Google search has finally failed me. I can't find the weight of a
1210 SMT capacitor so that I can guestimate its final temperature from the dissipation and the specific heat. All I can find is either zero grams or weights that include the packaging material.
Specifically, I need the approximate weight of a AVX 1210 SQ CB series cap: Yes, I know it varies with part value. I just need a 2 decimal place ballpark value for the weight. I would weigh some caps myself except that my scale and SMT cap stash are not easily available.
--
Jeff Liebermann jeffl@cruzio.com
150 Felker St #D http://www.LearnByDestroying.com
Santa Cruz CA 95060 http://802.11junk.com
Skype: JeffLiebermann AE6KS 831-336-2558
I tested a 7.5 ohm 1206 resistor. It exploded in 1 millisecond at 65 watts.
Here are some tests with a 1206 platinum RTD:
formatting link
formatting link
formatting link
It's probably in the ballpark of the thermal mass of a 1206 cap, maybe less if the cap is thicker. You can estimate theta and the thermal tau from those scope shots. The theta and tau depend mightily on what it's soldered to.
This must be RF if you care about a cap getting hot. And pulsed, if you care about the thermal mass.
--
John Larkin Highland Technology Inc
www.highlandtechnology.com jlarkin at highlandtechnology dot com
Precision electronic instrumentation
Yes, it's RF. Output low pass filter from a 300 watt RF power amplifier. That's about 2.5A rms output current into 50 ohms. The existing 20 year old design is both labor and component cost intensive. Management believes they can sell more boxes if I remove or downgrade such frills as cooling fans, shielding, 2 oz copper PCB, somewhat exotic magnetics, silver-mica caps, layout the PCB for pick-n-plop, etc... without also downgrading the specs, of course. Converting quality into junk is my specialty, so I'm doing the initial proposal (due last Friday, of course).
The existing radial lead silver mica caps work just fine, with an ESR of about 0.01 ohms at about 10 MHz. Very little temperature rise. Usually, there are two or more caps in parallel to divide the RF current. However, in order to use cheaper ceramic caps, the ESR will be about 10 times higher. The 10x increase in dissipation, combined with a reduction in physical size makes me worry. I don't have time for a bench test, so I'm doing a back of the envelope calculation to see if I'm going to get a working filter, a fire, or an explosion.
I'll assume 0.01 grams for the weight of a "typical" 1210, 0.6 watts for 60 seconds (36 Joules), and an adiabatic system, which means I can use a linear approximation. The specific heat of Barium Titanate is about 0.5 J/gK.
0.5 = (watts * seconds) / (grams * Kelvin)
0.5 = (0.6 * 60) / (0.01 * K ) K = (0.6 * 60) / 0.01 * 0.5) K = 7200 K temp rise per minute. Nice bomb which methinks approximates what you obtained with the resistor. I'll add the PCB and solder blob heat sinking (later) and see if the results are more reasonable.
I do have an expensive way out. Mica SMT caps such as: At least they have higher than 500v ratings as I haven't calculated if
500v caps are sufficient. Oh-oh. 300 watts into 50 ohms is about 490 volts peak. Looks like I'll be putting some ceramic caps in series. SPICE simulation if there's time.
I think I know what inspired this exercise: This is the back of the board: Ceramic caps everywhere at 1KW power level. Hmmmm...
Yet another all nighter coming. I'm getting too old for this...
--
Jeff Liebermann jeffl@cruzio.com
150 Felker St #D http://www.LearnByDestroying.com
Santa Cruz CA 95060 http://802.11junk.com
Skype: JeffLiebermann AE6KS 831-336-2558
If you ignore the metalization and end caps and assume it's all a fairly dense ceramic, say Alumina at ~4g/cm^3...
formatting link
Dimensions are 2.79 x 2.79 x 0.76mm = 5.91mm^3 for the 'B' case size.
At 4g/cm^3, that's ~24mg.
By way of comparison, the density of the TDK caps that Matt pointed to (COG 3216) is 5g/cm^3, so it checks out reasonably well.
Best regards, Spehro Pefhany
--
"it's the network..." "The Journey is the reward"
speff@interlog.com Info for manufacturers: http://www.trexon.com
Embedded software/hardware/analog Info for designers: http://www.speff.com
Good idea. I'm dealing with 0.6 watts and 0.1 ohms. So, if 65 watts blows the resistor in 0.001 sec, then 0.6 watts will blow it in 0.1 sec. Since the resistance is 75 times lower, the power dissipated will also be 75 times lower. That yields 7.5 seconds, which I think is probably high because I'm too lazy to convert the temperature range to Kelvin. The volume of the 1210 package is about 50% more than the
1206, yielding 11 seconds to explosion. This is not looking very good for using SMT caps so far.
If I was going to calculate more accurate times and temperatures, I would use the slope of your resistor curves to estimate the effects of radiation, PCB conduction, and air convection. However, I just want an order of magnitude ballpark number at this point. Besides, I'm now doing battle with SketchUp rehashing the package and giving the re-design review committee something they can understand and throw rocks at.
