Heat Pipe Reliability – What You Know is Wrong!

Last week I gave an IMAPS presentation on heat pipe reliability with Ross Wilcoxon from Rockwell Collins. Thought I’d give you the highlights in graphical form along with some comments.

Product reliability is determined by understanding the reliability metrics for each of the components and or sub-assemblies within the final product. But, does the current thinking about heat pipe reliability make sense?

Calculating System Reliability

 

How to Test Heat Pipe Reliability

 

Neither Ross nor I have never seen or heard about heat pipes failing… for reasons other than stupidity or planned obsolescence.

Reasons for Heat Pipe Failure

 

There’s a lot of conflicting market and research data about the mean time between failure (MTBF) of heat pipes.

Heat Pipe Reliability Conflicting Data

Here’s the conventional reliability assurance approach with heat pipes.

Conventional Heat Pipe Reliability Approach

Temperature and failure are correlated; chemists and semiconductor folks believe it, but does a much adhered to rule of thumb apply to heat pipes. Namely, every 10 degree Celsius temperature rise will double the failure rate.

Temperature and Failure are Correlated

 

In examining past research, it appears that some erroneous assumptions were made.

Heat Pipes and Arrhenius Equation

 

Ross plotted the findings from a heat pipe failure study and showed that they did follow an Arrhenius curve. Then he estimated the activation energy; a key portion of the Arrhenius equation.

Heat Pipe Reliability Data Follows an Arrhenius Curve

 

He then used the estimated activation energy of -4507 to determine failure rate predictions at various temperatures. He found that our much believed rule of thumb (double the failure for every 10 deg. C rise) was only true for one point on the graph.

Reliability of Heat Pipes Doesnt follow a Rule of Thump

 

As you can see below, the observations made in the Mochizuki study (that heat pipes follow an Arrhenius curve) should not have lead to the assumption that it followed the half-life rule of thumb. While the blue curve, below, seems to be a better approximation of heat pipe reliability in the field, it’s based on an erroneous assumption.

Big Differences in Heat Pipe MTBF

 

To determine a more accurate predictive test, we need to find a suitable activation energy. The problem is we’ve yet to get the most common type of heat pipes (water, sintered copper, copper) to fail in the lab.

Celsia Heat Pipe Test Data

 

Celsia is continuing its elevated heat pipe testing until we can determine a new activation energy. In the mean time, companies should remember so long as heat pipes are properly manufactured and tested for the most common early types of failures heat pipes should last in excess of 30 years. Here are some simple guidelines.

Heat Pipe Reliability Test Requirements

Here’s a specific example of how we test at Celsia.

Heat Pipe Testing Process

To summarize..

Heat Pipe Reliability Summary