Prototyping

Introduction

Prototyping is an important step to producing a working system and can be done effectively without spending excessive amounts of money on PCBs and components. Prototypes can be designed in such a way that changes can be made without having to re-spin PCBs but by simply changing resistor positions, running wires and cutting tracks. Often systems do not operate as initially designed but using the above techniques it is possible to get a working unit from a prototype. Once the problems have been sorted it is easy to then make changes to the PCB files and produce a working system.

To minimise the time required for a product to get to market there are steps that can be taken at the early development stage. Bluewater Systems Ltd has development boards available which enable customers to produce software code on a working system while their hardware is being developed. Once the hardware prototypes are available it is possible to download the software code onto these and test the system.

Prototyping techniques

It is important when designing a prototype that all the required components are available before the PCBs can be assembled. It is normally possible to request samples from reliable sources early enough to meet the build date. As this cannot be done with every component it is important to select components that are available with short lead times and can be purchased in lower than MOQs (Minimum Order Quantities). This avoids purchasing parts that may be removed from the design leaving excess material that is of no use. Components can normally be acquired in two to four weeks.

Component placement is important during prototyping because effective placement can make rework simpler. Footprints can be placed in a manner such that resistors can be moved so signals run down different paths. Using multiple footprints for similar parts can help if a problem is encountered with an unsuitable component. This allows one part to be removed and the other replaced for testing. Adding test access to bus and signal lines can be helpful to avoid having to run wires at a later stage to determine why the system is not behaving as expected. This test access can often be placed outside the final unit shape and cut off once it is no longer required.

Depending on the complexity of the PCBs it is normally possible to get a prototype board made in two weeks and assembled in two weeks after this. For simple boards with only two to four layers these lead times can be reduced to a week each. If the job is important enough it is possible to get complex prototype PCBs made in one week and assembled in one week but this does increase costs.

Once prototypes arrive back it is important to test power supplies before powering the entire system to avoid blowing components up prematurely. Having a current limiting power supply also prevents damaging the system. Having the right tools for the job can save a lot of time and avoid damage,

 

Bluewater Systems Experience

To demonstrate some of the techniques described above here are some examples.

When ordering components it is often a good idea to get more parts than required because if a problem is encountered you may need to wait until more parts arrive before you can continue testing the system. Ordering for volumes for twenty five pieces is a good idea when producing five to ten prototypes at a time. This provides spare parts and also puts you in the first price break bracket for some suppliers.

It is often a good idea to overlap parts when prototyping. Below are some pictures that show how this can be done:

Three pads for one resistor lets you move the resistor so the required signal goes down a different predetermined path.

Multiple footprints can be used when unsure of which part will do the best job. In this case we have added a footprint for a NAND Flash chip under a media card connector so either can be used.

Sometimes components need to be removed so the traces can be cut and reconnected to a more suitable position.

Test access can be carefully removed when it is no longer required.

Good tools are important. There is nothing worse than trying to remove a surface mount chip with one soldering iron.