SMD 0805 capacitors

[Viktor]

On my board I'm having tons of small 0805 capacitors (unmarked).

Measuring ESR one of them has almost 95ohms and on a similar board it's only 0.3ohm..

I'd like to replace it but there is no manual available so it's impossible to figure out the values.

How would I go about doing that on such a small capacitor? Would a regular DMM or LCR meter be able to measure the pico farads ?

And how fragile are these bastards to soldering? Is it fine to desolder and solder with a hot air gun?

 

[BobK]

What makes you think it is picoFarads? You can get up to 100uF in that size.

Did you measure the ESR in circuit? If so, the reading is meaningless.

Desoldering is most easily done with a hot air gun. Soldering is easier with a regular iron with a small tip.

If you don't have tweezers, get some.

Bob

 

[Viktor]

Glad to hear some feedback, I'm new to electronics and am very happy to get opinions.

Firstly, I haven't assumed any capacitance yet. It was more of a- what if scenarion.

Secondly, what's your argument for not being valid if measured in circuits?
Isn't the purpose of an ESR meter to use very low test voltages so semiconductors connected to the capacitor won't turn on, and therefore being able to measure in-circuit without having any impact on your readings? As long as there is no coil or another capacitor connected directly across the cap in parallel?

I had two identical boards, and measured most of the capacitors and got a near identical ESR measurement on all capacitors, except for one of those 0805 ones who had a 94,7ohm difference. ( That cap with 95ohm ESR (IN CIRCUIT) was next to a burned component. )

 

[BobK]

With something to compare and that large difference, you are right, it has identified a problem capacitor.

Bob

 

[(*steve*)]

Typically ESR measurements are only valid for capacitors above some value.

Smaller value capacitors will show a voltage drop sure to the capacitor charging which exceeds that due to the ESR. When you read 95ohms, you are probably measuring a small value capacitor and not seeing the ESR.

ESR can be measured in circuit. The actual value (assuming you're reading ESR) can only be lower than your measurement, so a high value is significant.

 

[Michael Studio1]

Typically ESR measurements are only valid for capacitors above some value.

Smaller value capacitors will show a voltage drop sure to the capacitor charging which exceeds that due to the ESR. When you read 95ohms, you are probably measuring a small value capacitor and not seeing the ESR.

ESR can be measured in circuit. The actual value (assuming you're reading ESR) can only be lower than your measurement, so a high value is significant.

Yes - this very much depends on the test instrument you're using. For my Lab I have a PeakESR70 which gives the read-out e.g. Cap = 9.6μF ESR = 1.08Ω. This is an actual test result from a 10μF SM cap on the Denon AVR-1912 I'm working on.
Michael Studio1 UK 13:44BST

 

[shrtrnd]

This is off subject, but I made the comment a couple years ago and think it might be a good time to repeat it.
I was buying SMD's for repair of a specific circuit regularly for years. Then I suddenly got a batch of the same exact part number in a smaller SMD size. I contacted the mfgr, and they told me the had a new composition, and were able to shrink the case size. They didn't bother changing the part number.
The reason I'm mentioning this again, even though it's not relevant to this particular post, is that I've found that some old original SMD parts, were larger case sizes then, and have been shrunk since then. In new designs I suppose that doesn't matter, but in the older electronics some SMD part numbers had larger case sizes than what's available now.

 

[kellys_eye]

They didn't bother changing the part number.

That's a big no, no in my books. Changing any parameter of a component makes it a 'new' component and the part number should change accordingly. In the case of changing SIZE, this is even more important as a physical size change could ruin any component placement (pick and place machines) or simply not fit the solder pads when making repairs (not exactly the case for 'adjacent' SMD sizes but, still, could be problematic).

 

[AnalogKid]

It sounds like you are trying to repair an SMT board for which you have no schematic, BOM, layout information, or reference designators. Not a good starting point. Do you have any information about the product? Can you post photos?

ak

 

[kellys_eye]

Is it fine to desolder and solder with a hot air gun?

A 'proper' SMD reworking hot air gun will be ideal but I suspect you may be referring to a simple DIYers hot air gun? In which case you're likely to blow a whole load of parts off the board!

As per @AnalogKid above, more details before you get yourself into bigger trouble!

 

[Michael Studio1]

That's a big no, no in my books. Changing any parameter of a component makes it a 'new' component and the part number should change accordingly. In the case of changing SIZE, this is even more important as a physical size change could ruin any component placement (pick and place machines) or simply not fit the solder pads when making repairs (not exactly the case for 'adjacent' SMD sizes but, still, could be problematic).

Well, as for changing size that could possibly be a problem which needs examining before attempting it. Two of the Electrolytics in the HMDI_B'D of the Denon AVR-1912 were SM's, but, because these weren't available, I craftily replaced them with through-hole types of the same capacity and Voltage but bent the wires in such a way as to enable soldering them to the original pads on the pcb - C799 & C831. - and they work. Michael Studio1 UK 20:26BST

 

[kellys_eye]

For the individual making isolated repairs it doesn't really matter that components change physically as there's always a way to make the new part fit!

 

[shrtrnd]

Yeah kellys_eye (still off-topic).
That was exactly my problem. The pads for the SMC device were larger and the smaller 'replacement' made me solder-bridge more than I liked. The manufacturer (it was a big one, I can't remember the name, but saved the email from them somewhere), said the smaller package could still dissipate the same wattage, so they so they made the business decision to just shrink the package so it would fit the newer, smaller, electronic circuits being designed.
They just didn't care to bother with changing the part number. I thought that was a bad decision, but they did it.
Late 1990's when they made the change.