Sample Size for PCB used in a Medical Device

R

Rob C

#1
I too have a sample size question.
we are working on a PCB in a medical device that has a fuse that blows during power up.(not always the first power-up) we have a population of 30 devices and during the build and test of these devices they are powered up approxiamately a dozen times. During the build of this population we have seen 9 occurrances of 'blown fuse'. we are increasing the rating of the fuse and would like to prove that the issue is resolved. currently, i only have four device left that can be used for testing for this condition. how would I determine how many trials (power-cycles) i should perform on each of the remaining four devices to prove the issue has been 'fixed'?
 
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Bev D

Heretical Statistician
Staff member
Super Moderator
#2
Re: Determining Sample Size - Filling a bag with three different types of candy

This isn't a straightforward sample size question. IF you had a completely homogenous process such that all devices randomly experience this failure then we could do a straightforward sample size calculation using exact binomial confidence intervals. However, we woudl need to know exactly how many power-ups were done - I'm assuming that once a failure occured on any particular device, the remaining "dozen or so" power-ups were not performed?

IF you have a device related causal mechanisim (soem devices are more susceptable to the failure than others, then the 4 remaining devices may be of no use in determining if you have improved the situation no matter how many power-ups you ran.

I would want to know more about the situation.
some starting questions:
  • What causes the fuse to blow? is it that some fuses are weaker than others or is it that some devices draw more current than others? These blown fuse things can be typical interactions
  • Do you have devices that are more susceptable than others? (Assuming that the device isn't ruined and you put another fuse in it, will it blow again in roughly the same number of power-ups?

The main response from my perspective is not the math but the nature of the Problem that is more important in determiening sample size.
 

Miner

Forum Moderator
Staff member
Admin
#3
This sounds like a reliability issue. Can you attach a data file that shows each unit, the number of power cycles that each failed unit saw when it failed, including how many power cycles the non-failed units have seen?

I can use this to characterize the failure distribution and provide better advice.

Note: You can also type the results in as shown.

Unit # Cycles Failed (Y/N)
1 5 Y
2 7 N

Bev D had some good suggestions. Reliability failures occur when there is an overlap between the stress distribution (load) and the strength distribution of the fuse. Bev D's questions pertain to the Physics of Failure. You need to understand the failure mechanism, stress distribution and strength distribution in order to improve the product.

Typical improvement approaches are to increase the mean of the strength distribution, decrease the mean of the stress distribution, decrease the variation of either/both distributions or a combination of all.
 
Last edited:

Kales Veggie

People: The Vital Few
#4
I too have a sample size question.
we are working on a PCB in a medical device that has a fuse that blows during power up.(not always the first power-up) we have a population of 30 devices and during the build and test of these devices they are powered up approxiamately a dozen times. During the build of this population we have seen 9 occurrances of 'blown fuse'. we are increasing the rating of the fuse and would like to prove that the issue is resolved. currently, i only have four device left that can be used for testing for this condition. how would I determine how many trials (power-cycles) i should perform on each of the remaining four devices to prove the issue has been 'fixed'?
I am not sure if the issue has been resolved. I would find out why the root cause of the fuse blowing (was the correct fuse specified, is the correct fuse used on the PCB, does the fuse meet its specification, was the PCB manufactured correctly, is the test fixture working correctly and does it simulate the operating environment).

Be careful with jumping to conclusions.
 

Bev D

Heretical Statistician
Staff member
Super Moderator
#5
So Rob C: you've gotten 3 pretty good responses. the ball is now in your court...
 

Marcelo

Inactive Registered Visitor
#6
we are increasing the rating of the fuse and would like to prove that the issue is resolved
What exactly are you increasing? I´m not sure if you are aware, but fuses have at least four key characteristics - voltage, current, operating speed and breaking capacity.

This "blows on power up" issue might be due to any of the four, but generally (and due to you mention of this not occuring to all products) it has to do with the operating speed or breaking capacity (since the main problem with power up is the transitory and quick peak tensions encountered).
 
I

icare2much

#7
I would reiterate that the root cause must be determined before any corrective actions such as changing the fuse size/rating are implemented.

Implementing the wrong solution could be hazardous.

Determine if it is a design issue or a manufacturing issue. If design; what tolerance analysis has been done to show peak load characteristics, and where are the verification test results to prove it? Why did the design engineer select the particular fuse?

Investigate!
 
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