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stefuel
Joined: 07 Mar 2006
Posts: 3353
Location: Green Harbor MA USA
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| Posted: Sat Jul 18, 2009 11:03 am Post subject: Tech question, SMT capacitors |
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I have a device that stopped working in front of my eyes. I took it apart to visually inspect it. I saw no obvious defects.
Upon further inspection I noticed that two caps did not have the usual tannish brown color on top, just plain silver color. I have never seen a SMT cap without this color on top. Is this a clue that the caps are blown? Thanks,
Chip
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Chip
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Nashou66
Joined: 12 Jan 2007
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Location: West Seneca NY
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Robert A. Hill
Joined: 19 May 2006
Posts: 182
Location: Simpsonville, SC
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| Posted: Sat Jul 18, 2009 1:48 pm Post subject: |
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Multilayer ceramic capacitors (mlccs) can experience cracking that can remove the cover layers and expose the internal metal electrodes. This nomally happens when the breakdown voltage is exceeded. We normally see this as a point defect - a "blowout" , but i have seen the entire ceramic cover layer crack and fall off of high capacitance devices. These components are under high voltage stress and have relatively thin cover layers. It's rare to see these defects in devices after the normal electrical screening during manufacturing.
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stefuel
Joined: 07 Mar 2006
Posts: 3353
Location: Green Harbor MA USA
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| Posted: Sat Jul 18, 2009 1:52 pm Post subject: |
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| Nashou66 wrote: | Were they the Square SMT caps or the round type? The squares I have seen in yellow and Black, not silver. But the round type that look like a regular cap are usually silver. Can you take a pic?
Athanasios |
These are the tiny rectangular caps (ceramic SMT) that you see in every day SMT boards. They are about 1/32 wide and abput 1/16th long. They are silver on the ends (where they solder) and tan on the top center.
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Heywood Jablome
Joined: 12 Mar 2006
Posts: 1548
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| Posted: Sat Jul 18, 2009 5:47 pm Post subject: |
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Chipper, I've still got my offices' LCR meter if you need to measure any caps. See my next OT thread for "What I did on my summer Saturday"
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stefuel
Joined: 07 Mar 2006
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Location: Green Harbor MA USA
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| Posted: Sat Jul 18, 2009 6:12 pm Post subject: |
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How do you test them without a schematic? They ain't marked...
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Chip
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Nashou66
Joined: 12 Jan 2007
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Location: West Seneca NY
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| Posted: Sat Jul 18, 2009 6:15 pm Post subject: |
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| stefuel wrote: | | How do you test them without a schematic? They ain't marked... |
You might get lucky and find they are shorted if they read 0 ohms with a multimeter, usually they have some resistance or it will read OL. if you have another similar board test the same part on the good board then compare but you should lift one end of the chip though before measuring.
Athanasios
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stefuel
Joined: 07 Mar 2006
Posts: 3353
Location: Green Harbor MA USA
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| Posted: Sat Jul 18, 2009 6:15 pm Post subject: |
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| Robert A. Hill wrote: | | Multilayer ceramic capacitors (mlccs) can experience cracking that can remove the cover layers and expose the internal metal electrodes. This nomally happens when the breakdown voltage is exceeded. We normally see this as a point defect - a "blowout" , but i have seen the entire ceramic cover layer crack and fall off of high capacitance devices. These components are under high voltage stress and have relatively thin cover layers. It's rare to see these defects in devices after the normal electrical screening during manufacturing. |
Well these are completely naked now...
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Robert A. Hill
Joined: 19 May 2006
Posts: 182
Location: Simpsonville, SC
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| Posted: Sun Jul 19, 2009 2:24 am Post subject: |
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Chip,
MLCCs are specified by size, capacitance and voltage rating.
1. Size is easy- just measure with micrometer: 0402= 0.040L x 0.020" long, 0603=0.060"x0.030", 0805=0.080x0.0.50",
1206=0.120x0.060, 1210= 0.120x0.100. These are the most popular sizes for consumer electronics but larger ones are still used by automotive and military.
