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25th November 2020

Circuit Board Test Points and How to Use Them

PCB Test Points and How to Use Them

Test points and why you should be using them right now.

Nothing is more frustrating than a PCB not working the way you want it to.

PCB test points are an excellent aid for hardware bring-up, firmware development and production quality control.

They make the product development process easier and help with programming, test, calibration, and setup when your PCB (printed circuit board) gets to volume production.

An article from the knowledge of Richard Fletcher, MD of the award winning electronics design consultancy Ignys Ltd.

What are test points?

A test point is a small area of exposed copper you can connect a probe to, either an oscilloscope probe for development or contact pin in production. These are usually on the bottom of the printed circuit board.  Whereas more complex or dense boards may have test points on both sides of the board.

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Should I label them?

Most PCBs need to go through some element of engineering test and validation. So if you have the room to label these meaningfully it makes life easier. Therefore make testing quicker for the engineering team during initial bring-up and debug. And if there is minimal room then identifying them by a simple numerical reference is also useful.

Test points

Which signals should have test points?

  • Power supply rails
  • Ground
  • Programming connections
  • JTAG ports
  • Serial communications busses
  • Reset lines
  • UARTs
  • Programmable references
  • Interrupt lines

Other signals can be of use to the development and production teams.  You’ll find this is especially important where a key signal is routed between BGA, QFN or LGA devices. As without planning there would be no access to the signal.

Choosing the optimum amount

You need just enough test points for the expected needs.  In addition the gold standard is to have a hardware bring up plan and a production test plan. And ensure each necessary signal has a test point.

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Test point hazards to watch out for

  1. Test points are an exposed copper area that could accidentally short to another test point and cause damage. Testing on the bench should ensure that the workspace is clean and has no stray wires, component leads or other boards.  Conductive enclosures and heatsinks need clearance from the test points to avoid issues.
  2. Too many test points – A scattergun approach to test points complicates boards, perforates power and ground planes and brings little additional benefit.
  3. Creates stubs on high speed or controlled impedance nets. Route test points, when they are needed, to minimise additional track stubs to prevent signal reflections degrading signal integrity and to avoid contributing to Electromagnetic Compatibility (EMC) issues.

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How have they historically been used?

The older school of thought.

In the past test points, or test lands, were usually put on each signal junction between every component. This included a big bed of nails tester to check value and positioning of each signal component on a pcb.

This often leads to the pcb design being bigger or having a higher layer count than needed which is more expensive on material costs. Additionally, extra time ends up being spent on the pcb layout.

Why should I consider using test points?

Of course, some businesses use no test points. This could be because the boards are so compact that there is no room and that alternate test strategies like boundary scan are more appropriate.  Sometimes test points have never been added to electronics in the product portfolio because of the products simplicity or because the expected sales volumes are very low.

Benefits for the initial design process

Adding appropriate test points during initial design opens up the possibilities of automating test where volumes increase, identifying quality problems or to make bring up a little easier.

Where there are no test points it’s possible to probe onto solder joints or onto larger component connections, but this can be less reliable and limits the test coverage.

Test points for product development

Test points are useful for development, bring-up and test. They are useful for hardware and software teams as you can solder a wire onto the test point to make monitoring easy during bring-up.  The same points can also be useful later to diagnose issues found during environmental testing.

Are they useful for quality control?

Quality control gold for the production line

The real value comes in the production environment where you can use test points for quality control or for programming, setup, or calibration. Repeatable automation of these tasks can speed up the process, reduce costs and de-skill engineering led tasks to allow non-technical staff to reliably use.

Do test points speed things up?

For the record…

Having test points can speed up development and more importantly provide you with quality control reassurance. It also gives you the ability to automate set up test calibration. Vitally it is also traceable.

Test points for traceability?

Using traceability to check a fault’s origin using historic data

Test results link to individual boards and products using bar codes and label printers. You can scan each board to access its test record. Testing with historic evidence is very useful if problems emerge later if the right measurements are recorded. You can look at test result trends to better understand if an issue is batch related, supply chain related or has other causes such as a bill of materials change or firmware update.

Why an instant fail is a good thing!

Sometimes knowing something is going to fail now is far better than getting an instant pass, why? Because of the time and money it saves.

During our careers as engineering in product development we have sadly often spoken with product owners who have no sub-assembly testing in place. This means that the full product must be assembled before any faults can be observed. This leads to costly and time-consuming reworks.

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Using test points to find hidden problems

Worse still are subtle problems that affect the products long term performance.  Sensor accuracy and power consumption are two issues you can catch t at the populated PCB stage but which may find their way into a customers hands untested. Getting right to the end and having to go back is not ideal to say the least!

Finding faults early using test points makes it cheaper and saves on cost and quality issues. It also gives you something you can’t measure. Peace of mind.

Often at the production stage it can be tempting to assume that testing processes are robust but using a test fixture takes this to the next level of certainty.

Using test fixtures to test at the manufacturing stage.

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Your own little advocate

If you outsource manufacturing to third party you can think of your production test fixture as part of your quality control team, almost like an employee, on your side sat on someone else’s production line, right before you pay for it.

If the product has been manufactured correctly then the unit passes the test and you have confidence, and proof, that all is well.  If the product doesn’t pass the tests then there is an early warning that there is something wrong.  This could be a manufacturing issue or a design issue.  Either way having the knowledge of an issue enables corrective action to be put in place and prevents poor quality product reaching your customers and potentially damaging your reputation.

Blog authorship: This blog was created with the knowledge of Richard Fletcher Ignys MD who has over 20 years’ experience in the electronics industry.

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