Flexible circuits (flex, flexible pcb, flexible printed circuit board) and printed circuit boards (pcb, hardboard) are usually populated with electronic components to perform their desired functionality. Soldering is the most common method for populating components. While the basic processes are very similar, the material set differences between a printed circuit board and a flexible circuit require different considerations during design and manufacturing. In addition, polyimide readily absorbs moisture and often require a prebake prior to exposing the circuits to soldering temperatures.
There are three basic types of soldering processes commonly used for both flex and hardboard:
•Iron soldering
•Wave soldering
•Reflow soldering
Iron soldering
Hand iron soldering is used in cases where the components physical or thermal properties do not allow mass soldering techniques or the volumes are low enough where fixturing and set up for mass soldering cannot be justified. The hand soldering process is similar for both flex and hardboard, with one key difference: The adhesives that bond the copper to the dielectric have different thermal properties. In hand soldering, the operator applies heat via a soldering iron while feeding solder wire. Since the adhesive for a flexible circuit needs to accommodate bending, it tends to be softer and may have lower temperature resistance. A skilled operator should be able to hand solder most components to a flex, but more care is required. Excess solder iron temperature or exerting too much pressure on the trace can cause movement or slippage. In some cases lower temperature solder wire can be used.
There are robotically controlled irons that are used in high volume applications and while robots can be more consistent than operators, they may not easily adapt to some of the nuances of handling flex.
Another option for flex is using an adhesiveless construction which eliminates the adhesive slipping issue. In addition, adhesiveless materials can operate at a higher temperature and are more robust when exposed to aggressive chemical environments.
Wave Soldering
Wave soldering is a mass application process normally used for through-hole components where a stuffed board or flex is transported over a wave of molten solder. The process and equipment for flexible circuits and PCB’s are very similar with some exceptions. The biggest difference is that since a PCB is rigid, it can be transported over the wave with a trolley system that holds the board by the edges. Since most PCB’s are rigid and rectangular, fixturing may not be required. Conversely, flexible circuits are generally not transported without fixturing during the process. Since there are so many possibilities of flexible circuit shapes and sizes, most fixturing needs to be built or adapted specific to an individual flex design.
The parameters for wave soldering such as preheat temperature, solder temperature and conveyor speed will vary between a hard board and flex because of material differences and thermal mass considerations.
Reflow Soldering
Reflow soldering uses the solder that is already in place and reflows it to attach the component. The solder can be applied in a variety of methods including printing solder paste, electroplating solder or dipping an unpopulated substrate into molten solder. Printing solder paste is the most common method for surface mount components. Lead free solder is used in applications to meet RoHS requirements.
There are a variety of reflow methods including infrared, hot air and convention. Conveyorized ovens are very common for transporting the material, but isolated reflow techniques can also be used such as solder irons or hot air nozzles.
In many cases flexible circuits will again require added fixturing for support and registration compared to what a hard board would require. Process parameters will vary based on thermal mass and material limitations. The reflow equipment used for both flex and pcbs is similar.
Reliability
While mass soldering to flex can be more complex than a hardboard, studies have shown that the reliability of the solder joints on properly soldered flex are the same or even better than printed circuit boards. Under hood automotive applications are a particularly harsh environment that have adopted flexcircuit technology. They are inherently more accommodating to the different thermal expansion characteristics between the substrate and components vs. a rigid PCB. Since a hard board is more rigid than a flex, components can experience a higher level of stress due to thermal expansion differences between the substrate, solder and component. In extreme environments, this can result in a fractured solder joint.
Summary
The components, processes and equipment for soldering flex are similar to those used for hardboards. The main differences are with fixturing, preconditioning and tooling. Process parameters also need to be different to allow for material and thermal mass differences.
