Previous in Forum: Toll Manufacturing   Next in Forum: about cutting tool consumption ratio
Close
Close
Close
10 comments
Rate Comments: Nested
Participant

Join Date: Sep 2007
Location: Angeles City, Philippines
Posts: 4

How To Reduce Coefficient Thermal Expansion

12/28/2007 7:09 AM

we have problem on crack ceramic capacitor, and i think reducing CTE is the best solution to this

__________________
"Be the change you want to see in the world." -- Mahatma Gandhi
Register to Reply
Interested in this topic? By joining CR4 you can "subscribe" to
this discussion and receive notification when new comments are added.
Guru
Popular Science - Weaponology - New Member United Kingdom - Member - New Member

Join Date: May 2007
Location: Harlow England
Posts: 16512
Good Answers: 670
#1

Re: How To Reduce Coefficient Thermal Expansion

12/28/2007 8:25 AM

It seems you are pre-supposing the solution? You can only change the CTE by changing the composition of the ceramic....

What circumstances cause it to crack? How is it mounted? What is it's physical size/shape? How are the connections made? What material are the connections?

Years ago I had a problem with a hybrid amplifer being very sensitive to the unit being flexed...it was mounted on 6 legs down each side, putting a zig zag bend in each leg completedly solved the problem. I likened it to standing with flexed legs when travelling standing up on a train...much smoother than standing rigid.

Dunno if this is any help

Del

__________________
health warning: These posts may contain traces of nut.
Register to Reply
Power-User

Join Date: Nov 2007
Location: Alberta Canada
Posts: 182
Good Answers: 9
#2

Re: How To Reduce Coefficient Thermal Expansion

12/28/2007 12:07 PM

AS Del said, before you can find a solution to your problem, you have to find the cause of the problem.

Is the CTE really the problem, or is mounting, shock loads, manufacturing defects etc the real cause.

Once you know the cause then you can find a solution. As Del also says (damn, why is he always right!) the only way to change the CTE of the material is to change the material.

__________________
Vote for something useful this time, vote to repeal the second law of thermodynamics!
Register to Reply
Power-User

Join Date: Sep 2007
Location: Sheboygan, WI USA
Posts: 372
Good Answers: 13
#3

Re: How To Reduce Coefficient Thermal Expansion

12/28/2007 11:51 PM

As Del stated putting a flex in the leg, I use a loop or dog-leg vs sharp bends which would stress the wires and in a high vibration environment you may need to fasten the cap to the board and put flex in the leads. A few years ago I wired ocean-going racing boats, lots of flexing of fibreglass and shock loads on wiring and hardware.

The alternative is that you have an electronic problem where voltage transients overheat the cap to the point it explodes or the leakage thru the cap is so high it heats up and explodes. Look at the circuit with an oscilloscope for transients or voltages exceeding the breakdown voltage the cap was designed for which would cause it to heat up and explode.

I have seen DC control circuits where the coils of relays discharging were causing problems, the resolution was discharge diodes, if your circuit has large inductor/coils being switched without a discharge circuit,,,you have a problem you need to address with disharge diodes.

The type of capacitor for the circuit or the capacity or the breakdown rating is more likely to break the cap then vibration unless during installation it is physically stressed somehow?

Just a few more thoughts to move you in a direction that by now may be obvious?

__________________
"I believe we are masters of our lives - we hold all the cards and it is up to us to use them right." Vesna Vulova - survived 33,000ft fall
Register to Reply
Power-User

Join Date: Apr 2007
Posts: 414
Good Answers: 19
#4
In reply to #3

Re: How To Reduce Coefficient Thermal Expansion

12/29/2007 12:59 AM

That's Vesna Vulović. See http://en.wikipedia.org/wiki/Vesna_Vulovic. Great story.

Register to Reply Off Topic (Score 5)
Guru

Join Date: Apr 2007
Posts: 3531
Good Answers: 59
#5

Re: How To Reduce Coefficient Thermal Expansion

12/29/2007 11:01 AM

The most common problems with ceramic capacitors that are surface-mounted in PCB are cracking of solder or separation of electrodes from the capacitor. These usually occur following temperature cycling that exceeds the capability of the mounting technique used, and are due to the TCE of the ceramic being much smaller than the TCE of the PCB. However, if the materials package, termination pads or process cycle are incorrectly defined, you can also see these problems during manufacture.

