Successful Endeavours - Electronics Designs That Work!

Printed Circuit Boards PCB


Printed Circuit Board Assembly

Also referred to as a PCA, the Printed Circuit Board Assembly follows on from Printed Circuit Board Manufacture. This is where the components are placed onto the Printed Circuit Board and the electrical connections formed.

In this post I will focus on volume manufacturing techniques. We also make Printed Circuit Board Assemblies in house by hand loading very small quantities. This is appropriate for prototypes and Niche Manufacturing quantities.

To start with, lets look at the 2 types of components we most work with. The first type is the Through Hole Component. These have pins that go through the PCB to make electrical connection. These components dominated PCB Assemblies until the 1980s when higher PCB loading density requires a change of technology. They are still widely used where mechanical strength, tall components, heavy components or high current levels are involved. An example is shown below with the connectors, relays, transformers and removable components as Through Hole with the Surface Mount Components toward the centre: 

Through Hole Technology

Through Hole Technology

The second type is the Surface Mount Component or Surface Mount Device and the overall process is referred to as Surface Mount Technology or SMT. These devices do not require holes through the PCB to mount them and so can be placed closer together and it also improves track routing options because tracks can run on the other side of the PCB without having to avoid the through holes. An example of all Surface Mount assembly is shown below in close up:

Electronics Hardware

Electronics Hardware

 Printed Circuit Board Assembly Process

The infographic below was provided by Algen Cruz of Advanced Assembly in the USA. Algen also provided a brief explanation to go with it and I have added that as well. You can click on the infographic to view a larger version. 

Printed Circuit Board Assembly

Printed Circuit Board Assembly

 “Design-for-Assembly (DFA), although not as well known as Design-for Manufacturing (DFM), needs to be taken into account during the design phase. And the first step in being able to design-for-assembly is to understand the assembly process. This infographic features this process by showing how a board goes from an unpopulated printed circuit board (PCB) to a final product, ready to be packaged and sent to consumers.” Algan Cruz

 

Successful Endeavours specialise in Electronics Design and Embedded Software Development. Ray Keefe has developed market leading electronics products in Australia for nearly 30 years. This post is Copyright © 2015 Successful Endeavours Pty Ltd.

Printed Circuit Boards

In our series on Electronics Design we have looked at the Electronics Design Process from Requirements Capture, Technology Selection, Component Selection, Schematic Capture and finally the Printed Circuit Board or PCB Layout. Now we have a design and the Electronics CAD files to make a Prototype. 

There are a number of steps involved in making a PCB and the following infographic provides an overview.

PCB Manufacture Steps

PCB Manufacture Steps

This infographic is courtesy of Newbury Electronics.

PCB Manufacturing Problems

That is a lot of steps. And there are things that can go wrong. The main pitfalls to avoid in the PCB Design Process are:

  • track widths too narrow
  • clearances between tracks are too small
  • acute angle entry to pads
  • component footprints have pins in the wrong place or the wrong size
  • component outlines are wrong
  • silkscreen or overlay over solder pads
  • via annulus too thin
  • mounting holes in the wrong place or the wrong size
  • PCB outline incorrect
  • PCB 3D profile doesn’t fit into the intended enclosure

 And there are a range of issues that can affect the PCB Manufacturing Process. These include:

  • misalignment of drill holes to tracks to PCB outline routing
  • internal cut outs missed / not routed
  • over etching or under etching of the copper
  • incomplete plated through holes
  • poor surface finish
  • poor FR4 and copper bonding or moisture ingress leading to delamination

Maybe you are wondering how a PCB ever gets made successfully? This comes back to undertaking the PCB Design with an understanding of both electronics engineering design principles and the process capability of the manufacturer into account. And when you get it right, the final product can be pretty awesome. A good example can be found at this post about making a Fine Pitch PCB

RGB LED Array Close Up

RGB LED Array Close Up

Next we will look at the PCB Assembly process.

Successful Endeavours specialise in Electronics Design and Embedded Software Development. Ray Keefe has developed market leading electronics products in Australia for nearly 30 years. This post is Copyright © 2015 Successful Endeavours Pty Ltd.

PCB Layout

After the Schematic Capture component of the Electronics Design  is complete, the logical connections for the electronics components have been determined. If the Electronics CAD package also supports it, you can add rules to guide the Printed Circuit Board Layout, also abbreviated to PCB Layout which we will use from here on.

The PCB provides both the mechanical support for the components and is many cases is a critical part of the circuit since the length of tracks, their thickness, their clearance from other tracks and the relative placement of components and tracks can significantly influence the final performance of the PCB. This is particularly true as power levels, clock speeds or frequency increases.

