PCB Design

Child Pages

Castellation
Design tips, the manufacturing process, images and more information about the PCB process of castellation (a.k.a half-vias, edge-plating, lead chipless carrier (LCC)).
Component Packages
This page contains links to all the component packages contained within the component database. This is where you can find dimensions, recommended footprints, soldering info, images and more about many popular through-hole and surface-mount component packages used in PCB design.
Decoupling And Noise Supression
ContentsBypass CapacitorsNoise From VFD’sBypass Capacitors Bypass capacitors are used to separate AC and DC signals from one another. Normally they are used to short out the AC component of a waveform and allow through the DC signal. A common type of bypass capacitor is a ‘decoupling’ capacitor. This is usually a 100nF (or more commonly, … Continue reading Decoupling And Noise Supression
Designators
Recommended dimensions, naming standards, the pros and cons of resetting designators, and more info about PCB designators.
Desmear And Etchback
Via residue issues, images, solutions, positive and negative etchback, and more information about the etchback/desmear process on PCBs.
EMI/EMC (Electromagnetic Interference/Compatibility)
ContentsOverviewContact vs. Non-Contact ProbesOscilloscope Probes Overview It is very difficult to reduce EM emissions by more than 20dB without serious design changes – Anon Contact vs. Non-Contact Probes Non-contact probes have the advantage of being easy to use. They do not require you to make and break electrical connections to test the circuit. They don’t … Continue reading EMI/EMC (Electromagnetic Interference/Compatibility)
General Routing Tips
Tips and tricks to help you with your PCB routing.
Getting Ready For PCB Manufacture And Assembly
One-off costs, re-occurring costs, and more information about the process of getting a PCB ready for manufacture and assembly.
How To Calculate Maximum Track Current
How to use the IPC-2221A or the IPC-2152 standard to calculate PCB track current. Online calculators are also provided.
Impedance Controlled Routing
Dielectrics, equations, microstrips, striplines, co-planar waveguides, microstrip dispersion, modes of propagation, the effect of vias and more information about impedance controlled routing.
IMS (Insulated Metal Substrate) PCBs
Thermal properties, good design practices, standard design constraints, example stack-ups and more information about insulated metal substrate (IMS) PCBs.
Layer Stackup
Cores, prepreg, impedance control, thermal properties, routing implications, design software and more info about PCB layer stack-ups.
Mouse Bites
Schematic symbols, PCB considerations and more info about mouse bites (a PCB assembly/manufacturing component).
Pads, Vias And Holes
Hole size, pad shape, micro-vias, blind vias, buried vias, thermal relief, annular rings, aspect ratios and more info about pads, vias, and holes on PCBs.
PCB Data Formats
Gerber files, ODB++, Gerber X2, IPC-2581B, file viewers, compatibility and more info about PCB data file formats.
PCB Design Checklist
A PCB design checklist covering schematics, circuit board layout/routing, versioning, releasing design files, manufacturing, assembling and documentation.
PCB Surface Finishes
HASL, ISn, IAg, ENIG, the benefits and dis-advantages of each, immersion vs. electro-less platings and more info about PCB surface finishes.
Prototyping
Breadboard, stripboard, development kits, design validation, grid generators and more info about circuit/PCB prototyping.
Schematic Tips And Tricks
ContentsMake Input Connectors Face Right, Output’s Face LeftSchematic NotesMake Input Connectors Face Right, Output’s Face Left This makes your design easier to follow. Things come in on the left, and leave on the right. Most EDA software allows you to flip (a.k.a mirror) schematic symbols (in Altium, press x while dragging the schematic symbol). Of … Continue reading Schematic Tips And Tricks
Soldering
ContentsOverviewManually Applying Solder PasteSolder Paste Dispensing MachinesPre/Post Soldering HeatersOverview Manually Applying Solder Paste Plastic syringe tips are usually better than metal ones, as they offer less friction to the paste, making it easier to control the amount of paste which comes out. Solder Paste Dispensing Machines If you are applying solder paste by hand (as … Continue reading Soldering
Soldermask Stencils
Stencil thickness, composite vs. stainless steel, etching process, aspect ratio, aperture size, approximate prices, stepped stencils and more info about PCB soldermask stencils.
Testpoints
Untented vias, voltage testpoints, current testpoints, purpose-built SMD clips and more info about PCB testpoints.

PCB Design Tools

Here’s a list of some useful design tools which come in useful when making PCB’s.

