soldered in the reflow oven prior to inspection. In order to adhere to the SMT process, all components must meet strict requirements, including the ability to withstand high temperatures.

Tape-on-reel packaging has become the preferred economical packaging solution for convenient placement when automatically feeding components. The tapes come in 24, 32, 44, 56, and 72mm standard widths. They are supplied wound on reels packaging many components in a confined space, and from there, they are inserted into the feeder unit of the pick-and-place machines.

Because feeder table space is usually limited, the preferred tape widths range from 24 to 56mm. The components should be packaged in the narrowest standard tape width that fits the product’s overall dimensions. PCB terminal blocks can then be used together with the other components of the LED printed-circuit board in a standard feeder. This permits the use of common placing routines, such as “chaotic” placing, without any restrictions.

Special Demands of the Placing and Soldering Processes
For most of these so-called “pick-and-place” routines, the free placing height is typically about 25mm. Pick-and-place machines with turret heads can have an even smaller free-placing height.

Selecting the lowest possible PCB terminal block height will ensure that the block can be used in as many pick-and-place machines as possible. This eliminates the need to program specific sequences—a complex procedure.

In order for the picking head of the pick-and-place machine to “pick” the PCB terminal from the tape, it must also have a smooth and adequately large suction surface. This means the machine can use a standard vacuum pipette to pick the component off the tape, rather than using costly grippers or a special pipette.

The single-most important property for plastics used in SMT production processes is the ability to briefly tolerate the high temperatures required to mount the component securely to the PCB. Depending on the particular requirement, polyamide (PA 4.6) or liquid crystal polymer (LCP) can be used. The changeover to lead-free processes has significantly increased the requirement for high-temperature stability. Many components that undergo SMT production have an upper temperature limit of 255 to 260 degrees Celsius.

The process capability of a component manufactured out of a specific high-temperature material must be qualified according to the IPC/JEDEC J-STD-020D standard. This standard defines Moisture Sensitivity Level (MSL) values, which determine the type of packaging and the processing in an atmosphere that is common for SMT processes. Some materials, if not precisely handled to the standard, can absorb atmospheric moisture that can cause components to blister, delaminate, or become deformed when subjected to the high-temperature soldering process.

For example, if a plastic absorbs a lot of moisture, the component would be classified as an MSL 6. These products must be dried and placed in an airtight package. After the packaging has been opened in the SMT production environment, then the components involved need to be processed within a certain time period.

Maintaining the so-called “open time” ties up resources and requires time-consuming checks in the SMT environment. It is simpler to process components that have an unlimited “open time” (MSL 1), as seen in LCP plastics. In this case, it is not necessary to observe when the packaging was opened.

Easy Handling
Solder contact surfaces of PCB terminals must comply with the same requirements as other components used in SMT processes. The PCB’s surface solderability is especially critical. Most specifications stipulate a solder surface coplanarity of between 100 and 200 μm. Coplanarity is the measurement of how flat a product is when placed on a flat surface. This is a key value because if the product is too skewed, proper connection to the board may not occur, leaving open solder joints.

This applies to the solder surfaces like anchor metals, as well as to the contact solder pin, since the component is connected to the printed circuit board through these surfaces. PCB terminals transmit electrical power or signals and, when the conductor is being connected (terminated), are subject to a mechanical load. A PCB terminal block must be able to withstand all of the mechanical forces while continuing to provide adequate electrical connection reliability at the solder contact area. Broken electrical solder joints will lead to faulty connections and increased resistance values.

Additional requirements arise from how the terminal is actually handled and used. Several options are available for implementing spring-cage connection systems. One connection method is the push-in spring technology.

This type of contact is extremely compact and allows conductors to be quickly connected as the prepped wire is directly inserted without the use of tools. The terminal point has to be actuated with a screwdriver only when connecting finely stranded conductors without ferrules, as well as when releasing the conductor. Taking into account all of the requirements from a processing-related perspective, (Table 1), the ideal PCB terminal block does not differ from established SMT components, such as SMT LEDs.

When designing LEDs, it is now possible to produce connection systems and modules in just one SMT process at a good price point. It is also possible to connect the conductors without any noticeable differences to a through-hole technology (THT) PCB terminal.

Push-in PCB terminals (Fig. 1) are especially useful when there are multiple wires to connect, such as in LED lighting technology. The push-in connectors can save considerable time compared with crimp connections or even other spring-cage terminals. The 5mm spring-cage terminals make it easy to insert and remove conductors of up to 20 AWG (Fig. 2), even in high-temperature applications.

Lukas Muth is the Combicon product marketing specialist at Phoenix Contact GmbH & Co.KG in Blomberg, Germany.

Andrew Bogaczyk is the Combicon product marketing specialist at Phoenix Contact USA. Learn more about miniature connectors for LEDs online.