clear flux under 1206 resistor

I have a 1206 component that need to have all of the flux removed under it to achieve low leakage.

The standard footprint with solder mask between the pads has shown a tendancy to hold flux even after a soak and swish in the solvent base flux washer.

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Looking at Johnashon dielectrics application note that tested slots, solder mask removed under the part and combinations of both that found that only removing the solder mask was good enough to sustaing high voltage across the part without flashover underneith due to flux. While my issue is not HV flashover i liken that to leakage currents.

Slots are fairly expensive and I will need this on ~ 12 parts in close proximity to each other. My concern is to compromose the strength of the board. Also I fear it would concentrate the bendinf force between the component lags and stress the solder joint.

I'm thinking of instead of putting a large slot, just drilling a unplated hole under the part to all the flux cleaner to flush under part and clear out any flux. This hole would need to be blown out after flux cleaning to removed the capliary action held cleaner but it should clean better than just removing the solder mask.

What so you guys think about handling a situation like this?

1) low cost 2) repeatable perforance for low leakage current 3) minimal compromise to board strangth.
Reply to
mook Jonhon
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We use a 10-mil slot extending 20 mils past the edges of the resistor, and that works with resistors well up into the gigohms, provided you use a leaded process with rosin flux followed by a good solvent clean.

If you align the slot along the long dimension of the board, the stress is reduced. If you're worried about it, you can put a C-shaped cut around the group.

Don't let water near the board, ever.

Cheers

Phil Hobbs

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Dr Philip C D Hobbs 
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Reply to
Phil Hobbs

We use RMA flux and a solvent cleaner for picoamp leakage apps. No slots or other layout tricks.

We clean by submerging the board in boiling solvent that has a deflux agent, for about a minute, then it gets washed well with clean (distilled) solvent with a spray wand. That works fine.

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If you use water wash, you'll have leakage problems.

You can also get low leakage with the proper no-clean flux, but it looks bad.

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John Larkin         Highland Technology, Inc 
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Reply to
John Larkin

Yikes, 10 mil slots? The poor fab has to carve those with needles!

Tim

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Reply to
Tim Williams

Both of our fabs (PNC in NJ and PCBway in Shenzhen) do 10-mil slots routinely, no extra charge. The slots are only 100 mils long for a 1206 part, so it's not like they have to cut the lawn with nail scissors. ;)

Cheers

Phil Hobbs

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Reply to
Phil Hobbs

se

!

It's just that it is a very small bit which is much more prone to breaking. Or are they using a laser???

Rick C.

Reply to
gnuarm.deletethisbit

Dunno. They're good enough at it that they don't charge extra, anyway.

Cheers

Phil Hobbs

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Reply to
Phil Hobbs

John Larkin wrote in news: snipped-for-privacy@4ax.com:

Hehehe... yeah... slots do not work (very well) when things get below a certain size.

Another thing we did is to apply solder bumps to the pads and stand the part on top of those, and reflow one side at a time keeping it floating. This yields a part that sits just a slight bit further off the board. That makes complete cleaning easy and picoamp leakage levels at their lowest.

Another great thing is a product called "Ensolve" It is a brominated solvent and when you clean an area with it then flush, there is ZERO remaining residuals. It ain't cheap stuff though. Best product for HV.

Reply to
DecadentLinuxUserNumeroUno

Hah, what if someone in India wants to buy one? (Singapore, Vietnam) (most of my problems have been with plastics and not resistors. Sockets, switches, connectors.) Through hole R's have never given me problems.

George H.

Reply to
George Herold

Through-hole parts are easy to clean, which is one of the main reasons that I often use TH feedback resistors in otherwise all surface-mount TIAs.

High humidity can be dealt with by conformal coating, plus or minus. On a hydrophilic surface, the equilibrium thickness of the adsorbed water is about one monolayer at 40% RH, and increases pretty rapidly from there, going off towards infinity at 100% RH.

Residual ionic contamination makes it much much worse. Those EU morons who insisted that everyone go lead-free have a lot to answer for.

Cheers

Phil Hobbs

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Reply to
Phil Hobbs

They work fine for 1206 and above, which is fine because you don't need that performance everyplace.

Picoamp leakage is actually pretty bad when it's coming via ionic conduction at a surface--the 1/f noise is way, way above the shot noise.

Never tried it.

Cheers

Phil Hobbs

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Reply to
Phil Hobbs

The solvent we use is something like $150 a gallon, but it's continuously distilled in use, and the refrig coils condense it to stop evaporation.

We use water wash for digital stuff.

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John Larkin         Highland Technology, Inc 

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Reply to
John Larkin

OK, I think in the future I'll make all resistors greater than 1Meg TH. (if I have space.) Cleaning up before inventory I found a bad of 1G 1/4 watt TH. brown, black, black, purple, gold. I have no idea where they came from.

Are we going to be able to sue them? I think they are responsible for getting the lead out of the tin used for switch bodies.. which lead to (gave rise to) tin whiskers and kilo bucks of time for us.

George H.

Reply to
George Herold

Wow, 1/4 watt--that's 16 kV! ;)

My biggest ones ere 50G, for the front end of a scanning surface potential tool from the late lamented Qcept. It used relays to switch between 1G and 50G feedback resistors, and I had to short out the 1G one when running 50G, because otherwise the Johnson noise current of the 1G would have coupled into the summing junction through the ~0.2 pF of the open relay contacts and destroyed the measurement. (Another win for doing the math in advance.)

It had a lead-lag network in the second stage to flatten out the response of both ranges, which was limited by the ~0.05 pF shunt capacitance of the resistors.

Wouldn't it be nice. They cost IBM tens of millions of dollars in lead-free solder process development, and the increased reflow temperature (and consequently higher residual stress) made it impossible to use chips larger than (iirc) 22 mm square, because the solder balls at the corners tore off, in spite of hard epoxy underfill.

It could be the mother of all class-action suits.

Cheers

Phil Hobbs

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Reply to
Phil Hobbs

1G is a more reasonable threshold if you don't water wash.

I have some 0805 1T resistors.

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John Larkin         Highland Technology, Inc 

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Reply to
John Larkin

Have you tried an ultrasonic cleaner? We have some high impedance stuff that has bias resistors with up to 350 V across them. We have to do several cycles of cleaning on some of the boards to get the leakage down.

You can then blow off the boards with compressed air.

But, the unplated hole might work well.

Jon

Reply to
Jon Elson

Water wash?

Yikes! Compressed air is usually wet and rusty.

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John Larkin         Highland Technology, Inc 
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Reply to
John Larkin

Lol! Yes, if you use the type of system they have in auto shops. It is simple enough to remove the water and dust with filters.

Rick C.

Reply to
gnuarm.deletethisbit

Certianly, ultrasonic cleaning apparatus requires water-based fluids (the cavitation of water is the effective phenomenon).

The same refrigerator/condensier technique you use on pricey solvent works on compressed air. Wet and rusty might be OK for some things, but compressed air can be suitable for fine painting.

In small quantities, tanks of nitrogen are good and clean, too. Airgas delivers.

Reply to
whit3rd

If the flux is water soluble, or if you use a saponifier on rosin to make it soluble, it will be pretty much impossible to get rid of hygroscopic ionic contamination under parts. That's OK for digital and low-impedance analog stuff, bad news if picoamps matter.

Ultrasonics can damage some parts.

We have a nitrogen generator system, to keep oxygen out of the reflow oven. That helps ROHS soldering a lot.

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But we don't blow anything on boards to dry them. I wouldn't trust that to get under all the parts.

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John Larkin         Highland Technology, Inc 
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Reply to
John Larkin

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