The assembly layer is used to represent the physical size of the device.It can be used when the mounter is welding.
Assembly layer can put a nominal value of the device, such as the value of resistance and capacitance .This is very convenient to assembly maintenance .
We will certainly encounter solder Mask and paste Mask when drawing the PCB design. Previously we vaguely are aware that solder Mask is a solder layer, a solder paste layer is a paste Mask.We do not care about it when using the protel.But when we use the cadence,it needs ourselves to make the PAD,so we must know the meaning of the two.
Solder Mask Layer: this is the reverse layer ! Some means nothing, and nothing represents there!
Solder mask is the place where is coated with green oil except the PAD(surface mount pads, plug-pad, via),which is to prevent the place where should not be tin-plated from being tin-plated when PCB is though the tin stove, so it is called solder mask layer or green oil layer.I think people who have seen the PCB should see this green oil.Solder mask can be divided into Top layers and Bottom layers.PAD should be exposed in solder layer.This is small circles or small square circle which we can see when we only display the solder layer,generally larger than PAD.Solder surfacely means solder mask layer,which is used to coat with solder material, such as green oil,to prevent the place where should not be welded from being tin-plated.This layer will expose all the PAD that is need to be welded, and the openings are bigger than the actual PAD.When generating the Gerber file,you can observe the actual effect of solder layer.When you draw a rectangle in solder masks layer which has top solder and bottom solder,the position in the rectangular frame is equal to openning window,where is not need to be coated with oil,where is the shiny copper.Solder Mask is painted green oil, blue oil, red oil, in addition to the welding plate and via which are painted, they will not be tinplated. Other should be coated with solder resist.And this solder resist has the colour of green,blue. red and so on.When painting cadence pads, solder Mask pad is larger 0.15mm (6mil) than regular pad .
Paste Mask layers [solder Paste protective layer] :this is positive.Some represents there and nothing is nothing.
It is for the components of SMD.This layer is used to make steel film .While the holes on the steel film correspond with the pads of SMD devices on the circuit board.When welding SMD,firstly make the steel film cover on the PCB (corresponding to the actual pad),then coat with paste.Scrape off the excess paste with a doctor blade and remove teh steel film,which makes the pad of SMD device tin-plated.Then make the SMD device attached on the paste by manual or machine.After the rellow,the welding the SMD device is finished. Usually the holes on the steel membrane is smaller than actual pad on the PCB printing.By specifying an extension rule to zoom in or out the protective layer of solder paste. For the different requirements of different pads, you can also set multiple rules on paste protective layer. The system also provides two solder protective layer, and they are top paste protective layer and bottom paste protective layer.When you draw a rectangle in paste mask layer which has top paste and bottom paste,the position in the rectangular frame is equal to opening window,and the machine sprays tin within this window .
Meanwhile Keep-out and Mechanical layer are also very easy to confuse., Keep-out is a painting frame, which determine the electrical boundary.
Mechanical layer is real physical boundaries. Positioning holes are made according to the size of Mechanical layer.But the PCB factory engineers generally do not understand this.So it is best to delete keepout layer before sending to PCB factory.It was occurred in the laboratory that PCBfactory has wrong cut of boundary because of not deleting the Keep-out layer.
We often encounter assembly layer and the printing layer in the PCB Gerber file. So what is meaning of the two layers?
Silkscreen: a plan view of the shape of the part.The silkscreen layer is the graphic symbols that are representative of device profile.When designing the PCB , the plot data often use this layer data. More appropriate to say that Silkscreen layer will be printed on the PCB board.
Assembly layer: PLACE BOUND TOP/BOTTOM, namely physical sharp graphics.It can be used in rules of DFA. DFM is short for “Design for manufacturing”.DFA is short for “Design for Assembly”This property is used to the layout and the assembly drawing.
We also often meet with the two words: positive and negative in the PCB . Positive and negative only refers to two kinds of different display effect on one layer.Whether this layer you set is positive or negative,the printed PCB board is the same.Just in the processing of the cadence, data volume, DRC detection, and the process of the software are different。Only one thing with two kinds of expression. Positive is that what you see is what you see.Wiring you see is the wiring.It really exist.Negative is that what you see, there is nothing.What you see is just the copper that is needed to be etched.
Positive film has the advantage that if we remove the components or vias, we need to shop the copper, and we have more comprehensive DRC check。Negative is the advantage that if we remove the components or vias, we do not need to shop the copper, and copper shop is automatically updated.But we have no comprehensive DRC check。
In the painting via pad ,holes are bigger 10mil (0.2mm) than the pin.Outer diameter is larger more than 20mil than holes, or the pad is too small and welding is inconvenient.