Careless reflowing

[itdontgo]

Hi,

I've got a prototype PCB here and have had to use used chips that have been hot-air de-soldered from old PCBs.  These have a pad on the bottom and so need some serious heat to move them!  I've had issues with these chips and could not find a fault with the rest of the circuit although it did sort of work.  I tried more carefully to desolder a 3rd chip and the circuit behaves much better.

Can you shift the spec of a chip with the hot air?  I was surprised because it still worked but did not start-up properly.  Anyone come across this?  My concern is that the PCB is on the edge of not working and a chip slightly out of spec stops it so the heat would have had to have shifted the spec considerably for me to be confident in it.

Michael

[KL27x]

Personally, the only things I have ever botched through high heat are ceramic resonators. On checking the datasheet for the max temps and reflow curve, whoa! Some of these things are VERY sensitive to heat.

it still worked but did not start-up properly.

I wonder if your chip relies on an internal ceramic resonator for a clock. Perhaps one on which the startup power-on enable timer relies. Maybe it got cooked.

Just vapid speculation, though. I've never botched an IC through hot air removal, before. (To my knowledge, anyway!)

[John_ITIC]

These have a pad on the bottom and so need some serious heat to move them!

You have to "soak" the whole board from the bottom with hot air (approximately 150 deg C). Then you don't need too much heat from the top.

[uncle_bob]

These have a pad on the bottom and so need some serious heat to move them!

You have to "soak" the whole board from the bottom with hot air (approximately 150 deg C). Then you don't need too much heat from the top.

Hi

There are a lot of variables here:

1) How much heat was used to remove the parts from their old boards?
2) Were the parts damaged before they were pulled (over voltage ...)?
3) How were the parts stored after they were pulled (ESD etc)?
4) How well are the temperatures on your hot air system controlled / set?
5) How is solder put down on the new board?
6) How was solder removed from the "pulled" parts?
7) Were leads bent / damaged on the pulled parts?

That's only getting into the obvious parts of the list. If you have parts with ground pads in the middle of the chip, there's a full list just for them....

Bob