The Electronic Schematic defines the electrical connections between components, the value of components such as resistors, capacitors and inductors, the type of semiconductors used (silicon chips) and the connectors that take signals and power on and off the PCB. Each item on the schematic has to be linked to a physical shape that will go onto the PCB. This is done by assigning a footprint to the schematic item.

Schematic Symbol

I will explain  it works. The Schematic Symbol for an FT232RL USB Serial Interface device is shown below. This is arranged with the signals conveniently placed to suit logical connections and to make the overall Schematic easy to read and understand.  The signal name is shown inside the symbol boundary, and the pin number of the IC package is shown on the outside. 

FT232RL Schematic Symbol

FT232RL Schematic Symbol

Schematic Circuit

So this  is the symbol for a single part, an IC or Integrated Circuit. The Schematic Circuit or Electronic Schematic shows the connections to the other parts of the circuit. Below we see USB connector wired up the the FT232RL IC and the power supply bypass capacitors. The logic level UART signals are shown at the top right. This section of the Electronic Schematic provides the logical connections for a USB serial interface.

FT232RL USB Schematic

FT232RL USB Schematic

PCB Footprint

 Before we can do the PCB Layout, we have to associate the PCB Footprint each Schematic Symbol will use. The PCB Footprint for the FT232RL IC is shown below.

FT232RL PCB Footprint

FT232RL PCB Footprint

This is one of the 2 possible footprints for the FT232RL. This one is a 28 pin SSOP package.

Once each Schematic Symbol has a PCB Footprint, we are ready to do the PCB Placement.

PCB Placement

 The first step is to create the outline for the PCB and its mounting points, then to place each PCB Footprint so it is in the correct place. For some components, such as connectors, there is a specific place it must go. For other components, there is more freedom to choose the position and there are groups of components that must be in a specific relationship to each other. An example of this are the power supply bypass capacitors which must go very near to the IC they are supporting.

An example of a completed PCB Placement is shown below. This is a USB to RS232 serial converter.

PCB Unrouted

PCB Unrouted

PCB Routing

Now we have the components where we want them, we turn on the autorouter and the PCB is finished. Sorry but I couldn’t help that. The autorouting features of most PCB Layout CAD software packages are never as good as doing it yourself. They can be useful for testing the ease of routing for a particular placement. There are a lot of manufacturing considerations that need to be taken into account and track size requirements, either for current carrying or voltage drop, can be hard to define from just the schematic. And example of this is the main system voltage such as VCC. In some parts of the circuit the required current is low so smaller track sizes are OK, whereas other areas need heavier tracks. It isn’t easy to define this at the schematic level because they are all the same signal or Net.

The PCB with the routing complete is shown below. The selection of track size is related to the current the circuit needs to carry. A good reference for determining the track size is provided by the standard IPC-2222A.

PCB Routed

PCB Routed

PCB 3D Cad Integration

 It is also important to make sure the PCB will fit into a mechanical enclosure. Most modern PCB CAD tools, such as Altium Designer which we use, can create full 3D models of the PCB. Shown below is an example of just the PCB without the components showing.

3D PCB View

3D PCB View

 So there we have it. A PCB taken from the completed Electronic Schematic through to a PCB Layout.

 Next we will look at prototyping our new PCB.

Successful Endeavours specialise in Electronics Design and Embedded Software Development. Ray Keefe has developed market leading electronics products in Australia for nearly 30 years. This post is Copyright © 2015 Successful Endeavours Pty Ltd.

Fine Pitch Printed Circuit Board

This example is from a project coming to the end of the Proof of Concept phase. So we have done the Electronics Design and also completed the PCB Layout. I can’t tell you what it does, but you don’t really need to know in order to appreciate the technology. This is an example of a Fine Pitch PCB or Fine Pitch Printed Circuit Board. And even better, it was made right here in Melbourne, Australia.

Pictures first.

RGB Light Emitting Diode Array

RGB Light Emitting Diode Array

Above we have the top surface of a Prototype PCB that drives a 16 x 16 or 256 RGB LED array. The size is 25mm square for the LED Array. You might also have realised that this is a custom RGB LED display. The display is driven as a row x column matrix. This top side has the 16 row drivers.

RGB LED Array Bottom Side

RGB Light Emitting Diode Array Bottom Side

This is the underside with the 16 x 3 = 48 column drivers. 

RGB LED Array Detail

RGB LED Array Detail

This shows some more detail where the Sea of RGB LEDs is sitting. They are in a staggered offset to reduce jagged edges on the image when it is displayed. 

RGB LED Arracy Close Up

RGB LED Arracy Close Up

 This final picture is a close up of the RGB LED array with a lace pin as a size reference. The RGB LEDs are 1mm wide and the pin head is a bit less than 1mm across. This is the smallest pin I could find.

