Is it ok to use the PCB as part of the structure of a product?

Question

The industrial and product designers I'm working with keep on coming back to the idea of using the PCB as an integral part of the physical structure of our products.

We have wall-mounted products and I am quite uncomfortable with this. My argument against is that: "It is not a good design principal, the PCB should generally only be supporting its own weight and not unnecessary things like the frame"

They argue that the parts being supported are relatively light. This even includes supporting a thin frame of stainless steel using double-sided tape directly to an empty (no tracks or components) part of the PCB, or to plastic PCB clips that are taped to the frame. Today they challenged my argument by saying "what design principal? show us where it says this"

These types of solutions can come up often from industrial designers because it solves a lot of cost problems for them, and also pushes some of the structural responsibility down to the PCB design.

In my opinion, products designed to "hang" off PCBs are typically proven and tested for mounting by the product designer. For example, a large GPU heat-sink is heavier than anything we plan on placing. However, If we are "hanging" our own parts of PCBs we are entering a whole world of design that I don't know enough about.

Perhaps someone can point me to some answers on these types of issues. It would be good to get some material on the forces that a PCB can take before solder starts cracking, etc. Or maybe someone has seen a production-product that uses the PCB as part of the structure? We are a small business, but our products are mass-production grade and we will need to pursue standards approvals such as CE.

Answer 1

PCBs are used as structural materials in many applications. If you make sure you load it in the right way - no bending loads, only tensile - it can be a very strong and stiff material useful for many applications. Also, you can put a lot of very fine detail in PCBs, allowing you to delegate a lot of mechanical complexity to a very cheap process. This may improve manufacturability and lower cost of your other mechanical components.

If you are intending to use PCBs as structural materials, make sure that:

• You keep in mind that rule number one of mechanical design is: orthogonalize the design. The easiest way to check this is to make sure that you can assemble the entire mechanical design without needing a million hands holding various different parts in place while you screw a certain part in place. Every step in the assembly should build upon the previous steps in a linear fashion and the end result of every assembly step should be a product that can be easily handled and manipulated.
• Even though you are using a PCB as mechanical as well as electrical interconnect (and thus your design is not orthogonal), try to decouple the functions as much as possible anyway. Do not lead mechanical stresses through (densely) populated areas, as the stresses may deform the PCB and cause microcracks. Use slots in the PCB intelligently to lead mechanical stresses around the populated area, through unpopulated 'less important' PCB material
• Use sleeved fasteners or very fine pitched threaded fasteners in your PCB, DO NOT use self-tappers. The PCB material as a whole is very strong, but the insides of unplated holes are very easily damaged, compromising the stability of the connection.
• Apply solder to the annular ring of mounting holes and use serrated rings to self-lock the fastener in place.
• very important: use vias in the mounting hole pads to 'nail down' the copper onto the board. Otherwise the annular ring will easily come loose under mechanical stress.
• Use appropriate board thicknesses and do some back of the envelope stress analysis. Under typical conditions you want less than roughly \$\epsilon = 0.001\$ strain on your circuit board. This is combined thermal and mechanical. Using the mechanical properties of your chosen board material, calculate the amount of strain you expect in your application. Thicker boards means you can take up proportionally more force for the same amount of strain.
• In applications where excessive strain is unavoidable, route your traces with round corners instead of sharp edges, use the smallest available component packaging and orient the packages in the orientation that can take up the most strain. Leaded parts can cope with more strain than leadless parts.

Answer 2

Unless your PCB is very thin, I would say these are the rules:

• strong in tension
• strong in compression perpendicular to surface
• very vulnerable to flexing

Flexing a PCB will damage the solder joints over time. Insufficiently stiff cases leading to PCB flexing was the cause of the Apple iBook product recall in the mid-2000s: the video circuitry developed intermittent faults.

If your product is something like a clock hanging from the wall, with a mounting hole in the PCB, that would be fine. If it's something that people are going to pick up in their hands, maybe by one side, and maybe bend, you'll want some stiffness from the outer frame.

Other considerations: some people think PCBs look cheap and ugly, which may be mitigated by non-green soldermask or gold plating. Exposed PCB also leaves you vulnerable to ESD, water or dirt ingress around the edges onto the rest of the board, etc. Obviously you can't use this in a product handling mains voltage; you should check the CE testing requirements to see if there will be problems with mains adapter powered products.

Possibly the main consumer product I can think of with exposed PCB is video game cartridges, which are very durable; but there it's surrounded by a hard plastic shell which provides rigidity.

Answer 3

The PCB material is glass fabric and epoxy resin based composite and has very, very good mechanical properties.

Using it as a part of the mechanical construction is, in my opinion, perfectly OK and usually leads to very elegant solutions.

The reason why it is not a common practice is because such approach needs very careful design and specialists of different profiles (electronics and mechanical engineers) to work together on the problem.

Such combined team is very hard to be created. That is why only very small teams can design such type of solutions.

Answer 4

I know that this is an old question, but here is a picture of an actual product designed by my father 50 years ago. A Vectrol "Red-Pac" high-current AC phase control.

Thick FR4 is quite strong. Do not try this with a phenolic board! And this was a power board, so the traces are thick.