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How hot should silicon run?

simonbramble

Active Member
You know you get those questions where you ask 10 people and get 10 different answers... This could be one of them.

How hot should a silicon device be run before you should start getting worried about its lifetime? Does it differ from device to device, say MOSFETs vs linear regulators and more integrated electronics?

I appreciate the Arrhenius equation states that lifetime drops off as temperature goes up, but are there any general rules of thumb that say 'Don't operate above this temperature if you want a lifetime of, say, 10 years'?

I am particularly interested in hearing from engineers who have designed high reliability equipment, life support, military equipment etc. Stuff that absolutely has to work for decades
 

ronsimpson

Well-Known Member
Most Helpful Member
What electronics? It makes a difference.
I pulled the data sheet on a power transistor. 2N3055.
The silicon should be from -65C to +200C. That is not the temperature of the case, that is the silicon. It takes some math to get to the case temperature.
You need to know what is the temp of the place your product lives at. Room temp is about 25C. Most products can be used in a much hotter place. When I worked for Samsung we think about a house for temperature. At HP/Agilent we make the electronics work over a much wider temperature range. When I was doing car electronics -40C was the spec but I have seen my product fail at -45 C. On the hot end a car can get really hot.
Simple answer. For consumer electronics I want most parts to be so I can put my finger on the part and leave it there. That is a very crude way of saying the part is 25C over a room temp of 25C. (50C) Transformers I run hotter. Capacitors less hot.

[the air is 25C, IC's case is 50C, the silicon is 75C, then you put the electronics in a plastic box that stops air flow and adds another 25C]

This gives me some room. Many consumer products are designed to run at a room temp of 50C.
Remember it is typical for a person to pile paper on top of the computer. Or put the electronics on a blanket. Or close up the product in a cabinet.
 

simonbramble

Active Member
Hi Ron. Thanks for getting back to me. Yes I am aware of theta JA and the calculation behind it. Funny you should say about the finger test, but I have always said that if you cannot hold your finger on a device for more than about 1/4 second, then it is running at about 50 degC. I dont like seeing (feeling, smelling?) components running above about 50 degC, but some engineers are OK with products running up to well above 60 degC or higher

The problem I have is that engineers always quote an 'acceleration factor' (from Arrhenius) whereby an IC will fail 30x more frequently at 100 degC than it will at 50 degC, but I dont have a number to start with (at 50 degC or 25 degC). How likely is the part to fail at 50 degC? If I had to design a product that absolutely had to work for 10 years, what is the hottest temperature my components can run at.

I am specifically interested in MOSFETs, but parts of smaller geometries are of interest too (controller ICs and other analogue components) which is why I was asking for rules of thumb from engineers who have designed equipment that definitely is not allowed to fail.

I am still interested in other thoughts from other members please...

Thanks

Simon
 

audioguru

Well-Known Member
Most Helpful Member
When I think about temperature changes then I think that they cause a device to expand and contract. Then eventually the device will crack or have fatigue.
If operated continuously at one temperature, even if very hot, I think the device will last much longer than if its temperature changes frequently.
 

Ian Rogers

User Extraordinaire
Forum Supporter
Most Helpful Member
I see the need to make things cheap... BUT!! something has to give.... I have repaired motor speed controllers on Japanese cranes... These are fit for about 6A... The motors get tired about 1 year on and start pulling 6+ Amps... The cost of production at 6A is quite a bit less than say 10A..

I retrofit slightly better components and old tired motors continue for at least another 2 years.... Running a device near to max temperature will derate the device over a period.. Take a humble LED... Run it at 10mA forever or 30mA for a good while.... The visibility isn't that impaired..
 

