PCB Design Perfection Starts in the CAD Library – Part 16

 Drafting elements in a CAD library part are not “Standardized” for specific values or sizes but there are recommendations that are coming out in the IPC-2610 series that include schematics, PCB assembly and fabrication. Documentation includes component outline and polarity markings for silkscreen and assembly. This article focuses on silkscreen and assembly Reference Designators.

 Every reference designator (Ref Des) originates in the schematic diagram and is transferred to the PCB layout via the netlist. They also appear in the Bill of Material that is exported from the schematic and passed to the assembly shop. The rules for reference designator assignment are established by the IPC-2512 publication. However the Ref Des size, font, CAD layer and placement location are left up to the EE engineer and/or PCB designer.

 Every CAD library part should have 2 distinct reference designators, one for the silkscreen and one for the assembly drawing. Both designators, in every CAD library part, are normally located in the center of the component body. The silkscreen reference designator is relocated outside the component body after the part placement is completed and approved by the design review panel. If via fanout and trace routing cause part placement nudging then it’s best to wait until that process is completed or duplication of effort will come into play. Also, if via hole sizes exceed 0.4 mm and they are not tented then it’s best to avoid placing the silkscreen reference designators over the via hole, as the ink will drop into the hole making the reference designator indistinguishable and eliminate the purpose of having the reference designator to begin with. If you are using large via hole sizes it’s best to wait until the PCB design passes the engineering routing review panel. Via sizes smaller than 0.4mm can be tented (covered) with solder mask and the placement of silkscreen designators can go directly on the via.

 The silkscreen reference designator height sizes are –

  • 1.0 mm – Minimum
  • 1.5 mm – LP Calculator Default
  • 2.0 mm – Nominal
  • 2.5 mm – Maximum

 The reference designator text line width is normally 10% of the height for good clarity and to prevent the characters from bleeding or blobbing together. The 0.15 mm height “Default” is what the LP Calculator uses but users can change the global setting values to any value or measurement system.  

 The assembly reference designators are different in the fact that they never get relocated outside the component body outline. Assembly reference designator height sizes are –

  • 1.5 mm – Default
  • 1.2 mm – 0.5 mm for miniature components

 Here are some chip component assembly ref des height sizes that scale down according to the body size  –   

  • 4520 (EIA 1808) = 1.5 mm
  • 3216 (EIA 1206) = 1.2 mm
  • 2013 (EIA 0805) = 1.0 mm
  • 1608 (EIA 0603) = 0.7 mm
  • 1005 (EIA 0402) = 0.5 mm
  • 0603 (EIA 0201) – 0.5 mm  

 Note: All assembly body outlines are 1:1 scale of the physical component with the exception of all micro-miniature parts smaller than 1.6 mm length. Parts less than 1.6 mm length are EIA 0402 and 0201. These 2 parts assembly outline has to be enlarged so that the 0.5 mm assembly ref des fits cleanly inside it.  

Also, most land patterns (CAD library parts) have the Lands (Pads) put on the assembly layer. This is true for all parts that are large enough to accommodate both the component leads and the assembly ref des without interfering with each other. When the component leads interfere with the assembly ref des, the component leads on the assembly layer are removed from the padstack. This includes all chip components, crystals, molded body parts and grid array parts with bottom only leads.  

 

 

See Figure 1 for a sample of a typical silkscreen with the reference designators relocated outside the part.

 

Example of Silkscreen Reference Designators

Figure 1: Example of Silkscreen Reference Designators

 

 See Figure 2 for a sample of a typical assembly drawing with the reference designators inside the part, exactly where they were put when the CAD library parts were built. While the silkscreen reference designators must be relocated to an optimized location after part placement is completed, the assembly reference designators do not require any movement or cleanup. Also notice in Figure 2 that the large parts have lands (pads) built into the padstack and the small chip components do not have lands (pads) on the assembly layer. The LP Calculator allows the user to turn on/off Land on Assembly because some people do not want any component leads on the assembly drawing; rather they only want closed polygons with reference designators inside.

