How to measure continuity of a single QFN without powering entire board?

Thread Starter

dignificent

Joined Nov 25, 2020
6

Papabravo

Joined Feb 24, 2006
16,183
Probably not. I don't think it would do much good since any un-powered device the QFN part was connected to would potentially cause it to misbehave. Your best strategy is to design for test.
 
Last edited:

dl324

Joined Mar 30, 2015
12,873
Welcome to AAC!
Are there Devices/Probes/Things that can measure single QFN operations, and without powering all SMDs on board?
It depends on the board design. Few are designed to allow testing components in-circuit; so most likely not.

Virtually all CMOS ICs have input protection diodes that will route any power applied to an input to the device's supply.
 

MisterBill2

Joined Jan 23, 2018
8,806
First question is why would you be testing only one device, and not the functionality of the system? If the PC board has been designed to allow an individual device to be tested, then you may be all set. I am not aware of that being done beyond developmental hardware, though, and so I very much doubt that is the case. So then the next question is just what do you need to either prove or disprove? and how important is it?
Is this for a production test or in the development phase of a design, or maybe to discover the cause of a bug that has started appearing?

Usually, if there is a question about some portion of a system, the hardware analysis begins with adding test points so that the portion in question can be watched while the whole system operates.
 

andrewmm

Joined Feb 25, 2011
1,496
So that's a SPI to CAN Bus interface.

I'm not certain as to what you want to do,

Is this chip on a board, and you want to check the chip is not blown ?
or are you wanting to check connectivity between it and other chips on the board ?

Are you trying to check the chip works as you expect ?

All are very different problems,

can you be a bit more specific, and give us some background please.
 

Thread Starter

dignificent

Joined Nov 25, 2020
6
Yea got ahead of myself and I'm newer to PCBA design.

Working in QA within SMT manufacturing.

Devices come in wheels and are fed to a machine that completes SMT processing.

Design for testing or design for testability (DFT) consists of IC design techniques that add testability features to a hardware product design. The added features make it easier to develop and apply manufacturing tests to the designed hardware. The purpose of manufacturing tests is to validate that the product hardware contains no manufacturing defects that could adversely affect the product's correct functioning.
After SMT processing the devices are all baked onto the board, however, the board is not in a state to be plugged and tested, therefore I could only imagine if any SMDs are faulty then they're going to be caught much further along when all boards are assembled to form the final system.

I see many boards include many different test-points in-circuit, and these perhaps are test-points available to complete some testing of the SMDs prior-after full system assembly of all components within a larger circuit.

I've found general information about using test-points to test a board at https://www.vse.com/blog/2019/10/01/understanding-in-circuit-test-vs-flying-probe-for-your-pcba/

input protection diodes that will route any power applied to an input to the device's supply.
Is this for a production test or in the development phase of a design, or maybe to discover the cause of a bug that has started appearing?
I'm looking for ways to discover defective parts that aren't going to operate correctly once mounted into the system on a single board. Each board is engineered to operate differently and seems there should always be some simple way to test each of it's more relied upon SMDs for proper functionality. I'm not sure if this testing can happen unless the board is powered as expected and some sort of test program is loaded. Obviously, for single resistors and capacitors, it's not of the same concern, it's more that I want to see the QFN read out to me in agreement with its datasheet before-after it's mounted.

Is this chip on a board, and you want to check the chip is not blown ? or are you wanting to check connectivity between it and other chips on the board ? Are you trying to check the chip works as you expect?
The QFN is being placed on the board anew from a reel, I doubt it's blown, but would like to somehow confirm it's connectivity is as expected and that yes it's communicating as expected with other SMDs. The board can be engineered or could enter a special process to be powered specifically to test the QFN as it exists in the circuit, but sounds like I can't just hook up to a QFN and send some chunk of 0s and 1s to verify in such a simple way, instead it makes better sense to verify a single QFNs datasheet reading in-circuit as part of the system. So then I need a testing suite of some sort designed to test any section of the entire circuit to know if a single device within the circuit is operating as expected. I believe I'm talking about Flying Probe Testing now, as I read about it:

The flying probe test (FPT) is an electrical test method, which makes a simultaneous in-circuit test (ICT) of the top and bottom of a module easily and flexibly.
https://www.protoexpress.com/blog/how-flying-probe-testing-works-for-pcb-assembly/

 

andrewmm

Joined Feb 25, 2011
1,496
I'll have a little chat then.

You are right. the sooner a fault is discovered in a production process, the less the cost.