Nigel Goodwin

Super Moderator
Most Helpful Member
I retrofit slightly better components and old tired motors continue for at least another 2 years....
For service purposes it's always best to fit higher quality components - as the parts costs is usually only a tiny fraction of the labour costs - so do it right, do it once. Different in production, where labour costs are small, and total parts cost is high - so fitting cheaper parts makes a big difference.
 

kubeek

Well-Known Member
Most Helpful Member
If the semiconductor works life at some elevated temperatuure, then you can use the arrhenius eqation to estimate the acceleration factor. But if the semiconductor is used sporadically and has significant thermal cycling, then you will be looking at much worse numbers.

The actual number vary based on the reliability standard that you use. We for example use SN29500. You can use the free MTBF calculator to get reliability data based on running temperature and voltage stress in transistors. https://aldservice.com/Download/download-reliability-and-safety-software.html
 
Really, you should consult the datasheet for each device you are using. The manufacture publishes its best estimate of maximum temperature based on its understanding of the device. It is not only that it is made of silicon (or other material) but also its geometry thermal resistance to the case and probably a few factors). That is why you cannot get a consistent answer.
 

simonbramble

Active Member
I have what I think is the definitive answer.

The Arrhenius equation states that failures accelerate as temperature goes up. However this equation only details failures at the *silicon* level. It does not take into account mechanical stress associated with the part heating and cooling and the device maybe expanding at a different rate to the PCB and thus, over time, causing poor solder joints etc. It also does not take into account corrosion effects that get worse as temperature goes up

Looking at silicon only, the Arrhenius equation states that a part is 943x more likely to fail at 125degC than at 25 degC. Arrhenius is based on exp (-qV/kt) where q = 1.602 x10E-19, k = 1.38 x 10E-23, T is Kelvin and V = 0.7)

So we know the acceleration factor, but we don't know what number to start with. Looking at the failure report of a random component (LTC3891) this details a statistical failure rate of 0.65 parts per Billion device hours at 55 degC. This report can be found from the landing page on www.analog.com.

So 0.65 failures per billion device hours is 1,538,000,000 hours per failure (taking the reciprocal of 0.65/1Billion). Divide this by (24 x 365) to give years per failure (or mean time between failures, MTBF) = 175,000 years.

So, at 55 degC you can run a part for 175,000 years before it will statistically fail.

At 125 degC, this goes down to 2,250 years.

So although, as temperature goes up, so do silicon failures, the numbers are minuscule.

However, as temperature goes up, mechanical and chemical problems start getting worse and these are far far more likely to cause a circuit to malfunction compared with anything dictated by Arrhenius.
 
Last edited:

KMoffett

Well-Known Member
Add in the lead-free debacle. But then, consumer goods are disposable in a few years at best.
 

gophert

Well-Known Member
Most Helpful Member
Looking at silicon only, the Arrhenius equation states that a part is 943x more likely to fail at 125degC than at 25 degC. Arrhenius is based on exp (-qV/kt) where q = 1.602 x10E-19, k = 1.38 x 10E-23, T is Kelvin and V = 0.7)

So we know the acceleration factor, but we don't know what number to start with. Looking at the failure report of a random component (LTC3891) this details a statistical failure rate of 0.65 parts per Billion device hours at 55 degC. This report can be found from the landing page on www.analog.com.
The in-house testing is approaching your calculated 2250 years (over multiple chips and packaging styles). Already at 18M hours...
 

simonbramble

Active Member
The in-house testing is approaching your calculated 2250 years (over multiple chips and packaging styles). Already at 18M hours...
Yes - I noticed that too. The theory seems to tie up with the practice. Thanks for the clarification. Reliable stuff indeed
 

audioguru

Well-Known Member
Most Helpful Member
The datasheet for some chips that have "thermal shutdown" say not to repeat the thermal shutdown often to prevent the heat expansion and cooling contraction from fracturing the chip.
 

simonbramble

Active Member
Just for completion for those who stumble on this post in future, thermal shutdown normally activates at 165 degC +/- 10 degC.

I agree that you should not get anywhere near that even at maximum ambient temperature. Even if you do, you are definitely exceeding the maximum junction temperature (TJMAX) of the device which is normally 125degC or 150degC
 

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