Technologies - PCB Design

Technology is the knowledge of how to create, produce, or perform some object or function. In PCB design the term technologies is no more than a categorization of values or capabilities of a manufacturer. These values are based on capabilities of the manufacturer's equipment and the overall process.

The three controlling points are etch, drill, and registration. Other capabilities influence the overall category, but these are the most important.

Previously, these technologies have not been clearly defined. Manufacturers have not bought into a category for fear of scaring off customers and displaying too much information for competitors to see. There are also no organizations or groups that record and organize such values. Therefore, during the creation of this book, a survey was taken of many PCB manufacturers, and the following categories were defined more clearly: conventional, advanced, leading edge, and state of the art (refer to the following section). As with all technologies, the values will change through time, and additional categories will evolve. 

These are the categories and their general definitions:

Conventional           

This is the lowest technology and is the most common. The general limitations of this technology are trace/space of .006"/.006" (for .5 oz copper), a minimum finished drill of .012"[.3048], and 8 to 10 layers maximum.

Advanced           

Advanced technology is a higher level of technology, limited to 5/5, a minimum finished drill of .008"[.2032], and 15 to 20 layers.

Leading edge           

Leading-edge technology is essentially the highest level of manufacturing that is commonly used. This technology is limited to about 2/2, a minimum finished drill of .006"[.1524], and about 25 to 30 layers.

State of the art           

State-of-the-art technology is not well defined because it is an ever-changing technology whose values will change with time and must be adjusted regularly.

Manual and Autorouting

Manual Routing

Route the following types of nets first:

• Most difficult
• Most complex
• Tight fitting nets first
• Very high current (primarily external)
• Very high voltage (primarily internal)
• Sensitive
• Noisy
• Separate analog and digital.
• Route busses.

Auto Routing

• Manually route those items shown in "manual routing" first, if necessary.
• Define attributes that are common only to the auto router.
• Define/select "Routine," "Do" file, "Route" file, or "Strategy" file.

After route completion:

• Manually clean up paths.
• Miter right angle corners.
• Run DRC/design rules to ensure clearances are met.
• Check annular ring.
• Change gates or parts.

Circuit Designer Requirements

Initial Planning

• Gather required information for part list, required component locations, and mechanical locations and requirements.
• Determine if all components are available in existing libraries. If not, use a component creation checklist.
• Select design template. 
• Save file by part number. 
• Enter design information. 
• Open/load necessary libraries. 
• Place components and wire together. 
• Note all current, voltage, high frequency, noise and circuits.
• Add a note like the following for the most commonly used trace and space in the board designs: "Unless otherwise specified, all circuits are less than .25A and 30V" (this works for 6/6; .006" trace & .006" space).
• Place power-pin table.
• Place "last used" and "unused pin/gate" table.
• Highlight power nets and check each sheet for connectivity.
• Check for design rules, such as single node net; no node net; unconnected pins; unconnected wires; or other.
• Generate BOM and compare against part list. 
• Add necessary notes. 
• Add sheet numbers. 
• Print and check schematic visually.
• Align/modify location, format and styles. 
• Place all nets into classes. 
• Generate netlist. 
• Archive libraries.
• Check for other concerns:

1. Are all IC inputs terminated as required? 
2. Do IC/components have necessary filter caps? 
3. Are main circuit and branch circuits clearly identified?

Printed Circuit Board Assembly Checklist

Create an Assembly Drawing

• Load title block on top assembly layer.
• Add page # to each sheet.
• Enter design info on assembly fields or designated areas.
• Load basic notes according to board type.
• Load silk Gerber, remove all exterior to the board and copy to top assembly.
• Draw side/bottom view of board (including parts and screws, etc.).
• Using parts list, add find number leader with quantity.
• Add notes as required and triangle find numbers as needed.
• Place additional tables for any wire(s) used. Always add note and triangle find number with basic note.
• Check for mounting hardware on all components and sub-assemblies. 
• Check soldering, shrink (all soldered terminals), glue locations, required fixture,mounting brackets, and    card guides.

Conformal Coating only

• Place phantom lines. 
• Place dimensions for conformal coat clearance with note find number. 
• Place serial number box block next to the serial number silkscreen. 
• Add appropriate additional notes and place find numbers 
• Place appropriate distribution statements and proprietary notes 
• Place revision sheet information on all layers. 
• Place revision status block on assembly drawing, top sheet. 
• Get assembly drawing(s) approval. 
• Compress Gerbers into drawing number.ext.

PCB Inspection after Fabrication

• Check initial look of board. 
• Note cleanliness and appearance.
• Check mask, including:
• Specified color 
• Specified thickness 
• Quality 
• Blemished 
• Pitting
• Is plating adequate? 
• Are holes centered in pad (annular ring and alignment)?
• Are hole sizes correct? 
• Are layers registered? 
• Adhesion test 
• Does appearance match artwork? 
• Does silkscreen match artwork? 
• Are overall board dimensions correct? 
• Is board warped? 
• Is any copper showing on board edges? 
• Trace width in tolerance 
• Check electrically the following items:
• Several plated holes front to back (less than 2 Ohms) 
• Longest trace from one end to the other (less than 5 Ohms per inch, or specified value) 
• Resistance between planes of different nets for shorting (should be open) 
• Test pads closest place for continuity (known opens should read open)

Defining Constraints for a PCB Layout

Define Constraints

1. Define board dimensions.
2. Define top and bottom board clearance.
3. Note dimensions of cutouts slots and unusable areas.
4. Define the board thickness.
5. Define edge clearance areas.
6. Define all slots and cutouts.
7. Define assembly requirement such as keying information.
8. Mark predefined component locations, including hardware, connectors, lights/LEDs, and switches.
9. Place polygon on mask layer for area that requires no mask.
10. Place keep-out (all layers or per layer) on area that require clearance from/for traces, vias, pads, and hardware clearance (by hardware-to-hole tolerance, hardware movement area, or hole tolerance).
11. Define requirements: IPC, Mil-spec, etc.
12. Determine assembly type for production, such as manual, automatic, or manual prototype-to-automatic. 
13. Determine servicing type:

•   No service/troubleshooting (throw away board) 
•   Low service (inexpensive components on the board, or easily swappable application, low serviceable location)
•   Highly serviceable (expensive components on the board or difficult to swap application, highly serviceable location)
•   Determine technology limitations and target technology.