Name Comment Picture Link
Saturn PCB Toolkit Very simple but extensive PCB design aid. And it’s free! Includes a via current calculator, trace width calculator, bandwidth calculator, differential pair calculator, mechanical data, conductor impedance calculator, unit conversions, planar inductor calculator, thermal resistance calculator and more! http://saturnpcb.com/pcb_toolkit.htm
Standard Resistor Calculator Invaluable when working out the closest manufactured resistor to the value you need in your design. There are tons of web-based calculators out there, and most let you choose a resistor series (e.g. E12, E24 e.t.c), or a precision (e.g. 5%, 1%). The higher the precisioin (1% is higher than 5%), the more expensive the resistor. http://www.daycounter.com/Calculators/Standard-Resistor-Value-Calculator.phtml
Cladlab Calculators And of course, there is this site’s own calculators, can help you choose component values when it comes to things like resistive voltage dividers, and low-pass RC filters.   http://blog.mbedded.ninja/electronics/general/online-calculators

Soldermask

All values are typical: Minimum Solder Mask Sliver = 0.1016mm (4mill) Solder Mask Thickness = 11um over copper, 20um over fibreglass (the difference is to that the resulting soldermask surface is flat) Common Solder Mask Colours: green (the most common), black, yellow, blue, clear, red and white Typical Dielectric Constant: 3-3.3 Note that the darker the solder mask colour (e.g. black), the harder it is to see the copper traces underneath, which in some cases makes it more difficult to debug a faulty PCB. The soldermask colour must be compatible with the silkscreen colour.

Silkscreen

The silkscreen is a printed layer which is used to convey information to the user(s) of the board. It usually contains outlines of components, component polarity indicators, component designators, test-point information, and the PCB designer/version. The silkscreen layer(s) helps greatly when manually placing components and soldering the board. The silkscreen is printed on-top of the soldermask, on one or both sides of the board. On really cheap PCB runs, the silkscreen is left-off altogether (because it is not a necessary layer for the board to function).  For good readability, it should contrast well with the colour of the soldermask. Common silkscreen colours are:

Silkscreen Colour Relative Cost (out of 5) Compatible Soldermask Colours
White 1 Green, Red, Black
Black 2 Green, White, Yellow
Red 4 White, Yellow

The silkscreen has to printed on either soldermask or bare PCB, it cannot be printed onto bare or tinned copper. For this reason, you cannot have soldermask ontop of component pads.

 For components that protrude of the edge of the PCB (for example, connectors, or right-angled switches/leds, see the picture to the left), it is useful to indicate this on the silkscreen layer with diagonal lines as shown in the image below.

Solder Paste

Solder paste is applied in the PCB manufacturing process just before the board is populated and soldered. It is the way that solder is applied in mass-quantities by a machine, and consists of a powdered metal solder suspended in a thick paste of flux. It is usually passed through a metal stencil which is placed ontop of the PCB, which has laser-cut holes in where the paste is to be applied. Since solder paste is only used when the board are to be populated by machine, it is normally not a concern when designing prototypes that will be soldered by hand, and is part of the “design for manufacture” process. For large areas that are going to be covered to solder paste (such as thermal pads on QFN packages), it is recommended that you “window” the stencil, as shown in the image below.

recommended-stencil-opening-for-solder-paste-on-qfn-package The recommended stencil opening for solder paste on a QFN package. Note the windowing to prevent the package from floating during soldering. Image from http://www.ti.com/lit/an/sloa122/sloa122.pdf.

Designators

Designators normally only face in two directions only (i.e. down-facing and right-facing).

close-up-of-0603-resistor-on-pcb A close-up of a 0603 resistor and it's designator on a PCB.

If you cannot fit the designators next to their relevant components (e.g. a dense PCB), you can place a group of designators beside their components with the same orientation/placement, and the user can still understand which designator belongs to which component. You can see this being done on PC components such as motherboards and GPU’s.

designators-seperated-from-dense-components-on-pcb Designators can be separated from dense components, as long as they are grouped and are in the same orientation as the components.

Estimating PCB Design Time

A good way of estimating the time to design a PCB board is by working out the utilization ratio. This is the total land area of all components divided by the total surface area of the board. A utilization ratio of 0.25 is easy enough to route, and the board starts getting harder as the ratio hits 0.50 and above. There is a good article, ‘Estimating PCB Design Time And Complexity‘, that explains this in more detail, and even has a program to work it out for you.

Clearances

There are many different types of clearances on a PCB board, and it can be really confusing sometimes to find out you can an can’t do in terms of component and track placement.

Copper To Board-Edge Clearance

Typical copper to board-edge clearance : 0.3048mm (12mill) This is the closest you can put copper to the board edge. This usually accomodates for slight board-misaglignment and tool in-accuracies when routing the the board into shape so that the router will not cut into the copper and leave ‘exposed copper’ ( a big no-no, usually). However, there are cases in which this rule is ignored, for example when you a making an RD shield or box from pieces of PCB, and need the copper to go right to the edge so that they can be soldered together into a box shape. This clearance is usually larger than the ‘track and gap’ clearance.

