What is the difference between Assembly, Placement and Silk outlines?

[sean87]

So, most of the PCB design software have layers named assembly, placement and silk (top and bottom) layers. What are the differences and use-cases? I assume that the silk outline is what will be printed on the PCB, then what about the others? when you send a PCB for manufacturing, which of them should you generate and send? Should they overlap? Is there any standard to follow? Can you give an example (e.g. 1206 resistor )

[Benta]

The PCB manufacturer needs the Gerber (.grb) and drill files (.drl) for simple PCBs, usually top layer copper, bottom layer copper, top layer mask, bottom layer mask, top layer silk and bottom layer silk as well as the drill file and the outline file.
For multi-layer PCBs with buried vias it gets a lot more complicated.
In all cases, ask the manufacturer exactly which files he needs.

Assembly and placement are either for your reference, or in the case of automatic pick-n-place machines, for them.

[KL27x]

Anytime u are using a component which needs orientation u need some mark. If there is no way to include it on silkscreen, u can put it on another layer., such as placement.

In the CAD u can decide what layers go in what gerber layer. The layer names dont really matter there.  If i need additional layer , i will name it something obvious and send it. The manufacturers arent stupid.

Example besides compnent placements might be area for stiffener on a fpc, or a dpuble stick adhesive.

[Benta]

If i need additional layer , i will name it something obvious and send it. The manufacturers arent stupid.

I like your gambling approach here. Personally, I'd not shoot dollars off on such a wild hope, but rather check first.
But it's your money...

[KL27x]

It's not a gambling approach. In general, along with quote submission, you fill in their requested specs. You send Gerbers. And you have space to fill in any additional information. You can add note what this is. But you can also name Gerber layer "bottom stiffener," and in notes specify spec for material and thickness. It is pretty obvious. Vs asking them, where to put and/or what name they prefer for a gerber layer for stiffener. It saves a round of stupid question/answer, at a minimum. (Due to innumerable disprepancies/inconsistencies/ambiguities present in most all human languages, it could eliminate several rounds of question/answer). It saves their time; it saves your time; and it eliminates the gamble of after multiple back and forth clarification/reclarification that "I'm pretty sure we're on the same page, by now." Gerber layer is a very unambiguous form of communication across language barriers. If a picture is worth a thousand words, a picture with standard scaling is worth more than a thousand words.

If you send an additional gerber layer and they can't figure out what it is or why you sent it, but they give you a quote and/or make your board, anyway, you should find a new manufacturer, lol.

I had one screwup in the past in assembly order. I had a RA LED, so I sent placement layer with a dot indicator of the cathode AND an arc to denote the direction the LED should face. They screwed up anode/cathode of the LED. When I referred them to the placement gerber I sent, they apologized and fixed it no charge, no fuss. All 1000 LEDs.

*This was a weird one. The LED had symmetrical pads which by datasheet/footprint it could be placed upside down just as easily as right-side up. They just put them on the way they came out of the cut tape. Perhaps it is my noob mistake to not figure out which way they come out of the cut tape, but this is not information that is available looking at datasheet or footprint, AFAIK. But they accepted responsibility because I sent the unambiguous, indisputable placement Gerber, on which they based the original quote. AFAIK, they removed and replaced these LEDs by hand.

[KL27x]

Bottom line is
Silkscreen is printed on the pcb. Unless you want assembly issue, you do not want to print silkscreen over exposed pads. Silkscreen may or may not be related to assembly instructions. It can be revision information, product ID, or graphics/ornamental, or just a bunch of swear words.

An assembly or placement Gerber contains information pertinent for placement/orientation of the components.

The board manufacturer needs a silkscreen Gerber(s), if you want a silkscreen. But they do not need placement/assembly Gerber, unless they're the ones assembling the board. They will gladly make a board with no orientation information regarding the components.

The company assembling your board may need additional information in form of a Gerber, even though they are not necessarly manufacturing the board, and this information may not have anything to do with the actual pcb manufacturing process.

If for esthetic or security thru obscurity, or lack of space, you may elect to even put all your component designators in a placement gerber which will not be any part of the final board (e.g. will not be in the silkscreen).

There is not so good standardization of placement/orientation info in standardized library parts. Some vendors/components will have graphics that overlay the pads and are not meant to be put in a silkscreen, while some are designed to be part of a silkscreen if desired. These graphics can appear in any number of obscure layers within your PCB software. You have to sort them into the desired Gerbers (if any), yourself.