Fine Pitch PCB Technology

Now for some technical details:

  • 4 mil track width (that is 0.1 mm)
  • 4 mil clearance (that is also 0.1 mm)
  • 0.25 mm via hole diameter

 The Prototype PCB was manufactured by PCB Fast in Seaford. We use them for our Prototype PCBs because they still manufacture in Australia. And that is part of our focus, maintaining manufacturing in Australia. So I was very impressed with the work they did and thought this was a great way to show what they can do. I was also impressed with the spirit of adventure Kevin and Leeanne had in taking this one on.

One day I’ll be able to tell you what it was for.

Successful Endeavours specialise in Electronics Design and Embedded Software Development. Ray Keefe has developed market leading electronics products in Australia for nearly 30 years. This post is Copyright © 2015 Successful Endeavours Pty Ltd.

Schematic Capture

Schematic Capture is the process of defining the logic connections between different components in an Electronic Circuit. At the end of the process you have a diagram or Schematic of the circuit. That’s a complicated way of saying that it shows the connections between the selected components. We use Altium Designer as our EDA or Electronics Design Automation tool. 

That is a lot of links but this is an important part of the process. Get this wrong, and you have a product that doesn’t work.

Electronics Schematic

Electronics Schematic

This the Electronic Circuit Schematic for a 5VDC Switch Mode Power Supply, also known as SMPS. It can deliver up to 0.5A and includes a number of novel features to reduce noise and ripple. The RC damper across D5 is one of these. The other is the 82R series resistor that limits the maximum current through the charge pump diode C14. The measured ripple is less than 1mVRMS.

I’ve gone into a bit of detail because this shows how effective Component Selection can lead to a great outcome. We started with the design objective of a non-isolated power supply to get a 5VDC rail for our circuit from the incoming 12VDC rail. I wanted an efficiency above 80%, low noise, small footprint and low cost. So we looked at a wide range of suppliers including some like Texas Instruments, or TI as they are usually referred to, who have tools on their websites that will select suitable components for you. In this case they didn’t have a suitable offering but Microchip did. 

And the Schematic above is the result of the Component Selection process, review of the datasheet to get the circuit requirements for things like calculating the output voltage feedback divider (R10 and R12) correctly. And now we have our Schematic ready for creating the PCB Layout

Altium

Altium – EDA

Altium have a comprehensive tutorial on the whole process using their tool at Get Started With PCB Design.

Successful Endeavours specialise in Electronics Design and Embedded Software Development. Ray Keefe has developed market leading electronics products in Australia for nearly 30 years. This post is Copyright © 2014 Successful Endeavours Pty Ltd

Design For Manufacture

Electronics products almost invariably have a Printed Circuit Board , PCB, on the inside. This is one of the most common things we do, designing the Printed Circuit Board on the inside on the product. Now designing a Printed Circuit Board so it works correctly is one thing, but if you are going to make them cost effectively in volume then you have to consider the manufacturing options at your disposal. To achieve Low Cost Electronics Manufacture requires every aspect of the design to be considered. The following video covers the basic issues very well:

So the things to focus on are:

  • Use SMT as much as possible,
  • Reduce the number of components by using more highly integrated circuits,
  • Reduce the variety of components so the number of reels is reduced,
  • Ask the PCB loader about their standard panel sizes.  If you can adjust the PCB size to suit them then it will reduce their costs,
  • Work with component types that the PCB loader can handle
  • Work with components that you can buy in suitable quantities

Ray Keefe has been developing high quality and market leading electronics products in Australia for nearly 30 years.  For more information go to his LinkedIn profile at Ray Keefe. This post is Copyright © 2011  Successful Endeavours Pty Ltd.

 

Carbon Nanotubes replace Solder Pads

Recent breakthroughs in nanotechnology could change the way Printed Circuit Boards, PCBs,  are made and this could start happening soon. If you either the Design PCBs or Manufacture PCBs then you will want to keep up with this new technology that uses Nanotape PCB Pads .

Here is a picture showing how the NanoTape structures differ from conventional Solder Pads. Although they look the same from the outside, the internal geometry shows the higher thermal and electrical conduction created by the Carbon Nanotubes. This can significantly help in product miniaturisation and for high power designs where both the enhanced thermal and electrical performance will improve the efficiency.

NanoTape replaces solder pads

Like all new technologies, there will be teething problems but this has the potential to overcome the Tin Whisker issues that plagued PCBs before the introduction of Lead to Solder and which have emerged again with the move to RoHS compliance and the use of Lead Free Solder.

Ray Keefe has been developing high quality and market leading electronics products in Australia for nearly 30 years.  For more information go to his LinkedIn profile at Ray Keefe. This post is Copyright © 2011  Successful Endeavours Pty Ltd.