 

 

Example of Assembly Reference Designators

Figure 2: Example of Assembly Reference Designators

 

 

 

 

Table 1 contains list of the standard reference designators from the IPC-2612 standard for schematic symbol generation.

 

Standard Reference Designators for Schematic Symbols

Table 1: Standard Reference Designators for Schematic Symbols

 

 *These class letters would not appear in a parts list as they are part of a PCB and not an active electronic component.

 **Not a class letter, but commonly used to designate test points for maintenance purposes.

 Note: The above list is not exhaustive. See the standard list of class designation letters in ANSI Y32.2/IEEE Std 315, Section 22 and the Index.

 

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5 comments on this post | ↓ Add Your Own

Commented on May 31, 2011 at 8:17 am
By Penn Linder

I have had comments that the assembly drawing text size (e.g. 0603 is 0.5mm) is too small to read on a paper copy. I can’t increase the size of the text due to a lack of whitespace. What do you anticipate most people will do to accomplish a readable drawing? Perhaps scale the assembly drawing? Or perhaps expect whoever needs to read the small text to use an electronic copy and zoom in?

Commented on May 31, 2011 at 8:38 am
By Tom Hausherr

Assembly drawings are normally created into a PDF file. You can scale the assembly drawing to any size and create a clean “searchable” document. I don’t know of anyone creating hard copies anymore on paper. The smallest ref des size is 0.5mm and can be easily recognized if you scale the assembly drawing 2:1. I know this is very small, but the components are small and often packed onto a PCB.

Commented on August 29, 2011 at 10:18 pm
By Lawrence Joy

If the IPC standard (IPC-2612) does not simply defer to ANSI/IEEE 315 clause 22 as far as assignment of class designation letters, and both of these are ANSI approved, then it seems to me there is a conflict and someone doesn’t know how to apply what already exits.

Here are my comments about the list you show as Table 1 coming from IPC-2612:
–C is the class letter for a capacitor, whether the capacitor is fixed, variable, or consists of multiple pieces (network).
–CN If you want to add this write to Standards Association of IEEE.
–GN General Network–No! “Z” is the class letter for a general network.
–R is the class letter for a resistor, whether the resistor is fixed, variable, or consists of multiple pieces (network).
–RN If you want to add this to the “official” list write to the Standards Association of the IEEE.
–P is not the male part of a mating pair of connectors but is the most movable part of a mating pair. See ANSI/ASME Y14.44-2008. This answers how you ref des a sexless connector.
–J is not the female part of a mating pair of connectors but is the most fixed part of a mating pair. See ANSI/ASME Y14.44-2008. This answers how you ref des a sexless connector.
–TZ for transzorb–No! Use “D” if the device is of the Zener type or “RV” if varistor type.
–L for ferrite bead–No!
–E besides a terminal the class letter “E” is for an antenna, miscillaneous electrical part, ferrite bead, and other items.
–N is not a class letter but used with the location coding method of applying reference designations.

Here is a question I know the answer to. What class letter would you use for a part that consists of a potentiometer and a SPST switch? In other words a volume control and an on/off switch controlled by the same mechanical shaft.

Commented on January 3, 2012 at 10:16 am
By Bob Wilson

Hey Tom! I notice that there is a mistake in the standard reference designator list. In the Y14.44 it defines naming a connector with a J if it is a fixed connector and a P if it is movable. It has nothing to do with gender. This has caused endless grief in aerospace where a connector plate has a “P” designation engraved in it and they have to scrap the plate. From Y14.44 sect 2.1.5.3
(a) The movable (less fixed) connector of a mating
pair shall be designated P
(b) The stationary (more fixed) connector of a mating
pair shall be designated J
Maybe I’ll get a chance to meet up with you at APEX. Stay well.

Commented on April 15, 2013 at 3:47 am
By Shailesh Zagade

I need guidance on below subject:
I have a PCB panel with V groove, which is suppose to be spitted using round cutter/PCB separator machine.
When cutter moves on V groove, some SMD near groove goes fail/faulty functionally.
What is minimum distance that is to be maintained for SMD from V greeve?

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