The chicken or the egg? The box or the PCB?

ERNTEC Pty Ltd

Thursday, 01 October, 2020



The chicken or the egg? The box or the PCB?

Begin with the end in mind

PCB first, or standard off-the-shelf enclosure first?

In many cases when a designer is working on a new product, their focus is on the PCB design.

The designer has many things to consider, such as general layout, complexity, number of layers, EMC serviceability and placement of the required components, to build the circuit. Further down the track, when they have a PCB that works, they start to think about its packaging (the enclosure).

This often leads to frustration and lots of time spent trying to find an enclosure that properly accommodates the PCB. In many instances a modification needs to be made to the PCB to fit the enclosure. This modification can be costly, which means the designer will have to settle for a less-than-suitable enclosure. This can have an adverse effect on the appearance and may not meet the deployment requirements.

If the enclosure is thoroughly thought about and planned for at the early stages of the design cycle instead of the end, time and money will be saved — and sometimes a lot of money.

The smart way would be to define the material, size and looks required, then investigate the standard off-the-shelf enclosures available and where all I/O connections will be located on the PCB.

Why? Cost and appearance!

Firstly… cost

Modifying a standard electronic enclosure will almost always require holes and some form of printing or labelling.

An enclosure generally has six sides — plenty of real estate to work with, right? True, but be wary of the costs if you plan to modify every side.

Modification

In general, the holes in the enclosure are required to accommodate all the I/O requirements like cables, connectors, switches, LEDs, etc.

For a professional finish, the holes are best made on a CNC routing machine. Should you decide to have holes in multiple faces of the enclosure, the cost increases due to multiple set-ups for programming, clamping, feeding and tooling.

The most cost-effective approach to take is to try and have all the control I/Os on one surface (normally the front) and generally, if the device requires power input, the rear. In other words, limit the number of faces that need modification.

It is important to ensure your PCB design considers internal features like mounting bosses, PCB slots or strengthening ribs, etc.

Minimising the number of faces of the enclosure that need to be machined provides considerable savings on the finished product cost.

Labelling/marking

Typically, I/Os will need to be labelled for user interface and you will also want to brand your finished product with your logo and/or company name.

The most common forms of labelling for electronic enclosures are screen printing, self-adhesive graphic overlays (generally made from polycarbonate material) and digital printing.

As a designer you will need to define the finish you require, as the cost impacts vary substantially depending on the quantities being manufactured.

Screen printing when done well provides a sharp defined look but set-up costs are high, particularly if you are printing many colours. As this process exerts some force on the enclosure, a special jig needs to be made to hold the enclosure in place to prevent it from moving. If more than one colour is required, then each colour needs a separate screen. Also, each colour needs to be properly cured before applying the next colour. So it can be quite time-consuming.

Polycarbonate overlays are costly for small quantities, as they require selective screen printing or digital printing onto the label. Once printing is complete, an adhesive layer needs to be attached to the label to allow for assembly onto the enclosure front or rear.

Digital printing directly onto the enclosure material is the newer kid on the block and is more efficient and cost-effective than screen printing or printed overlays. This is because the initial cost to print, prepare artwork, layout, dies, etc can be much less.

For this type of labelling, set-up is straightforward. With the right artwork and a flat face to print on, you simply manage placement and alignment of the enclosure on the bed and print. Multiple units on the print bed can keep overall costs down, but the overall area of the artwork is what defines the print time. An example is where you might have a logo at the top and some text at the bottom of the face. The print time here is the actual number of passes to cover the entire face, not just that needed for the logo and text.

Additionally, each feature to be labelled may require separate set-up. For example, if the connector openings have been consolidated, then that is one face print. If the brand names and logos, LED lights, etc are grouped on the front cover then that is a second face print. Grouping them together on one face will save costs but is not always practical. This is something to think about at the start of design.

Importantly, both modifications and labelling will need the appropriate drawing details in the required format, so that the manufacturing processors can be carried out.

Secondly… appearance

There are many elegant and practical enclosure solutions available from several manufacturers, so you will almost always find one that suits your requirement.

Spend time to do your research and you will be pleasantly surprised by the enclosures available. Depending on volumes and material, some manufacturers will provide alternative colours in specific styles of enclosure. Be careful: these are often non-stocked options.

The range of material varies, so ensure that you choose the correct material that meets the environmental conditions of where the equipment will be deployed.

Examples of environment conditions to consider are exposure to dust and water (IP rating), vibration, wind, chemical resistance, temperature and ultraviolet exposure.

Many designers use standard available products for the application; however, if you are planning for high-volume sales, a customised solution should be considered. Custom enclosures require special tools to be manufactured and minimum production runs to be produced for each batch.

In summary

Choosing the enclosure before determining the layout of the PCB in many cases is a more cost-effective way of designing a product. The more effectively you plan the project, the lower the unplanned hidden costs.

And why is it smarter to select the box at the beginning? It is easier to size the PCB to suit the box than the box to suit the PCB.

For more information, click here.

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