Analysis of common terms in PCB! Don't be wrong again!

- Jan 06, 2020-

Analysis of common terms in PCB! Don't be wrong again!

We will definitely encounter solder mask and paste mask when drawing PCB. We used to know that solder mask is a solder mask layer and paste mask is a solder paste layer. When we use protel, we do nt really care, but when we use cadence If you want to make your own pads, you must understand the meaning of the two.

 

Solder Mask [Solder Mask]: This is the inverse display layer! Some means no, and no means there is.

It is the place where the outer layer of the PCB pads (surface-mount pads, plug-in pads, vias) is coated with green oil. It is to prevent tin from being soldered when the PCB passes through a tin furnace (wave soldering). Local tin is called solder mask (green oil layer). I think that anyone who has seen a PCB board should see this layer of green oil. The solder mask can be divided into Top Layers R and Bottom Layers. The Solder layer is to expose the PAD. This is the small circle or square circle we see when displaying only the Solder layer, which is generally larger than the pad. (Solder surface means solder mask, which is used to coat it. Green oil and other solder resist materials, so as to prevent the solder from being contaminated in the place where soldering is not needed. This layer will expose all the solder pads that need to be soldered, and the openings will be larger than the actual solder pads.) When generating the Gerber file, you can observe Solder The actual effect of layers.

Draw a solid rectangle on the Solder Mask Layer (with TopSolder and BottomSolder), then this window is equivalent to opening a window (no oil, no oil is shiny copper!) Solder mask is green oil, Blue oil, red oil, except solder pads, vias, etc. (can not be coated with solder?) Others must be coated with solder resist, this solder resist has green blue red. When drawing a 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.

This layer is for surface-mount (SMD) components. This layer is used to make a steel film (sheet), and the holes on the steel film correspond to the solder joints of the SMD device on the circuit board. When surface-mounting (SMD) devices are soldered, first cover the steel film on the circuit board (corresponding to the actual pad), then apply the solder paste, and scrape off the excess solder paste with a doctor blade to remove the steel film. In this way, the solder paste of the SMD device is added with a solder paste, and then the SMD device is attached to the solder paste (manual or placement machine), and finally the soldering of the SMD device is completed by a reflow soldering machine. Generally, the size of the aperture on the steel film is smaller than the actual soldering on the circuit board. By specifying an expansion rule, the solder paste protective layer can be enlarged or reduced.

For different requirements of different pads, multiple rules can be set in the solder paste protection layer. The system also provides 2 solder paste protection layers, which are the top paste protection layer (Top Paste) and the bottom solder paste protection layer (Bottom Paste) Draw a solid rectangle on Paste Mask layers (there are TopPaste and BottomPaste), then a window is opened in this rectangular frame, and the machine sprays solder into this window. [Actually, a window was opened by Wave soldering will be tinned]

 

Keepout and the mechanical layer are also easy to confuse.

Keepout, draw borders, determine electrical boundaries;

Mechanical layer, the true physical boundary, the positioning holes are made according to the size of the mechanical layer, but engineers in PCB factories generally do not understand this. Therefore, it is best to delete the keepout layer before sending it to the PCB factory (the laboratory has previously occurred that the keepout layer has not been deleted, causing the PCB factory to cut the border).

 

In the PCB, assembly layers and silk printing layers are often encountered. So what do these two layers mean?

Screen printing layer: The outline plan view of the part. The screen printing layer refers to the graphic symbol representing the outline of the device. When designing a PCB, this layer of data is often used for light drawing data. More appropriately, Silkscreen lay will be printed on the PCB board.

 

Assembly lay Assembly lay: PLACE BOUND TOP / BOTTOM, that is, physical appearance graphics. Can be used for DFA rules: DFM / DFA, is DESIGN FOR manufacturing (M) / DESIGN FOR assembly (A). This property is used for layout and assembly drawing. It is when the board parts are provided to the CHECK staff to check whether there are any problems or other uses. SO IT ISN'S PRINT BOARD. Silk screen is definitely necessary, but the assembly layer is not necessary (personal understanding)

 

The silk-screen layer is for those who manually load the pieces, and it is for those who adjust the boards.

The assembly layer is an assembly layer, which is used to indicate the physical size of the device. It is only used when the placement machine is soldered. The assembly layer can hold the nominal value of the device, such as the value of resistors and capacitors. This is convenient for assembly and maintenance.

 

The two words positive and negative are often encountered in PCBs. Positive and negative only refer to two different display effects of a layer. Regardless of whether you have a positive or negative layer on this layer, the resulting PCB board is the same. Only in the process of cadence processing, the data volume, DRC detection, and software processing are different. Just two ways of expressing one thing. The positive film is, what you see is what you see, wiring is wiring, it really exists. Negative film is, what you see, nothing, what you see is exactly the copper skin that needs to be corroded.

 

The advantage of positive films is that if moving components or vias need to be re-copped, there is a more comprehensive DRC verification. The negative film has the advantage that the moving components or vias do not need to be re-copped, the copper is automatically updated, and there is no comprehensive DRC verification.

 

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

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