N.B. that there should be never be issues even for military or for automotive requirements if you can use capacitors whose maximum dimension is 2-mm or less. That means that, if your problem is because the design uses capacitors with larger dimensions it may be that the most economic approach would be to use a multiplicity of smaller components. If this is not practical, the situation can usually be improved by using the mounting techniques proposed in other postings, but these can sometimes introduce excessive series inductance, or make the equipment vibration sensitive. Only in this case will it make sense to use a substrate that provides a lower TCE; various substrates can provide better TCE match to capacitor ceramics, ranging from ceramic boards (so-called "hybrid packaging") to multilayer substrates that use low-expansion alloys (e.g. Kovar) as dimensional stabilisers.

If you will provide information that clears up the uncertainties we have regarding your problem, we may be able to provide clearer (and simpler) guidelines.

Register to Reply
Power-User

Join Date: Jun 2006
Location: Birmingham, Alabama, USA
Posts: 313
Good Answers: 7
#6

Re: How To Reduce Coefficient Thermal Expansion

01/02/2008 2:53 PM

You have received a lot of interesting suggestions, but you have not gotten a direct answer to your question yet (interestingly, that is often the case on this board). The first zero CTE ceramic was Corningware. There may be others now. Zero CTO was achieved in Corningware by mixing very small particles of two ceramic materials- one with a positive CTO and one with a negative CTO. The application it was developed for was radar-transparent missle nose cones. (At least that's how I remember the story from Materials class decades ago.) It should be possible to purchase the material from Corning or someone else for use in your fabrication facility.

Bill Morrow

__________________
Bill Morrow
Register to Reply
Guru
Popular Science - Weaponology - New Member United Kingdom - Member - New Member

Join Date: May 2007
Location: Harlow England
Posts: 16512
Good Answers: 670
#7
In reply to #6

Re: How To Reduce Coefficient Thermal Expansion

01/02/2008 4:05 PM

You have received a lot of interesting suggestions, but you have not gotten a direct answer to your question yet (interestingly, that is often the case on this board).

I think I answered at trick 1.

__________________
health warning: These posts may contain traces of nut.
Register to Reply Off Topic (Score 5)
Guru

Join Date: Apr 2007
Posts: 3531
Good Answers: 59
#8
In reply to #6

Re: How To Reduce Coefficient Thermal Expansion

01/02/2008 5:09 PM

The question didn't actually state what part of the system would require the reduced TCE. Given the usual environments in which ceramic capacitors are used, it's most unlikely that it would be desirable to reduce the TCE of the capacitor. In any case, a capacitor is a layered structure of metal and ceramic**, so the TCE of the capacitor is not simply the TCE of the body material. Then, you need specific dielectric properties (usually high er) for the capacitor body, whereas the ideal for the nose-cone would be to have er as small (close to unity) as possible. You also need a material to which the conductor will adhere.
**As far as internal differential expansion is concerned, this layered structure is capable of withstanding temperatures up to 1300-OC; terminations etc would be destroyed long before that temperature is reached.

Randall didn't answer the question as posed - but failing provision of additional detail I think we should assume that his answer would be the most appropriate solution to the actual problem.

Register to Reply
Power-User

Join Date: Jun 2006
Location: Birmingham, Alabama, USA
Posts: 313
Good Answers: 7
#9
In reply to #8

Re: How To Reduce Coefficient Thermal Expansion

01/02/2008 6:06 PM

Yes, you are probably correct. I did not mean to attack anyone with my comment:

"you have not gotten a direct answer to your question yet (interestingly, that is often the case on this board)".

I was just making an observation. Indeed, it is a good thing that there is such a wealth of information here that people can often see past the specific question and address the real issue/need.

:>)

Bill

__________________
Bill Morrow
Register to Reply
Guru

Join Date: Apr 2007
Posts: 3531
Good Answers: 59
#10
In reply to #9

Re: How To Reduce Coefficient Thermal Expansion

01/03/2008 6:07 AM

Nicely put. Sorry if I misinterpreted your previous contribution

Fyz

Register to Reply Off Topic (Score 5)
Register to Reply 10 comments
Copy to Clipboard

Users who posted comments:

bmorrow492 (2); bubbapebi (1); CoronaCameraMan (1); Physicist? (3); user-deleted-1105 (2); Zaphod2Headed (1)

Previous in Forum: Toll Manufacturing   Next in Forum: about cutting tool consumption ratio

Advertisement