The answer is a layered approach,

A fact I find it hard to teach, is Quality is primarily about repeatability,

If I have two production lines , one that makes 80 percent products with the same fault, and one that make 80 percent good , but with 20 percent random faults, technically the line with the 80 percent fault all the same has a higher quality !!

It works on basis that once the single fault has been fixed, then its repeatable, where as the 20 percent random is who know what's at fault.


So in production,

a) The blank PCB is from a reputable house and is electrical tested

b) The PCB is well looked after, its inspected when you receive it, to make certain you can solder to it.

c) All your components are from an authorised distributor, handled and stored correctly, baked if needed .

d) I'll skip over this, but your assembly process needs to be "correct" / quality proven.

e) Once assembled, optical inspection of joints and are correct part in correct place.

Then you on to testing the PCB.

The two basic methods of this are test points and JTAG,
but both of these have to be built into the board and defined by the board designer.

In general,

low cost items and/or mass produced items such as phone PCBs, are just plugged in and tested,

High cost items, tend to have involved test harnesses and / or JTAG testing.
 

MisterBill2

Joined Jan 23, 2018
8,806
Another option is a functional test of the board. What is needed for that is to check that for some given inputs the correct outputs occur. At the same time, the power supply current can be monitored, because that should be quite repeatable within component tolerance variations. So there does need to be a fair level of communication between the system designer and the test designer. And the test designer does need to understand what the board is doing. Thus the whole thing is a team effort. And at all of the more successful places I worked the design group was a team. The ones that had all kept separate were the ones that often failed.
 

Papabravo

Joined Feb 24, 2006
16,183
Another option is a functional test of the board. What is needed for that is to check that for some given inputs the correct outputs occur. At the same time, the power supply current can be monitored, because that should be quite repeatable within component tolerance variations. So there does need to be a fair level of communication between the system designer and the test designer. And the test designer does need to understand what the board is doing. Thus the whole thing is a team effort. And at all of the more successful places I worked the design group was a team. The ones that had all kept separate were the ones that often failed.
This is where the original design comes back into play. At the dawn of time (ca. 1970), we filled the available memory with functional code. There was literally nothing left over: ROM or RAM. Fast forward 50 years and memory is now so cheap they can practically give it away. It would not surprise me to find a system with 20% of its code space devoted to functional tasks and 80% devoted to debugging and testing.
 

MisterBill2

Joined Jan 23, 2018
8,806
Actually I was thinking of the various boards that I had designed, and how each board could have a functional test prior to being used. Some tests involved a digital input while others needed an analog input for a digital output.
 

Thread Starter

dignificent

Joined Nov 25, 2020
6
:oops:Thanks!
@Papabravo @MisterBill2 @andrewmm

All replies are super helpful and lead me to this over-view of types of PCB testing mentioned:
https://blog.matric.com/pcb-testing-methods

Also, I just found out we have a brand new Flying Probe Machine that we'll be training on!

I remain curious about Functional Testing as it seems to be the secret sauce for adjusting the testing suite per design so to produce better testing results:

My curiosity mostly surrounding modifying-refining-simplifying the requirements for Functional Testing such as:

  • External pieces of equipment
  • Fixtures
  • Requirements for UL, MSHA, and other standards

.. Modifying-Refining-Simplifying so to better produce the advantages such as:

  • Simulates the operating environment, minimizing customer cost
  • May eliminate the need for expensive system tests
  • Can check product functionality -- anywhere from 50% to 100% of the product being shipped, your need to check and debug it
  • Pairs well with other tests, such as ICT and flying probe
  • Great for detecting incorrect component values, functional failures, and parametric failures

Seems from the looks that Functional Testing is engineered for to test certain boards, but can a more universal equipment-fixture be developed that can adjust to multiple boards? I see this video of a functional PCB test and it makes me wonder..

Could a 3D printer perhaps be generating a new Functional Test or modifying a previous one in real-time to test multiple boards? The curiosity never ends:D If I end up developing anything new, I'll certainly share here. Enjoy the day!
 

MisterBill2

Joined Jan 23, 2018
8,806
The challenge of creating a useful functional test is understanding just what the system under test is supposed to be doing, and then providing the inputs to get the anticipated outputs. So the testing system is telling the story while checking for the correct responses at each point. The big challenge is to find the set of inputs that exercise all of the inside parts in the minimum of time. A ten minute test is simple, getting it down to a ten second test without missing anything is the challenge.
 
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