Anyway, I don't think it's going to matter. The high voltages and moderate heat dissipations involved will probably require at least 6 SMT caps to replace two existing radial silver mica caps. Even with parts prep and pick and plop assembly savings, the estimated costs are about the same. In other words, there's no cost savings in using SMT caps. I haven't priced out the mica SMT caps, which offer some cost benefits.
Midnight, and no end in sight...
--
Jeff Liebermann jeffl@cruzio.com
150 Felker St #D http://www.LearnByDestroying.com
Santa Cruz CA 95060 http://802.11junk.com
Skype: JeffLiebermann AE6KS 831-336-2558
Hey you're in luck... you don't have to weight it (and then figure out what it's made of.) The heat capacity of everything* is about 3.0 J/(K * cm^3) so just determine the volume.
George H.
ok I have numbers for a few materials, but I couldn't find them in my note book. (All at ~300K) Cu = 3.4 Al = 2.4 Fe = 3.5 Al2O3 = 3.0
That's assuming the cap is folating in space with nowhere to get rid of its heat. And a cap is probably tougher than a thickfilm resistor, where the thin element gets all the heat.
An 0603 resistor will be reliable at 0.6 watts continuous if it's soldered to some chunky copper pours. It's generally the same substrate thickness as a 1206, and has the same l/w ratio, so its hot-spot temp is about the same *if* the end caps are kept cool.
How thick is your cap? The thicker it is, the better heat conduction to the end caps.
A couple of smaller caps in parallel would be conservative, 0.3 watts each if the net capacitance stays the same. The cooling geometry is better than one bigger cap.
Sideways caps, 0612 instead of 1206, are another way to keep things cool. The solder joint to the pcb copper is bigger and the thermal path from the part hot-spot is closer to the end caps.
Use low ESR RF-type caps to keep the dissipation down. You knew that.
--
John Larkin Highland Technology Inc
www.highlandtechnology.com jlarkin at highlandtechnology dot com
Precision electronic instrumentation
"John Larkin" wrote in message news: snipped-for-privacy@4ax.com...
^ ^ ^ ^ "to make a salt of folic acid" (folate)..? Nevermind.
I doubt thickness helps much, BaTiO3 ain't no Al2O3. The conductivity is much worse.
Special low-ESR types might have enough metal in them to be slightly more conductive? Can't imagine by much though (it's not like polypropylene (a good insulator) comparing metallization to foil). If they use completely different construction (are the CTEs close enough to wrap with Al2O3, or embed some inside?), I have no idea.
Yes, though 500V ratings was mentioned elsewhere... beats me if anyone makes 'em like that. At least, it'd be more like 1218 than 0612. Which would help on the power handling anyway.
PCB traces wick away an amazing amount of heat. For continuous operation I'd add it in earlier rather than later. If you're running it for 60 seconds I wouldn't worry about the heat capacity of the part at all -- I'd worry about it's thermal conductivity to the board.
--
Tim Wescott
Control system and signal processing consulting
www.wescottdesign.com
Please read the column heading: "Total weight(gram) including packaging".
More later... now typing furiously.
--
Jeff Liebermann jeffl@cruzio.com
150 Felker St #D http://www.LearnByDestroying.com
Santa Cruz CA 95060 http://802.11junk.com
Skype: JeffLiebermann AE6KS 831-336-2558
I have and ATC100B designers kit and collections of porcelain cap samples that I've accumulated for the last ummm... many years. I use their RF cap handbook as a reference guide[1]. I think they're the best thing ever for RF. Every time something has insufficient Q or ESR (same thing) and gets hot, I initially drop in an ATC porcelain cap to fix it. Just one problem, the price. In quantities of 1000, I pay about 50% more for porcelain caps than for the existing silver mica (CM05 CM06) caps at the same 5% tolerance level. With about 40 caps per filter board, that can become expensive. I would love to use these, but it's unlikely.
In the past, I've used ATC porcelain caps mostly because nothing else would work. The problem is that ATC porcelain caps are somewhat of a single source item. There are other porcelain cap manufacturers (AVX/Kyocera AQ series) but when I tried these as a 2nd source, there were small differences that required adjusting other component values.
Looking at the web page, I see some new caps that I've missed. I'm not familiar with the ATC 800 series: which look like they might be useful in the power amp.
Tomorrow please... I'm falling asleep at the keyboard.
[1] I just requested a new copy as I can't seem to find my old one.
--
Jeff Liebermann jeffl@cruzio.com
150 Felker St #D http://www.LearnByDestroying.com
Santa Cruz CA 95060 http://802.11junk.com
Skype: JeffLiebermann AE6KS 831-336-2558
The table is not totally useless, you're just procrastinating. You need to work backward from the maximum temperature rise tolerable to effective heat capacity required of a 1210 volume to a weight.
I really think your order of magnitude ballpark number would be to use the power dissipation of a similar-sized SMD resistor. Your "11 seconds to explosion" notion is only valid if the capacitor simply does not dissipate heat -- once you get much above the thermal time constant of the system, it's power dissipation that matters, not heat capacity.
--
Tim Wescott
Wescott Design Services
http://www.wescottdesign.com
ElectronDepot website is not affiliated with any of the manufacturers or service providers discussed here.
All logos and trade names are the property of their respective owners.