12

Apart from oxidisation and tarnishing, silver migration is also a serious concern, to the point that alloys are required to mitigate this. However alloying silver also means that the advantageous increase in conductivity with respect to gold is somewhat lost. This might be the reason why silver has not taken over gold for wire bonds. OTOH: nowadays, ...


11

At this writing, there are two "answers" that are total guesses - and wrong at that too. These comb structures are as you might expect to see when you want to induce breakdown at a precise location and into controlled structures rather than else where in the chip. These are in the TOP metal layer, the combs are there to give lots of sharp edges to promote ...


7

Gold doesn't corrode. Silver will oxidize and tarnish, reducing conductivity and potentially breaking the wire. Gold doesn't react significantly with any common component of air or the plastics used in ICs, even at elevated temperatures, and so it's the material of choice for bond wires.


5

They are probably large p-MOS and n-MOS transistors that are used for ESD protection on the bonding pads. Here is a reference that shows various bonding pad designs in detail (in general this information is not easy to come by- IC manufacturers seem to treat ESD protection as a kind of trade secret). Image taken from the above pdf: I don't recall Microchip ...


2

Depends on the process obviously. Aluminium wire bonding processes I've seen had the aluminium wires bonded using ultrasound. Never used any reliefs, never got any complaints. Then again, the chip was so high pitch and had a copper void under it that you wouldn't get any real thermal reliefs. Check with whoever does the bonding would be my advice.


2

But what about the bond wires? Is it possible that they desoldered/detached? The bond wires generally aren't soldered to the chip or to the package. They're pressure welded. If the bond wires are embedded in the plastic package material (I believe they will be for most QFN package types), and you heated the plastic beyond its glass transition temperature, ...


2

how can I safely bond them ... Safely for what? For the units themselves against lightning? Or for humans operating or servicing the units? Or for the building, against fire risk? You have no choice but to bond them correctly for humans and for the building, which means to the protective ground that's run with their power supply. There may be other ...


2

If you're just talking about mounting the die for a one-off prototype, you can do that freehand with a vacuum tool or tweezers and a small dot of epoxy (silver epoxy is common due to heat transfer and the fact that it's conductive). This was how they did it back before the automated die attach machines. Yield isn't very good, but it works. Ask whoever the ...


2

According to documents I have from a PCB vendor, a typical spec for wire-bondable electrolytic soft gold is and 1.97 \$\mu\$in (min) gold over 188 \$\mu\$in (min) nickel. These are called out as IPC minimum values, though I don't have the IPC documents in front of me. ENEPIG (electroless nickel, electroless palladium, immersion gold) plating can also ...


2

Bond wires connect the external pads to the chip die itself. Traditionally they're gold. Presumably some new manufacturing technique allows the use of cheaper copper. I doubt this will be externally detectable; maybe it makes some difference to the expected lifetime and corrosion resistance of the chip.


1

With some caveats, it is actually possible to test if the bond wire is connected if you remove it from the PCB. First of all take a known good IC. Then use a volt meter in diode mode. Connect the black probe to IC GND and the red wire to one of the pins you want to test. On the known good IC note the forward voltage shown in diode mode. You may also want to ...


1

There is a large body of knowledge about bonding with gold wire. Much is known about process and reliability. Gold wire can be stored in air and remains bondable for many months after purchase. It is the most convenient type of wire for setting up a process the first time. Some gold wire types are extremely malleable lending themselves to being bent into ...


1

A lot of wire bonding uses gold wires (aluminium is popular too), gold has the nice property that you simply squash two gold parts together and they bond like chewing gum. No heat needed, because there's no heat, having thermal reliefs has no effect. Reliefs are only useful when you need to get a pad hot without having to heat the whole pcb or when there's ...


1

While there are conductive adhesives that are useful in some specific circumstances, their electrical performance is far inferior to a soldered joint, and they are not suitable for general PCB repair or rework. Therefore, the kind of tools you are imagining have never been developed.


1

Usually heated fuming nitric or sulfuric acid is used to remove epoxy encapsulant. A properly passivated chip won't be damaged by this, and you'll still be able to read any text that is visible with a microscope. That's assuming it's epoxy. If the material is flexible (such as silicone rubber used in some markets) you might need a different set of ...


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