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I tried to re-use a costly Integrated circuit which includes dual port ram inside. I removed it from unused previous board by carefully using heat gun.

The problem is that the other part of the ram works fine, but the dual-port ram part always reads as 0x00. The interface is SPI and other part of the ASIC works just as fine, and the same firmware on the other board that was build from new IC works fine.

Is it possible that only that part of IC damaged from using heat?

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    \$\begingroup\$ Well, if the IC looks damaged after being removed with a heat gun, that gun would be #1 on my suspects' list. \$\endgroup\$ – Dmitry Grigoryev Sep 13 '16 at 9:27
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Yes. Manually using a heat gun (even a temperature controlled one) is a risky operation. The pins of an IC are designed to dissipate heat to the PCB so it is quite possible to have one side of your chip below the thermal failure tolerance while the other side is above that threshold, leading to potential damage to the silicon substrate in an isolated area.

Sounds like you got lucky in that you can see the damage straight-away rather than having an intermittent failure that might have only become apparent in the field.

There are products out there that make manual desoldering easier by dropping the melting point of your solder by forming a new alloy during desoldering. One such is ChipQuik but I have never used it myself for large chips. This makes it possible to melt all the solder simultaneously at a much lower temperature than normal (temps as low as 90 Celcius are possible, which is not likely to damage any IC I can think of).

(I don't work for the ChipQuik company by the way :))

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carefully using heat gun

LOL! Do you carefully use a lawn mower to trim your fingernails too?

Hot air can be used for soldering, but has to be properly controlled and applied. Without a hot air soldering station with the right tip and the right temperature and airflow settings, you're just slicing deli meant with a chain saw.

Consider anything you removed from the board with the chain saw heat gun to be junk. Toss it and move on. Don't waste any more time and frustration with abused parts.

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    \$\begingroup\$ If you must use a chainsaw at least make sure it's an ESD-safe one. \$\endgroup\$ – Wossname Sep 13 '16 at 11:18
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It's dead Jim.

"Careful" and heat gun don't usually belong in the same sentence. You can buy nozzles for desoldering heat guns that match various chip types which are significantly less likely to cook your chip in the progress. Hot air is directed to component pins.

Just regular lead solder liberally applied over all the component pins makes it easier to desolder. It's not going to instantly coagulate with more volume and a pb based solder has a lower melting point.

Another basic tool is a PCB heater that will raise the PCB temp up to desired temperature so it's easier to (de) solder things.

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    \$\begingroup\$ I saw a guy on the street in Hong Kong who replaced BGAs inside iPhones using a hot air gun and returned the now working phones to their owners for a fee. I was impressed. \$\endgroup\$ – winny Sep 19 '16 at 11:39
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If anything you need a heat gun with variable heat settings and many different attachments. I use a smaller has pen gas torch if its a big job There are a few heat guns reviewed here but be carefull a heat gun can burn a hole in a wall let alone a circuit board.

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    \$\begingroup\$ No you can't use these, they are the wrong tools. You have to get one specifically meant to be used for de-soldering, which controls the air flow and the temperature like a soldering iron does. The term you are looking for is "de-soldering station", not "heat gun". \$\endgroup\$ – Lundin Jul 5 '18 at 8:07

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