Pins and packages

A datasheet states limits about a part. A schematic states connections to that part’s pins. Joining the two looks like a matter of matching pin numbers. It is not, and the reason is physical rather than notational. This page starts from the article on the bench and works outward to what a tool has to model.

In short, a pin number belongs to the package, not to the chip. The same silicon is sold in several bodies, each wiring it to differently numbered legs, so a number means different things in different bodies. A pin name survives the change and a number does not.

What is actually in there ¶

An integrated circuit is a stack of five objects rather than one, and the words for them get used interchangeably in conversation, which is where the confusion starts.

Layer	What it is
die	The silicon itself, a rectangle of processed wafer a few millimetres across. Every number a datasheet prints is ultimately a fact about this.
bond pad	A metal contact on the die’s top surface. One electrical terminal of the circuit.
bond wire	A wire thinner than a hair, running from a bond pad out to a lead.
lead	The metal leg that solders to the board. Also called a pin, a leg, or a terminal.
package	The moulded body plus its lead frame: the whole black rectangle you can pick up.

The die is the part. The package is how it was shipped. A vendor sells the same die in a handful of packages, because a phone needs the small one and a lab instrument wants the one you can solder by hand. Each body also lands on its own footprint, so picking a different one changes the board as well as the part.

Here is a real one, the TXB0104 in its 14-lead TSSOP body, drawn the way the datasheet draws it.

The same part, three bodies ¶

The TXB0104 also ships as a 12-lead UQFN and a 12-ball DSBGA. Same die, same behaviour, same datasheet. The pin table prints one column per body, and reading across a row follows one terminal between them.

Terminal	Function	PW (TSSOP-14)	RUT (UQFN-12)	YZT (DSBGA-12)
VCCA	supply in	1	1	B2
VCCB	supply in	14	11	A2
B3	data I/O	11	8	C1
B4	data I/O	10	7	D1
OE	input	8	12	C2
GND	ground	7	6	D2
NC	no connect	6	absent	absent
NC	no connect	9	absent	absent

Three things fall out of that table, and each one breaks a plausible-sounding shortcut.

Number 11 means different terminals in different bodies. In the TSSOP it is B3, a data line. In the UQFN it is VCCB, the main supply input. A tool that joined a design to a datasheet by pin number, on a part seeded from one body and placed in another, would compare a power rail against a data line’s limits. It would not error while doing it, because both are real terminals with real limits. It would report that everything looks fine. That failure is worse than no answer at all, so the name serves as the primary key and the number is only ever a tie-breaker.

A number is not always a number. Ball-grid packages designate by row and column, so A2 and B2 are pin “numbers” in the DSBGA. Anything storing a designator as an integer has already lost.

A name is not unique. NC is printed on two terminals of the 14-pin bodies. Large parts do this with real terminals too, printing VDD on three pins that may carry different limits. So the name channel resolves most cases and needs the number to settle the rest.

One pad, several legs ¶

A terminal on the die can reach more than one leg of the same package. This is not a labelling convenience. Three physical reasons drive it, roughly in order of how often they decide the matter.

Current. A bond wire and a lead can only carry so much before the resistance and the heating become the circuit’s problem. A part drawing several amps fans its supply and ground out across several legs in parallel.

Heat. The legs are the die’s main escape route for heat, out into the board’s copper. More ground legs means a better thermal path. The exposed metal pad on the underside of a QFN is that idea taken as far as it goes.

Inductance. At high switching speeds, several short return paths behave better than one long one. It costs a leg and buys real margin.

So the multiplicity runs in two directions at once. One die terminal has a different number in each body, and it may have several numbers within one body. Both are addressing rather than anatomy, and the terminal is still one terminal.

The rule that follows is asymmetric, and worth stating on its own. One number identifies one terminal. One terminal may hold many numbers. A package cannot send leg 5 to two different places, but it can bring three legs to the same place.

What this means for a tool ¶

Anything joining a design to a datasheet has to pick which channel it trusts, and the choice is not symmetric.

• Match on the name. Both sides copy it from the same pin function table, the datasheet directly and the symbol library by transcription, so it survives a change of body.
• Use the number to break ties, and only inside a body you have actually identified. It is the answer to the name’s one weakness rather than a competing key.
• Refuse when the two disagree. That is a repackaging mismatch or a wrong symbol, and either channel taken alone would have produced a confident wrong answer.
• Refuse when a name is ambiguous and no body is known. Reporting nothing is recoverable. Reporting about the wrong terminal is not, because nothing downstream looks wrong.

A design carries both channels already. A schematic connection names a component by its reference designator and a pin by its designator. The component resolves to a part type through its section, and that part type’s pin carries a name alongside the designator. So the join has a name and a number on both sides, and the only question is which one leads.

How Agni models this, and the exact precedence its resolver implements, is in the datasheet layer. The wider map from circuit concepts to software ones is in the software analogy.