The Flexperts
What Kind of Solder Mask is Used on Flexible Circuits?
First differentiate between rigid-flex and true flex.
As is often the case with flex circuits, knowing which solder mask to use on flexible circuits is somewhat of a trick question, one with several answers. The decision boils down to circuit construction and design intent.

To start, there are several ways to insulate circuits in the flex world. These include solder mask, coverlay and coverfilm. In most cases, the designer may simply note solder mask per IPC-SM-840 and leave the rest to the fabricator. This allows the fabricator to use the proper mask in the proper setting.

When making a design decision, first differentiate between rigid-flex and true flex circuits.

Let’s cover the easiest one first: rigid-flex. Typically, a rigid-flex construction will have solder mask applied to the external rigid layers to insulate all external traces, as well as define surface mount or BGA pads. It may also provide mask dams between pads to reduce the potential of solder shorts at assembly. This solder mask usually is classified under IPC-SM-840 as a type H solder mask, which denotes a high-reliability solder mask. These are the most common solder masks. Normally green in color, they can be modified for other colors, as desired. It is worth noting that if the color deviates from the as-formulated green option, there may be feature resolution and web size tradeoffs. This is because the additives used to change the color impact how the mask material absorbs light energy during the imaging process. As a result, the fabricator may need to ask for some relief for other colors.

Typically, the flexible portion of the rigid-flex does not use solder mask. The flex will use coverlay/coverfilm material, usually per IPC-4203. This is a laminated material completely covering the flex region of a rigid-flex. In some cases, openings in the flex coverlay expose solderable pads or edge connector traces, like a ZIF (zero-insertion force) connector pattern.

For “pure” flex circuits, that is, one- or two-layer or multilayer flexes, the decision gets a bit cloudier. All options are now on the table.

It is still most common for flexible circuits to use coverlay over the outer layers of a flex circuit. This is usually the default on all Class 3 applications, as well as any high-voltage applications. Coverlay material provides the highest level of dielectric strength, as well as flex endurance. It also is more rugged than solder mask. The overall thickness ranges from 0.001″ (25µm) to over 0.006″ (150µm). The adhesive thickness on the coverlayer material is selected to ensure complete conformance over and around all the conductors.

While coverlay is very good, in some conditions solder mask can provide an advantage over coverlay. When using solder mask on a flex circuit, we need to ensure a flexible formulation is used. IPC-SM-840 defines these as type “FH.” This is because standard solder mask materials are brittle and will immediately crack and chip off a flex circuit once bent. They are not formulated to be bent.

The flexible solder mask is typically green or amber in color and has the same electrical properties as standard non-flexible options.

When is solder mask preferred on a flex circuit? In many commercial applications, solder mask can provide sufficient protection while doing so at a lower total cost. The material cost is lower and the processing cycle time is shorter. Therefore, solder mask is often used in these situations.

In other cases, solder mask provides an even thinner overall coating relative to coverlay. This may be important in a very constrained design. Because it is thinner, it is then assumed the part can be bent to a tighter bend radius. This is not always true. Coverlay can withstand far tighter bend radii than solder mask, and more total bend cycles overall.

Usually, flexible solder mask is used when the radius is relatively generous and in a “bend-to-install” situation. That said, we have built applications that use flexible solder mask in a dynamic way that work just fine. In these cases, the bend radius is larger, and the angle of the bend is much less than 90°.

SMT on flex may require a hybrid strategy.
Figure 1. SMT on flex may require a hybrid strategy.
HDI features may also drive the need for solder mask on flex. Standard coverlayer material is a combination of film and adhesive, which is first drilled, and then laminated over the etched pattern. The adhesive in the coverlay does flow and may encroach on pads. In addition, since it is drilled, openings in the coverlay can be any shape desired, provided they are round. We don’t have square drill bits. This can be a problem with rectangular SMT features. In addition, a minimum of 0.010″ of webbing between openings is required, which can drive the manufacturer to request large gang access openings, leaving no webs between pads. Solder mask can resolve these issues, which are becoming a driver for solder mask on flex.

What if you have a flex with SMT features that has a bending challenge too? Often a hybrid approach comes into play. In these cases, standard coverlay is first laminated in the bending regions of the part. Then, solder mask is applied in the areas with SMT patterns. The two materials overlap in areas where they meet. This provides the best of both worlds, with minimal additional cost.

An extra piece of advice: If your circuit requires a UL flammability rating, a one- or two-layer flex using adhesiveless copper-clad and solder mask may not pass UL flame testing. Adhesiveless copper-clads are inherently inflammable and thus contain no flame retardant. The flame retardant in the flexible solder mask may not be sufficient to self-extinguish. To get around this in UL applications, UL 94V0-rated adhesive-based copper-clad laminates should be coupled with flexible solder mask to meet UL flame testing.

Beyond this, additional solutions for flex can be employed for high-temperature applications, high-speed applications, and others. Your fabricator can leverage its experience to help you make the best decision for your specific design and end-use.

Nick Koop headshot
Nick Koop
is director of flex technology at TTM Technologies (ttm.com), vice chairman of the IPC Flexible Circuits Committee and co-chair of the IPC-6013 Qualification and Performance Specification for Flexible Printed Boards Subcommittee; nick.koop@ttmtech.com. He and co-“Flexpert” Mark Finstad (mark.finstad@flexiblecircuit.com) welcome your suggestions.