Workbench Wednesday 05: Surface Mount Rework Tools

Thank you, James. I tried the setup you described to measure a 10uF electrolytic and 100 nF ceramic capacitors. The IR was about 12 Mega Ohms for the electrolytic and 200 Mega Ohms for the ceramic.

By the way, I also found an answer to this frequently asked question on TDK's website: https://product.tdk.com/info/en/contact/faq/faq_detail_D/1432616871058.html

I am glad you are part of the community!

koudelad  wrote:
Is there a way to easily measure the Insulation Resistance? I only heard about measuring Capacitance and ESR.

Of those three (IR, Cap, and ESR), Insulation Resistance is the easiest to measure. Apply a voltage for 5 minutes, then measure the current. Use ohm's law and you get the insulation resistance of the dielectric.

The inverse property is leakage current. It measures the quality of the dielectric. An ideal dielectric would be infinite ohms since no current should pass through it. However, all dielectrics are far from perfect. So by measuring the leakage current, you know its condition.

For ceramic capacitors, the dielectric type, determined by temperature coefficient, will vary. For X5R/X7R it'll be in the mega-ohm range while C0G will be in the giga-ohm (or even tera-ohm) range. Cap manufacturers usually specify the IR limit as a function of the capacitance and rated voltage. (Lower rated voltage means thinner layers, so more leakage. Higher capacitance means more layers, so more leakage.)

Thank you very much, James, for all the information. Definitely something I haven't heard about before.

Is there a way to easily measure the Insulation Resistance? I only heard about measuring Capacitance and ESR.

koudelad  wrote:
 
Did I understand correctly that I shouldn't solder ceramic capacitors using a soldering iron, but only a hot air gun? 

Ideally, yes. Touching the terminals of a ceramic capacitor with the soldering iron can thermally crack it. As the nickel electrode heats up, it expands and separates from the surrounding dielectric. With nowhere to go, it causes a crack. When an MLCC with a thermal crack is cross-sectioned, the cracks run parallel to the electrode plates. (Whereas a flex crack runs 45 degrees from the "bottom" of the termination towards the outer termination wall.)

So... does that mean you can't use a traditional tip or that all of your capacitors are cracked? No.

When hot air isn't available, heat the solder blob on one of the pads and push the capacitor into the solder. While the process still causes a thermal gradient, the lack of direct heat from the iron gives time for heat to spread. When soldering the opposite terminal, keep the iron on the PCB pad and let solder wick down from the capacitor terminal. Lots of flux is necessary.

Is it possible to test? Not easily. Just like any other component, an in-circuit test can be difficult. Ideally, you want to measure the Insulation Resistance (IR), Capacitance, and ESR. The IR is probably most critical because it'll tell you if there are any cracks. The way cap manufacturers do it; they'll cut the PCB around the capacitor and then soak the assembly in alcohol under a vacuum. After a few hours, if there are cracks, moisture seeps in and causes a shift in IR.

Did I understand correctly that I shouldn't solder ceramic capacitors using a soldering iron, but only a hot air gun? I currently do not have any hot air gun at home (nor the solder paste, which is usually used for a reflow process). Is there any chance to check the capacitors I already soldered without removing them?

Explosions during hot air rework is often moist that got in the component cavities.

That's a good point. When I worked for the capacitor company, we called it "pop-corning." Anything with an epoxy enclosure (e.g. surface mount chip tantalum or ICs) absorb moisture. When moisture experiences reflow temperatures. it converts to steam, and destroys the part. For production reflow, keeping the parts dry is best, which is where the JEDEC Moisture Sensitivity Levels (MSL) come from. Most epoxy parts are MSL 2 or 3.

When using hot air, my experience has been if you're using too much heat, you'll end up pop-corning parts while removing them. Using lower heat and giving the moisture time to egress, prevents the pop-corning from happening. Of course, the best approach is to bake the entire assembly before removing parts, but that isn't always practical.

is +5 to +35°C and less than 75% in relative humidity.

An important distinction is whether these environmental conditions apply to the heat from reflow, solderabiilty of the component leads, or operational life of the component. For MSL 2 or higher parts, the environmental conditions are usually limited by the MSL recommendations, which are meant to prevent thermal damage (more appropriate term for pop-corning) during reflow.

For MSL-1 or non-MSL parts, environmental storage conditions are such that the leads will remain solderable for a certain period of time.

In the case of a traditional aluminum electrolytic capacitors, like the EDX and EDV series from KEMET, the storage condition is recommended to preserve the dielectric layer and life of the electrolyte. There's no concern over environmental moisture ingress. The paper separators are already impregnated with significant moisture. Outside air isn't going to add much.

Aluminum polymer capacitors, on the other hand, have a different story. PEDOT absorbs moisture faster than a Sham-Wow. Worse, that moisture can cause the polymer to oxidize, raising the capacitor's ESR. So in the case of polymers, the storage conditions are to prevent pop-corning AND prevent degradation of the cathode layer. (Those that oxidation process is relatively slow in terms of the lifetime of the capacitor.)

Explosions during hot air rework is often moist that got in the component cavities. Dry components made for reflow luckily don’t do that .

Production houses have a dry oven where sensitive components go through before being sent to the solder cycle.

I have hybrid ICs here that require a day long pre-bake.

(the spec of the electrolytes mentioned above says :

KEMET's E-series aluminum electrolytic capacitors should not be stored in high temperatures or where there is a high level of humidity.

The suitable storage condition for KEMET's E-series aluminum electrolytic capacitors is +5 to +35°C and less than 75% in relative humidity.

KEMET's E-series aluminum electrolytic capacitors should not be stored in damp conditions such as water, saltwater spray or oil spray.

)

Did you happen to measure the higher clock frequency?

Hi Doug, I did. It was about 11.3 MHz.

Did you happen to measure the higher clock frequency?

Question from YouTube:

Any suggestions for the correct way to rework surface mount electrolytics? Hot air tends to make them explode, and it’s next to impossible to get tweezers on the leads. Some YouTube videos suggest just twisting them off the board with pliers. Yeah, no. Maybe some boards can withstand that, but not the old stuff I work on

If hot air is exploding them, you’re using too much heat or too much air flow. Surface mount electrolytic caps are designed for reflow temps. but you’ll notice on larger cans, their max temp can be lower than other components. The JEDEC standard profile ( J-STD-020) peaks at 260°. Here is an example from KEMET's EDK series.

Smaller cans (260°) peak higher than larger cans (250°).

With all that said, consider the construction of a SMT electrolytic. The can is mounted to a deck. Again, lets use the KEMET part as an example.

The leads are on the deck. So you only need to heat the deck area.

I’d recommend using a narrow nozzle and heating either side of the lytic with a focus on the deck, not the can.

oh. and of course, use lots of flux!