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There is a circuit board assembly in almost every product with an on/off switch. Just sitting in your office, you are within arm's reach of at least four or five items with printed circuit board assemblies (PCBAs) inside of them—computer, calculator, phone, thermostat, digital clock, fitness band. And that's just within your office! Stepping out of work into daily life, you'll encounter products with PCBAs at every turn—there are countless circuit board assemblies in your car, in your home, in your doctor's office, at your local gym. Put simply, they are everywhere.
The proliferation of circuit board assemblies in so many different product types wouldn't be feasible without the use of rigid-flex and flex circuits. Could we appropriately put a rigid circuit board assembly into a small ankle monitor? Or a solar panel? Or a wearable infant safety device?
Figure 1: Cathy Cox, process engineer, and John Mielke, supervisor for selective solder and wave solder, discuss the first time build manufacturing plan for a rigid-flex assembly in the SMT area of Saline Lectronics.
Flex circuits have changed the way product development engineers can design and package their electronic products. Flex applications have opened the doors for PCBAs to move out of square, box enclosures and fit into small, tight, even oddly shaped three-dimensional spaces that can withstand harsh vibration and multiple flex uses.
These thin, flexible circuits have completely revolutionized the use of PCBAs in certain applications. And while these flex assemblies may perform in the same way a traditional rigid PCBA does, they have their own set of assembly rules and manufacturing nuances.
“Flex circuit designs can make some things more painful for contract manufacturers,” commented Dave Cusumano, VP of Engineering at Saline Lectronics. “We’ve come up with a variety of techniques to overcome specific flex circuit assembly hurdles.”
Flex circuits don’t play by the same rules as rigid applications in the assembly process—they’re light, bendy, and can be heat sensitive. To ensure optimal quality and guarantee that the board will not be damaged while inside manufacturing equipment, a flex circuit must have strong support.
In most cases, depending on PCB size and how they are panelized, any PCB that is thinner than 0.032” requires a pallet for the assembly process. This will help to ensure that the board doesn’t bend or get damaged during manufacturing. Pallet design is critical to appropriately support a flex assembly, and it’s important to identify a supplier who understands the nuances of flex circuit behavior in different machines.
At Saline Lectronics, the manufacturing and engineering teams work closely with hardware vendors to design the ideal pallet for each specific flex assembly. In the case of double-sided flex circuits, Lectronics’ process engineers build the perfect pallet to accommodate top and bottom configurations. This can be tricky depending on the differences between the two sides, but a single pallet design is far more cost effective.
Since flex circuits are so easily manipulated and extremely light, it can sometimes be difficult to ensure that they remain flat on the pallet. If the flex PCBA doesn’t stay flat and fully supported under the component pads, the air cushion under the flex can act like a trampoline and cause components to bounce off during the placement process. To avoid this from happening, Lectronics’ specifies a certain material that holds the flex circuit flat during the assembly process to guarantee that components are being placed appropriately.
“Pallets are the most important thing for flex circuit assembly,” said Jason Sciberras, manufacturing manager at Lectronics. “Flex circuits are completely manufacturable if done right—design is right, panelization is right, and tooling is right. We’ve manufactured high-volume, complex rigid-flex assemblies with micro-BGAs, QFNs, and 0201s at 31,000 units per week. With the right support structure, it’s easy to do.”
Figure 2: Dylan Stringer, SMT operator, verifies the solder paste inspection results on a DEK Horizon 03iX SPI machine.
Pallets also help to ensure that the flex assembly is presented to the machines at a consistent height. In the case of solder paste printing applications, it’s crucial that the assembly is at a specific height for even distribution of paste. If the assembly doesn’t enter the machine properly, the paste won’t print correctly and gasketing issues can arise, which will cause manufacturing defects later in the assembly process.
In the case of SMT pick-and-place machines, the pallet for the flex circuit ensures that the assembly is also presented to those machines at the ideal height. High-speed SMT placement machines don’t leave much room for error, so the flex assembly must be adequately prepared and supported for those fast placements.
Beyond tooling support, manufacturing engineers and technicians need to pay attention to the soldering heat applied to flex circuits. Since flex circuits are typically very thin, there’s nowhere for the heat to go besides into the barrel, so thru-hole parts must be soldered at lower temperatures. Hand soldering a flex circuit can be challenging, and requires an experienced technician to utilize the perfect technique.
“When hand soldering a normal, rigid assembly you can touch the PCB for three to five seconds with a temperature range of 600–800°F,” commented Cathy Cox, process engineer at Lectronics. “With certain flex circuits, you can only touch that board for one to two seconds at most, and the heat is much lower at about 580°F.”
In the case of testing flex circuits, the same support and tooling rules apply. All flex circuits require appropriate backing to support the assembly during the testing process. In the case of flying probe testing, a special custom fixture should be developed to provide that support to the PCBA when it’s being probed.
For in-circuit testing (ICT), a rigid backing is usually designed into the clamshell fixture. While this can add mechanical challenges for ICT development, it’s needed to ensure safe testing of the flex circuit. Additionally, low force probes should be used to avoid any unnecessary damage to the flex’s fragile surface.
“We also make special provisions to guarantee the orientation of flex circuits during the testing process,” commented Tom Newman, test engineering manager at Lectronics. “Flex assemblies don’t always have tooling holes to use as guides, so it’s important to use a fixture or cut-out that the flex assembly can sit into perfectly.”
Traditionally it can also be difficult to access test points on flex assemblies. Without exposed thru-hole vias, test technicians have limited access to probe the assembly. Additionally, designing in appropriate test access points is sometimes overlooked with flex products. Electrical engineers should design in appropriate test points during the product development stage.
De-panelizing flex assemblies can also be tricky. In many cases the only option for de-panelization is by hand, which can lead to poor quality control and repeatability issues in manufacturing. Laser de-panelizing is gaining popularity, and while much more precise than hand cutting, it’s also a far more expensive solution. However, as laser cost continues to go down, these machines should become more commonplace.
Lectronics’ engineering team works closely with bare board fabrication suppliers to optimize the array design and perimeter connections within the flex panels. They have specifically developed proprietary techniques to improve and optimize the de-panelization process where special tooling is no longer required.
As flex circuits continue to solve design constraints for space, heat, weight, and bend requirements in a variety of product applications, electronic manufacturing suppliers will develop new manufacturing parameters to assemble these flexible products. While flex circuits can be temperamental and play by their own rules at times, all these thin, fragile, bendy, heat-sensitive circuits really need is the right support system.
This article was originally published in the March 2017 issue of SMT Magazine.