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omit stencil apertures to hand solder or paste, reflow then solder.

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#84532

omit stencil apertures to hand solder or paste, reflow then solder. | 25 February, 2020

I am currently weighing up my options for resistors that will be fitted to a fully assembled PCB (PTH&SMT). Do I omit the stencil apertures leaving exposed pads (ENIG) so they can be hand soldered with fresh solder. Considering shelf life for solderability, how would it be affected post reflow? Is it better to apply solder paste and reflow with the rest of the Surface mount then solder components? I can't make up my mind and would appreciate any advice in case of things I am overlooking? Thanks!

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#84533

omit stencil apertures to hand solder or paste, reflow then solder. | 25 February, 2020

First question would be why are you hand soldering these resistors?

I would expect ENIG to be perfectly solderable after reflow and quite some time later.

If you leave the apertures out of the stencil and then decide to automate fitting them, you'll need a new stencil (or to reprocess that one if your stencil house does that).

No-clean paste will leave a protective laver of inert flux on your unused soldered pads, again probably perfectly usable. However depending on technique/tools used the person manually soldering these resistors might wick off the solder from those pads before the manual stage (e.g if they're doing it with a soldering iron).

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#84534

omit stencil apertures to hand solder or paste, reflow then solder. | 25 February, 2020

The reason the resistors are fitted at a later stage is to complete the circuit but the values that will be fitted depends on the frequency range desired. My preference would be to leave them free of solder, limiting the options for the process used, which in my mind will be with an iron. If they are already soldered it may lead people to use various techniques based on preference.

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#84536

omit stencil apertures to hand solder or paste, reflow then solder. | 25 February, 2020

If you get the stencil made with all apertures included then you can blank some off with sellotape. Saves you buying 2 stencils should you require them machine fitted in the future

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#84538

omit stencil apertures to hand solder or paste, reflow then solder. | 25 February, 2020

I think I'd opt for a stencil with everything already cut and then tape it if that didn't screw up your stencil gasketing.

Then you just have to define the process for adding the optional parts and you can decide to use those apertures or not.

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DWL

#84539

omit stencil apertures to hand solder or paste, reflow then solder. | 25 February, 2020

Are you running a no clean process, or are you washing your PCBs? One caveat with ENIG is sometimes washing with hot DI water can effect solderability later on.

I would also talk to your solder technicians and see what their preference is. Some prefer to have a flat surface to solder to while others prefer to just add a little flux and reflow the existing solder. Which ever way you go, make sure your board is clean afterword. Even no clean flux can cause problems with sensitive RF circuits if left on the board.

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#84550

omit stencil apertures to hand solder or paste, reflow then solder. | 25 February, 2020

Flat surface is the number one choice for fresh part soldering. We did not discuss details about resistor size and location, but to keep them flat I will suggest no apertures on the stencil.

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#84553

omit stencil apertures to hand solder or paste, reflow then solder. | 26 February, 2020

"One caveat with ENIG is sometimes washing with hot DI water can effect solderability later on."

How so? (Genuinely curious)

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#84559

omit stencil apertures to hand solder or paste, reflow then solder. | 26 February, 2020

I'd agree with getting a stencil fully cut, and taping the pads you don't want paste on.

But, I'd also check with the people that will be doing the hand soldering to see what they prefer. Some people prefer soldering on bare pads, some prefer solder to already be there. If the solder is already there, the rework can be done with a hot-air gun, rather than soldering irons; which might make that stage faster.

Cheers, ..rob

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#84563

omit stencil apertures to hand solder or paste, reflow then solder. | 27 February, 2020

I want to avoid peoples preferences as they will differ and will not be a deciding factor. A heat gun will introduce risk to surrounding PTH components and having to add protection for those, will limit any gain in productivity. There is no question of taping apertures the boards will always be built in this manner.

My original post was pretty ambiguous.

It's a question of what is the best process for the longevity of the shelf life for a populated PCB, for pad's that will be soldered at a later date.

My assumption is the best route to take with that in mind is to cover all exposed pads with solder.

However, this leads me on to other concerns, which I can combat in other ways, so it really isn't a big deal.

So I was wondering, how long would an ENIG PCB that has been through a reflow process, hand fitting of PTH and cleaning remain solderable for: 1 Pads that remain free of solder? 2 pads that have been soldered?

Thanks to you all for taking the time to share your thought's.

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#84565

omit stencil apertures to hand solder or paste, reflow then solder. | 27 February, 2020

In both cases, it will depend on storage conditions, unless you do something horrible to them (or as previously mentioned, wash them) months & months. For boards we put through SMT and then stash for THT loading in smaller qty's I don't think we've ever seen that become an issue.

There are some that would argue that soldering resistors with a single tip soldering iron introduces thermal stress across the resistor and that tweezer irons are better suited if an air pencil or focused IR rework station is not a good fit.

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DWL

#84572

omit stencil apertures to hand solder or paste, reflow then solder. | 27 February, 2020

@Phil, not sure if this link will work, if not try googling the name of the .pdf:

http://www.circuitnet.com/news/uploads/3/Comparing-Soldering-Results-of-ENIG-and-EPIG-Post-Steam-Exposure.pdf" target="_blank">http://www.circuitnet.com/news/uploads/3/Comparing-Soldering-Results-of-ENIG-and-EPIG-Post-Steam-Exposure.pdf

Years ago, I battled this problem. ENIG PCBs reflowed with active WS flux. We built one side, and then washed them because they were going to sit on a shelf for a while before we could build the other side. When we build the other side, we had terrible non-wetting problems on the pads. All we did to them was wash in DI water in a batch style cleaner.

We had this problem on and off for years. If an ENIG board went through reflow, washed in a hot DI water batch cleaner (no solvents), then reflowed again, the second side reflow would sometimes exhibit non wetting issues.

I did't have the resources to do an in depth analysis on the root cause. My theory was since these were low cost Chinese boards, the FAB cheaped out on the gold and the nickle leached through and oxidized. I never understood why washing in hot DI water would be a catalyst.

The above linked paper at least proves its not just me, and I'm not crazy.

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#84597

omit stencil apertures to hand solder or paste, reflow then solder. | 3 March, 2020

Thanks for the info. In a past life I ran everything WS and washed in a batch wash using saponifier. I'm surprised I never saw anything then as we would regularly wash boards assembled on only one side.

Unless it's specifically washing in DI only.

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