Removing IC with rework: when to lift off?

[komatsu]

When using a hot-air rework station to remove an IC from a PCB - how do you know when do you "lift" the chip off?

i.e. how do you know when the connections have completely melted?

[c4757p]

Apply a constant, gentle pull. It's ready to lift when it lifts. If you don't have an easy way to apply a constant pull, apply a constant, gentle nudge from the side with tweezers or something, and then pull it when it moves.

Emphasis on the "gentle", particularly if the pads are small.

[komatsu]

ok, that's a good tip...do you have to wait for the "crackling" sound?

[c4757p]

...crackling sound?

I seriously hope you mean that you've applied flux and it's boiling.

If yes, no. (Well, it'll happen first anyway.)
If no,

[wraper]

Cannot agree with c4757p, especially for bga. You'd better try to push it gently time to tome and see if it starts moving. Then heat it a 3-10 seconds more, especially larger chips, then remove. If doing constant push all the time it is easy that hand will slip and it is just catastrophic, especially on crowded boards. There will be a mess as solder around particular is melted too.

[komatsu]

no flux applied - is there really need? none of the YouTube videos of ICs being removed show flux being applied?

btw, its a hard drive PCB

Don't worry I am only practicing ( they are redundant PCBs )

[wraper]

I usually apply flux. That make job easier. Especially if you want to reuse the part.

[komatsu]

What type of flux would you use?

[wraper]

I prefer tacky flux. It lasts longer and does not burn out instantly when you are heating the chip.

[c4757p]

Cannot agree with c4757p, especially for bga.

Techniques are definitely a bit different for BGA.

If it's easy to slip and knock things, than you have misinterpreted "gently" by at least an order of magnitude

[wraper]

If it's easy to slip and knock things, than you have misinterpreted "gently" by at least an order of magnitude

When there are other parts at <1mm distance from the part you are pushing, it is risky to put there tweezers by itself. It is not so easy to precisely hold tweezers in one point, especially when producing force, even smallest. It is just very hard to control the force precisely for a prolonged time. Quiet often will happen that what you thought was tiny force, was able to push surrounding parts. Of course not much an issue when desoldering huge SO-8 ICs and 0603 resistors.

[rob77]

doing a knock-knock with tweezers/needle/whatever from side will tell you it's ready if the chip moves a little bit during knocking it from it's side,  it means all pins/balls have the solder melted and it's safe to lift.

a good habit is to cover the PCB in the proximity of the chip with kapton tape - to avoid knocking off passives and other chips (and avoid unnecessary heating of those). it's not funny to solder back knocked off 0402 components - been there, and i'm not lazy  to place the kapton tape any more

[komatsu]

What does the flux do?

[curiosu]

The flux gets under the IC, it helps spreading the heat evenly, it helps with keeping the balls "in one place" and don't spread around due to surface tension and also prevents oxidation.

[Ryano]

What does the flux do?

Same thing in rework as it does in new construction. Help transfer the heat and prevent oxidization. When pulling a part it will reflow sooner and stay wet longer with flux. For hot air pulls I prefer to use a Rosin RMA flux pen as it is thinner then most liquid flux and doesn't cake the board as badly. If you don't have that handy you can wash the board with some IPA and then mix in a little thicker flux and achieve the same result.

+1 for using Kapton tape to mask off surrounding components. This is a life saver when working high density boards.  The tape will help prevent "blow off" of the surrounding parts. I've been in situations where I've had to apply enough heat that the Kapton was singeing, but the underlying components stayed stuck down solid.

Show us what kind of package you are trying to remove and I'll tell you how I would approach it. There are different techniques for different packages.

 

[pickle9000]

I don't know what these are called but they are useful. Basically a bit of spring wire with a handle. They prevent pushing too hard. You do need to bend them to get a decent grab. To use a bit of pressure (just a little bit) in the direction you want to go and the chip will lift as soon as the solder melts.

[komatsu]

thanks for the great replies so far.

What temp AND air flow to use on hot air rework station?

Do you go for Max. Temp and Max. Air Flow and how long 60 seconds?

[helius]

your air flow will be constrained by any small parts like SMD resistors in the area. too much air and you will blow them around. otherwise the more air the better.
temperature should be roughly 30C higher than the solder liquidus to avoid damage
you must move the tool around the area continuously, letting it stay in one place can cause damage

[komatsu]

I was just practicing some more.

Using temperature of 400C ,  it took approx 1min and 15 secs before chip came loose

Not surprisingly chip became extremely HOT. Was 400C overkill? Would this temp damage chip?

[pickle9000]

Stress (on devices and the pcb) can be greatly reduced by using a board preheater. They are worth every penny.

[komatsu]

But is a pre-heater really needed just to remove one IC?

[pickle9000]

But is a pre-heater really needed just to remove one IC?

Needed no, as long as the device is small and the board is not too thick, nor should it have a large ground plane.

If you have a look at the datasheet for a device it will give you soldering information. If you have a new device or one your planning to save check out the datasheet.

Large devices really do need a preheater for removal to reduce component damage. There is also the issue of lifting a pad.

I have seen some amazing work done with a hot air gun, no preheater. I've also seen makeshift preheaters using heating pads, a lamp with a grate on top and so on.

If you have a toaster oven set it at a low temp (150F) warm up a practice board then give it a go. Place the board on top of the oven for the actual removal work. It will keep it warm long enough for you to try it out.

[tautech]

But is a pre-heater really needed just to remove one IC?

No it's not required.

But if you intend to do lots of rework they are valuable.

One thing that has not been mentioned in this thread is lead free solder and the increased temp needed to melt it.
Nearly all commercial manufacture is done with it today.
But this stuff doesn't make rework as simple as it should be.

I personally curse the stuff because of the additional thermal stress everything must suffer, not to mention how easy leaded solder is to use by comparison.

But of course Lead is a noxious element  that we must avoid for our good health's sake.
Never mind it has been part of history for 1000's of years and it's affects well known for most of modern time.

[pickle9000]

- I hate lead free solder.
- I was told the rosin smoke is more harmful than lead but that washing your hands was a good idea so you don't ingest the lead. A fan on the bench is a good idea.
- Rosemetal is a low temp alloy that can be used to help remove chips.
- I really hate lead free solder, really.

Rose metal on ebay, I didn't look hard so I'm sure it can be found with cheaper shipping than Russia.

http://www.ebay.ca/itm/Roses-metal-Rose-metal-Bismuth-Lead-Tin-alloy-220-g-/281570225039?pt=LH_DefaultDomain_0&hash=item418ee4ab8f

[komatsu]

Lead-free components are not liked on this thread I see!

When doing the real thing (i.e. not a redundant PCB), is 400C too hot a setting on a hot-air rework station to remove a chip?  Will I damage the chip? (it contains adaptive firmware data which  must be subsequently transferred onto another PCB).