Is there a good tutorial on reworking SMT parts with exposed pad?

[rfmerrill]

Sorry if this is the wrong subforum but it seemed like the one that fit best.

I can find a good number of tutorials on reworking SMT parts but not a lot on how to remove and replace parts with exposed ground pads on the bottom. Does anyone know of any resources for this? Thanks!

[mrpackethead]

preheat the board on an ir-heater to 140-150C..  Flux and hotair

[Rerouter]

Preheat as much as you can, even if its just going over the area with a hot air gun in a circle pattern for say a minute, low airflow and about 3-4cm away to give decent coverage,

when you feel the part should be almost up to temp, give it a light nudge with your tweezers, when it easily moves in all directions, slide it off the pads if possible instead of directly lifting, the surface tension is quite high on those exposed pads,

Flux can help, but I would preheat as far as you can first, then apply the flux, and continue with hot air

The preheating can mostly be skipped if you have a serious enough hot air gun with a big enough nozzle, but the cheap ones need some help. (If the nozzle is not as large as the chip, then yours is not the serious type)

[rfmerrill]

Something I'm curious about: How does flux help when you're just removing a part with hot air? Does the oxide layer act as a thermal insulator or something?

The problem I ran into when I was trying this at work was that I could only get it free by pushing it sideways, but the IC is surrounded by passives on all sides and it was basically impossible to avoid knocking them out of place. Just how things go I guess?

Also for putting in the new part I assume there's not really a better choice than using solder paste for the exposed pad. The only tube of solder paste we have at work seems like it's kinda useless (dispenses a whole lot at once and it seems like the grains of solder are huge). Is there like a solder paste buying guide or something?

Thanks all for the replies so far!

[Rerouter]

Both lead and tin oxide are much higher melting points, this is why if a joint is dry, adding more heat just makes the problem worse as you expose more of the mixture to oxygen,

Lead free mixes tend to have similar issues, where generally the tin will separate leaving other metals to react,

These oxides are also better insulators, which make it hard to get heat into the joints.

The flux strips the oxides back to there base metal at much lower temperatures than there melting points, freeing them up and letting them re-dissolve into what we call solder, this outside film of fresh solder finally gets thick enough to rejoin the solder that was under the film and the entire joint can become a fluid.

This dissolving action is why you have to be careful with gold plated PCB's, if you don't let the solder react with to  dissolve the plating a bit so there is not a high concentration right at the boundary to a pad, it can suffer from brittle fatigue and crack off the PCB later on,

If you cannot slide, try a twist, reduce the amount of surface area the pad is stuck on by.

As to solder for the new pad, clean all the pads with desolder braid, give them a go over with a soldering iron and back wick the excess that ended up on the pad so only a thin coating remains, that is enough,

Why clean the pads? if your mixing different blends of lead free solder you can create a weird alloy with a lower melting point and is less resistant to brittle fatigue.

[rfmerrill]

When doing some cursory research it seems that solder paste is quite perishable and so I'd only order it when I know I'm going to use it. That's a pain because everywhere I can find it for sale gouges you for fast shipping and I'm sure there are no brick-and-mortar stores to buy it even in silicon valley :/

The particular chip I'm dealing with is a 28-TSSOP with exposed pad underneath. I'm wondering do I:

• Tin the center pad on the board, use hot air to get the chip on, then solder the pins with an iron
• Tin the center pad on the board, solder the pins with an iron *then* use hot air to flow the center pad
• Just tin all of the pads and put it on with hot air as you would with a QFN

Also wondering if it makes sense to heat the board before putting the chip on or if I should only start heating after?

Thanks for the info about oxides btw.

[Rerouter]

I just use a solder roll for any small scale rework, and only buy a tube with a nozzle when I have a large batch of new boards to run

Tin all the pads, make sure the center pad has barely anything, lay down some fresh flux (I like gel flux in the syringe applicators) heat up the area with hot air until you see all pads have melted and are shiny. place chip on, continue heating, and nudge the chip a little until your happy its collapsed to be flush with the board, and is aligned with the pads. If its correct it should be self centering for the most part.