Flex circuits can use rigid board finishes, but some of them crack under pressure the moment you ask them to bend.

Is it possible for flex circuits to have the same final finishes available for rigid PCBs?

Yes, you can use any of the final finishes available for rigid PCBs, but whether you should use them depends on the application. Some standard finishes used on rigid PCBs are not suitable for flex applications. Here we go over some of the common and emerging finishes and the impact of using them on flexible circuits.

ENIG (electroless nickel immersion gold). This is far and away the frontrunner for final finishes used on flex, and there isn’t even a close second. This finish provides excellent solderability and shelf life when stored properly. Also, due to the sheer volume of flex and rigid PCBs utilizing this finish, ENIG offers good performance at a reasonable price. The most common reasons ENIG is not the first choice for the final finish on flex are:

• The application requires wire bonding.
• The application is extremely price sensitive (trying to shave pennies or less).
• Very thick gold is required.
• The plated area is in or near a flexing area. (More on that below.)

Nickel is brittle and does not fare well when flexed. The typical result of bending a nickel-plated trace or contact is that the nickel will develop cracks and those cracks will propagate through the copper below. “High ductility” nickel chemistries are available, but those are high ductility compared to other nickel chemistries, not relative to copper. In my opinion, high ductility nickel is really bad, and non-high ductility nickel is really, really bad. I personally never design a flex that contains nickel or is in close proximity to a flexing area.

Figure 1. While popular, nickel-bearing finishes are unsuitable near flexing areas.

ENEPIG (electroless nickel, electroless palladium, immersion gold). This finish is common in flex when wire bonding is performed. ENEPIG offers good solderability and shelf life, in addition to being easier to perform wire bonding than ENIG. ENEPIG is more expensive than ENIG due to the extra plating steps, and it is not used to the same extent as ENIG. It should be less expensive than electrolytically plated gold due to the lower gold content, however. Like ENIG, ENEPIG has a nickel layer and is not suitable for flexing areas.

Electrolytic gold. Electrolytic gold comes in two flavors – hard gold and soft gold. Both typically have a nickel underplate to prevent diffusion between the plated gold and base copper. Soft gold is very pure, whereas hard gold has other metals such as nickel or cobalt added to increase hardness and durability. Soft gold is usually preferred for wire bonding, while hard gold is used on connection surfaces that undergo multiple insertions and extractions. Since the gold thickness is significant, expect to pay a premium for either finish. And as with ENIG and ENEPIG, the nickel underplate of electrolytic copper makes it unsuitable in, or near, a flexing area.

OSP (organic solderability preservative). These are very thin coatings of compound that prevent bare copper from oxidizing. The time these materials maintain copper’s solderability ranges from weeks to months. The main driver for using this finish is cost. In most cases, OSP is the least expensive final finish.

Immersion tin. ImSn is typically used to save money, or if the application cannot have nickel. It tarnishes quickly and does not provide long-term solderability.

Immersion silver. ImAg is rarely used on flex circuits. As you would expect, it quickly tarnishes, which makes it less solderable. Since it is rarely used, you will also pay a premium for this suboptimal storage life. The advantage silver offers is for high-speed designs because there is no nickel underplate required. But there are newer finishes (covered below) that offer high-speed performance as well as long-term solderability.

HASL (hot air solder level). This was the king of final finishes before ENIG dethroned it over a decade ago. It is actually rare to see HASL finish specified on anything other than legacy designs. Most flex (and rigid) PCB manufacturers do not even maintain the equipment to apply this finish. When a solder finish is specified, the best option is to just solder paste and reflow sans components. HASL is a significantly higher-priced finish than most others.

EPAG (electroless palladium, autocatalytic gold) or IGEPAG (immersion gold, electroless palladium, autocatalytic gold). These are newer, similar finishes that solve the issue of nickel underplate and gold thickness. Palladium may not have the ductility of copper, but it is a big improvement over nickel. I would not push bend ratios of an EPAG-plated area to what the rest of the circuit is held to, but again a big improvement in bending performance over ENIG. Also, since the final gold finish is autocatalytic, gold thicknesses previously only available with electrolytic gold can be achieved.

DIG (direct immersion gold). As the name implies, this is gold deposited directly on copper through an immersion process. Since the deposit is immersion and not electrolytic or autocatalytic, the deposited layer is very thin. This by itself is not an issue, but that thin layer of gold will diffuse into the base copper over time, permitting the copper to oxidize once it is no longer protected. How long does this take? I have heard from various sources that this can happen in a month or take a year. This is not widely used due to the unknown shelf life.

As you can see, any final finish can be applied to a flexible circuit, so weigh the performance and cost implications to determine which best fits your application.

Mark Finstad is director of engineering at Flexible Circuit Technologies (flexiblecircuit.com); This email address is being protected from spambots. You need JavaScript enabled to view it.. He and co-“Flexpert” Nick Koop (This email address is being protected from spambots. You need JavaScript enabled to view it.) welcome your suggestions.