Surface finish on PCBs and why it matters for reproduction carts

There are many types of surface finish used in production for printed circuit boards. To the end user, it may seem as though there are essentially two type of finish: a silver-coloured finished and a gold-coloured finish. In reality, the silver-coloured finish is probably HASL (hot air surface levelling) while the gold-coloured finished could be either ENIG (Electroless Nickel with Immersion Gold coating) or Hard Gold plating. In this post I aim to describe the use of these finishes in the context of game cartridge PCBs and what an informed buyer should look for in reproduction cartridges.

Why Are There Surface Finishes?

The surface finish on a PCB plays two important roles: it protects the underlying copper from corrosion and it provides a solderable surface.

Types of Surface Finish

Hot Air Surface Levelling

Hot air surface levelling has the major advantage of usually being the cheapest available finish when ordering PCBs. For this reason, it is a very popular choice. HASL finishes are also very easy to rework which is excellent for prototypes. However, HASL finishes are essentially a thin layer of solder applied on top of the copper. The mechanical strength of the finish is very poor. Therefore, this type of finish is not well-suited for edge connections like a game cartridge where the PCB finish is relied upon for mechanical contact with a socket connector.

Electroless Nickel with Immersion Gold

ENIG finishes are used when surface planarity and oxidation resistance are important for the design. Surface planarity is important for PCBs with large BGA packages since ENIG finishes are much flatter than HASL finishes. The thin layer of immersion gold applied on top of the nickel plating protects against oxidation since gold is one of the least reactive elements. This type of finish, while not ideal for edge connections, is definitely a better choice than HASL for a game cartridge.

In the picture below, an ENIG board (left) and a HASL (right) board were both subjected to 100 mating cycles in an edge connector. Both the ENIG and HASL finishes show clear wear from the mating cycles. However, while the ENIG finish appears to only have changed colours, the HASL finish is clearly being scraped off by the mating connector since it is not strong enough to resist the mating strength. More cycles would completely destroy the HASL finish and expose the underlying copper to oxidation.

Hard Gold

Hard Gold finishes are used when a durable surface and protection from oxidation is required. This type of finish is ideal for game cartridge which will be subjected to many mating cycles.

In the picture below, a HASL board (left) and a Hard Gold board (right) which I routinely use for prototyping are pictured. I have no exact numbers but I would estimate that both have been subjected to 500+ mating cycles. The HASL finish is clearly destroyed and a significant portion of the underlying copper is exposed – this board will oxidise. The Hard Gold board looks like it’s brand new.

Conclusion

HASL finish PCBs are absolutely not meant to be used as game cartridges. They do however have their place, especially in unexposed PCBs where mechanical connections to the PCB are not required. But, in every case, always ensure the game cartridges you buy, wherever you buy them, are finished with Hard Gold. At the very minimum, refuse to purchase any game cartridges which do not have a gold-coloured surface finish. If ambiguous, ask the seller what finish the PCB has – an informed seller will know and will be happy to tell you!

René

Electronics engineer and retrogaming fanatic!

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