2. Cap will be impossible to determine if they are shorted without analysis. Otherwise LCR meter will work.
3. Voltage rating is inherent in chip design (dielectric type and thickness). Common values are 3, 6.3, 10, 16, 25, 50, and 100 volts. Without schematic, destructive physical analysis and comparison to determine which manufacturer could help. Let me know if this is worth it to you. I work for a large cap manufacturer and can have my lab take a look at one if you send it to me.
TCC (Thermal Coefficient if capacitance) is importance for some applications and is dielectric specific. ESR (equivalent resistance) is very low for all MLCCs and dependent on cap, but very low ESR and low series inductance designs are possible. Low ESL chips are commonly terminated in the long sides instead of the ends for example.
Last edited by Robert A. Hill on Sun Jul 19, 2009 10:17 pm; edited 1 time in total
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macgyver655
Joined: 22 Aug 2007
Posts: 8508
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| Posted: Sun Jul 19, 2009 3:12 am Post subject: Re: Tech question, SMT capacitors |
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| stefuel wrote: | I have a device that stopped working in front of my eyes. I took it apart to visually inspect it. I saw no obvious defects.
Upon further inspection I noticed that two caps did not have the usual tannish brown color on top, just plain silver color. I have never seen a SMT cap without this color on top. Is this a clue that the caps are blown? Thanks,
Chip |
Hey Chip. No service manual for this device? What is it anyways?
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macgyver655
Joined: 22 Aug 2007
Posts: 8508
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| Posted: Sun Jul 19, 2009 3:14 am Post subject: |
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| Robert A. Hill wrote: | Chip,
MLCCs are specified by size, capacitance and voltage rating.
1. Size is easy- just measure with micrometer: 0402= 0.040L x 0.020" long, 0603=0.060"x0.030", 0805=0.080x0.0.50",
1206=0.120x0.060, 1210= 0.120x0.100. These are the most popular sizes for consumer electronics but larger ones are still used by automotive and military.
2. Cap will be impossible to determine if they are shorted without analysis. Otherwise LCR meter will work.
3. Voltage rating is inherent in chip design (dielectric type and thickness). Common values are 3, 6.3, 10, 16, 15, 50, and 100 volts. Without schematic, destructive physical analysis and comparison to determine which manufacturer could help. Let me know if this is worth it to you. I work for a large cap manufacturer and can have my lab take a look at one if you send it to me.
TCC (Thermal Coefficient if capacitance) is importance for some applications and is dielectric specific. ESR (equivalent resistance) is very low for all MLCCs and dependent on cap, but very low ESR and low series inductance designs are possible. Low ESL chips are commonly terminated in the long sides instead of the ends for example. |
Hey Robert. What company do you work for? If I may ask.... 
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stefuel
Joined: 07 Mar 2006
Posts: 3353
Location: Green Harbor MA USA
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| Posted: Sun Jul 19, 2009 4:03 am Post subject: Re: Tech question, SMT capacitors |
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| macgyver655 wrote: | | stefuel wrote: | I have a device that stopped working in front of my eyes. I took it apart to visually inspect it. I saw no obvious defects.
Upon further inspection I noticed that two caps did not have the usual tannish brown color on top, just plain silver color. I have never seen a SMT cap without this color on top. Is this a clue that the caps are blown? Thanks,
Chip |
Hey Chip. No service manual for this device? What is it anyways? |
No service manual. It's a Crestron CPC-CAMI pan tilt zoom power head interface board. It was a Ebay purchase. I bench tested it when it got here and it worked fine. I installed it at a camera location in my driveway and calibrated. It worked perfectly for a couple of days. It started to act funny so I did a re-calibration. that's when it failed. It's supposed to swing and tilt in one direction till it hits the mechanical stops, then swing in the other direction to the other stops, then go to the center position. It checked the first stops, swung to the second stops and when it reached the second stops, it died. The power LED is still on but the manual controls are not funtional and it won't respond to a call for calibration. There is also a "NET" LED that tells you the unit is connected and configured to the Crestron system. That is on but flickering. It was flickering when it worked but now it's doing it more. I originally thought it was a weak LED but now I'm not sure. I have tested the power head it's self and it works fine. I can pan and tilt with relays but I'd like to fix the controller as it has presets and variable speed not to mention the fact that I spent the better part of a day programming it 
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