PCB Technical - Have you missed the common nouns in PCB?

What is the role of the assembly layer in PCB design, and what is the difference from the silk screen layer?

The silk-screen layer is for the person who puts the pieces by hand, and also for the person who adjusts the board.

The assembly layer is the assembly layer, used to indicate the physical size of the device, and is only used when the placement machine is soldering.

The assembly layer can put the nominal value of the device, such as the value of resistance and capacitance, which is very convenient for assembly and maintenance.

We will definitely encounter solder mask and paste mask when drawing PCB. We have always been vaguely aware that solder mask is a solder mask and paste mask is a solder paste layer. We don’t care much when using protel, but when using cadence When you want to make your own pads, you must understand the meaning of the two.

solder Mask [Solder Mask]: This is the anti-display layer! Some means nothing, and nothing means yes. It is the place where green oil is applied to the outer layer of the PCB pads (surface mount pads, plug-in pads, and vias). It is to prevent the PCB from being tinned when the PCB goes through the soldering furnace (wave soldering) The place is tin, so it is called the solder mask (green oil layer). I think anyone who has seen a PCB board should see this green oil. The solder mask can be divided into two layers, Top Layers and Bottom Layers, Solder The layer is to expose the PAD. This is the small circle or square circle we see when only the Solder layer is displayed. It is generally larger than the pad (Solder surface means the solder mask layer, which is used to coat the green Solder mask materials such as oil to prevent the solder from being contaminated in the areas that do not need to be soldered. This layer will expose all the pads that need to be soldered, and the openings will be larger than the actual pads); when generating the Gerber file, you can observe Solder Layers The actual effect. Draw a solid rectangle on the Solder Mask Layer (TopSolder and BottomSolder), then the rectangular frame is equivalent to opening a window (without oil, it will be shiny copper) solder Mask is painted with green oil, blue Oil, red oil, except for pads, vias, etc. can not be coated (can not be coated with solder), all others must be coated with solder resist. This solder resist has green, blue, and red. When drawing the cadence pad, the solder mask is 0.15mm (6mil) larger than the regular pad.

Paste Mask layers (solder paste protective layer) this is a positive display, there is nothing there is nothing. It is for surface mount (SMD) components. This layer is used to make steel film (sheet), and the holes on the steel film correspond to the solder joints of the SMD device on the circuit board. When soldering surface mount (SMD) devices, first cover the steel film on the circuit board (corresponding to the actual pad), then apply the solder paste, scrape off the excess solder paste with a scraper, and remove the steel film In this way, solder paste is added to the solder pad of the SMD device, and then the SMD device is attached to the solder paste (manual or placement machine), and finally the SMD device is soldered by a reflow soldering machine. Usually the size of the aperture on the steel film is smaller than the actual solder on the circuit board. By specifying an expansion rule, the solder paste protection layer can be enlarged or reduced. For the different requirements of different pads, multiple rules can also be set in the solder paste protection layer. The system also provides two solder paste protection layers, namely the top solder paste protection layer (Top Paste) and the bottom solder paste protection layer (Bottom Paste) Draw a solid rectangle on Paste Mask layers (TopPaste and BottomPaste), then a window is opened in this rectangular frame, and the machine sprays solder on the window. In fact, the stencil has opened a window. The wave soldering is tinned.

At the same time Keepout and Mechanical layer are also easy to confuse. Keepout, draw the border, determine the electrical boundary, the mechanical layer, the real physical boundary, and the positioning holes are made according to the size of the mechanical layer, but the engineers of the PCB factory generally do not understand this. Therefore, it is best to delete the keepout layer before sending it to the PCB factory (there has been a situation in the laboratory where the keepout layer was not deleted, which caused the PCB factory to cut the wrong boundary).

The assembly layer and the printing silk layer are often encountered in the PCB. So what is the meaning of these two layers?

Silk screen layer: the outline plan view of the part. The silk screen layer refers to the graphic symbols that represent the outline of the device. When designing the PCB, the light drawing data often uses this layer data. More appropriately, the Silkscreen lay will be printed on the PCB board.

Assembly lay: PLACE BOUND TOP /BOTTOM, that is, physical shape graphics. Can be used for DFA rules: DFM/DFA, it is DESIGN FOR manufacturing (M)/DESIGN FOR assembly (A). This attribute is used for layout and assembly drawings. It is when the parts of the board are all up and available to the CHECK staff to check whether the parts have problems or other uses SO IT ISN\’S PRINT BOARD. The silk screen is definitely necessary, but the assembly layer is not necessary (personal understanding).

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The terms positive film and negative film are often encountered in PCB. Positive film and negative film refer to two different display effects of one layer. Regardless of whether you set up a positive film or a negative film on this layer, the PCB board produced is the same. Only in the process of cadence processing, the amount of data, DRC detection, and the processing process of the software are different. Just two ways of expressing one thing. The positive film is, what you see is what you see, the wiring is the wiring, and it really exists. Negative film is, what you see, there is nothing, what you see is exactly the copper that needs to be corroded. Therefore, the positive and negative film technology cannot say that that technology is necessarily better than the other. For example, the Jiefang factory uses negative film technology, which controls the accuracy and tolerance of the circuit better than the industry. The holes are made without metallization, because the holes are sealed by negative film, so the unsealed holes directly contact the liquid and will not retain copper, so it is better to make the holes without metallization. The positive film process is the most commonly used in PCB factories. It has a long history and mature process. It has good adaptability and processing methods for many unconventional processes, such as semi-hole processes, such as edging processes and so on. The advantage of the positive film is that if the moving components or vias need to be re-plated with copper, there is a more comprehensive DRC verification. The advantage of the negative film is that there is no need to re-lay the copper to move the components or vias, the copper is automatically updated, and there is no comprehensive DRC verification.

When drawing through-hole pads, the hole should be 10mil (0.2mm) larger than the pin, and the outer diameter should be more than 20mil larger than the hole. Otherwise, the pad is too small and it is inconvenient to solder.