Copper (Track And Gap) Clearance

The copper to copper and copper width clearance are usually have the same value. This is commonly called the ‘Track and Gap’ clearance. Typical Values:

  • 0.5mm – Homemade PCB’s
  • 0.2mm – Professionally printed PCB’s
  • 0.1mm – High-end PCB’s

 

Multi-Layer Routing

Route on the top or bottom layers if possible, especially when prototyping, this makes debugging and rework easier to do (you can’t cut a track on one of the inside layers!).

EMC Considerations/Noise Supression

This is a big topic, and has it’s own page here.

PCB Antennas

RF Antennas

In some situations, antennas for wireless communications can be formed directly on the PCB, with no need for an external component. They are normally used with carrier frequencies in the UHF range (300MHz-3GHz), due to these have frequencies having the appropriate wavelength for PCB construction. They are very cheap, and easy to tune, at the expense of sacrificing some communication range relative to a external antenna. PCB antennas are commonly made for:

  • RFID
  • 2.4GHz communication (e.g. ZigBee, Wi-Fi, ANT+)

Stress Relief Antennas

Antennas can also be used for stress relief during soldering for stress-sensitive parts, such as accelerometers and gyros. Un-used pins on the stress-sensitive component package are routed a minimum distance from the pad, just like the used pins, so that when soldering using reflow or infrared techniques, all the solder on the pads melts and solidifies at the same time, reducing the stress on the component.

routed-accelrometer-footprint-showing-thermal-relief-on-not-connects-for-stress-relief Routing un-used pins on a accelerometer component package a minimum distance away from the IC to reduce stress when infrared/reflow soldering.

Undesired Antennas

Any PCB antenna that isn’t for radio transmission or stress relief is probably a bad thing. These are cause by tracks end in the middle of nowwhere, and are often created un-intentionally when routing the PCB.

Heatsinking

See the Heatsinking page.

Mounting

Don’t forget to consider how the PCB is going to be mounted. One of the most common things accidentally left of a PCB. Normally all it requires is four holes (un-plated) in the corners for screwing in bolts/supports or just enough free PCB space to stick on rubber feet. M3 bolts and nuts are commonly used. Remember to consider the implications of using metal bolts next to/on-top of electrical signals. There are nylon bolts and washers when insulation is needed. Lock-tight can be good to prevent bolts/screws from undoing. The following picture shows the four mounting holes on a PCB. Notice one is underneath a component (the Xbee module).

pcb-showing-mounting-holes A PCB, showing the four mounting holes. Note that one is underneath a component (the Xbee module).

Standoffs are good for providing clearance between the PCB and the mounting surface. You can buy plastic or metallic standoffs for most electronic suppliers, and they come in round or hex shapes. Electrical enclosures usually come with standoffs built into the structure.

spherical-rubber-pcb-feet A stick-on spherical rubber PCB foot.

Below is an example of a SMD standoff from Pem Engineering.

reelfast-smd-standoff-example An SMD standoff from Pen Engineering. Image from http://catalog.pemnet.com/viewitems/uts-and-spacers-standoffs-surface-mount-type-smtso/t-nuts-and-spacers-standoffs-br-type-smtso-unified.

Desiccant

Desiccant (a moisture absorbing compound), can be a good thing to use if your PCB is at risk of condensation.

dessicant-on-oven-tray Desiccant about to be dried on an oven tray.
See the Desiccant page for more info.

Making It Look Good

It could be said that PCB design is an art form. Personally, I take great pleasure in making a PCB that got the looks as well as the functionality. And it’s helpful that most steps toward making a PCB look good actually tie in well with good design practises anyway.

This is an example of a PCB which I really like. It is called “The Mojo“, and it is a Arduino-like FPGA development board. I love the black and green colour combination of the soldermask and silkscreen respectively.

the-mojo-fpga-pcb "The Mojo", a Arduino-like FPGA development that looks really nice. Image from http://embeddedmicro.com/products/the-mojo.

Late Cycle Rework

“Late cycle rework” is the term given to PCB reworking done at the late stages, just before being sent of to the PCB manufacturer. Late cycle work is normally a bad thing, it is estimated that work done in this stage cost 10x as much as it does in the earlier stage.

Panelization

Panelization is when you combine multiple PCB designs onto one PCB board, with the normal intention of saving costs associated with producing individual PCB’s. Most PCB manufacturers support panelization and will route/v-score each design out for you, so you can separate them easily.

Altium supports panelization. There is also software called GerbMerge, an open-source tool which merges Gerber files together.

Sending It To A PCB Manufacturer

For a review on different PCB manufacturers and their capabilities, see the PCB Manufacturers section of the Electrical Suppliers page.

PCB Rework

Below is an example of a screwed PCB after too much rework was done on a 0.5mm pitch QFN package.

qfn-footprint-failure-after-too-much-resoldering The PCB was wrecked after doing too much re-soldering on a 0.5mm pitch QFN footprint.

PCB Design Checklist

Click here.