Introduction to PCB, Basics & Designing

Terms & Acronyms

• PCB – Printed Circuit Board.

• Tracks – Form the connections between component.

• Pads – Used to make connections between your component and the track with solder.

• Layers - Everything is drawn on a layer, layers can have different physical properties. Example layers are: Top layer, bottom layer, Top overlay, bottom overlay, Mechanical 1, Keep Out.

• Plated Through Holes (PTH) – Drilled holes that are plated with conductive material (usually copper). 

• Footprints – A collection of pads and outlines which describe an individual component’s physical layout on a PCB.
• Design Rules – Rules specified by the designer to ensure PCB integrity (more on this later).
• Vias – Used to make connections between different
  layers of the PCB.
• Silkscreen – A protective layer over the top of the
  board, makes soldering easier.

Some more doubts to clear......

FAQ

1. Should I use metric or imperial?

2. What size drill-holes should I be using?

3. What size should my tracks be? What sort of clearance should I have  

    between tracks?

4. The tutors tell me Auto-route is the Devil, is this true?

5. Help! I can’t find a footprint for my component!

1. Should I use metric or imperial?

• Remember: Metric measurements are in (milli)meters, imperial  

   measurements are in thou (thousandths of an inch).

• For Team Project PCB's we have decided on the following important rules.

• 1000mil/thou = 1 inch ≠ 1000mm.

• Drill Holes – USE METRIC!

   

2. What size drill-holes should I be using?

• If a panel to be manufactured has more than 15 different hole sizes, then 

   the COST increases and possibly even the LEAD TIME. So try to use as few  

   as possible (within reason).

Suggested Hole Sizes

Part	Suggested Hole size
Vias	0.6mm
IC Legs, TO-92 Transistors, 1N4148 Diodes	0.9mm
Resistors/Capacitors/ and many other components	0.8mm
Diode Legs (1A, eg 1N4004), IDC connectors, TO-220 transistors	1.0mm
Diode Legs (3A, eg 1N5404), many connectors	1.25mm

4. What size should my tracks be? What sort of clearance

    should I have between tracks?

• A track should not be narrower than the minimum manufacturing limit.    

   However, it is recommended that for simple designs you should stick to    

   12thou (0.3mm) as a minimum.

• Thicker tracks are needed if you want to carry significant current. Thin tracks   

   will get hot, and cause voltage drops if significant current flows.

• For your projects, there’s no harm in using larger tracks.

5. The tutors tell me Autoroute is the devil!

• Using Autoroute is a design decision of your own to make.

• Using Autoroute is a lazy option (you should be spending more time learning  

   how to create PCBs, not less). “Would you trust Microsoft Word to 

   automatically format

  

6. Help! I can’t find a footprint for my component!

                                      "MAKE YOUR OWN"

(More on this soon)

General PCB Design Tips

PCB Design Tip 1: LABEL YOUR PCB

•Include your TEAM NUMBER & COURSE CODE

 Put it on Top Overlay

PCB Design Tip 2: Be careful with your power nets

•Place decoupling caps near microcontrollers and other IC's, to

help reduce noise on your power-supply line. How much

capacitance? If the datasheet don’t specify an amount, start

with 10-100nF, then measure your supply-line using a CRO.

PCB Design Tip 3: Orientate polarised components sensibly

PCB Design Tip 4: Where did that component go?

Make use of the PCB filter

PCB Design Tip 5: Make sure you draw a PCB border on Mechanical Layer 1

OTHERWISE YOUR PCB MAY BE REJECTED!                                  

 PCB Design Tip 6: If you make mistakes on your board, it is possible to correct them with PCB hacks.

• Cut tracks with a sharp knife.

• Bridge tracks/pads with fly-wire.

• File/cut your board down to size. Wear a mask.

• Drill new mounting holes (if you forgot them!).

PCB Design Tip 7: Use oval shaped solder pads instead of circular

pads.

  PCB Design Tip 8: Use long solder pads for surface mount

 devices.

PCB Design Tip 9: Learn how to use the Bill of Materials tool! 

You can keep track component details such as:

• Part Manufacturer

• Supplier (and cost)

• Supplier order number

• Datasheet URL

• etc

PCB Design Tip 10: Spend lots of time setting up your PCB rules!

Eg

• Set your Power Nets to have a minimum track

width of 18mil.

• Set your ground plane to have a larger

clearance constraint than usual.

PCB Design Tip 11: Make sure you check the PCB manufacturing

rules!

                              Or your board will not be accepted!

PCB Design Tips

• Use a sensible number of drill-hole sizes. (Consolidate with other teams).

• Don’t forget mounting holes!

• If your PCB has a background layer, delete it!

• Take your time (and allocate a lot of it). You can’t do a PCB design the night before a batch is due.

NUMBER ONE PCB DESIGN TIP

"PRINT YOUR PCB TO SCALE AND PLACE COMPONENTS ON TOP OF YOUR PRINTED PCB".