Surface Mount Technology I

In this project, we will be following the processes of converting a circuit diagram, into a Printed Circuit Board (PCB) Design. The type of PCB we will be designing will have surface mounted components; known as Surface Mount Technology (SMT). This is where the electronic components are placed and soldered on one side of the FR4 board. Let's get started!

The circuit we will be working on is an ASTABLE circuit. This is a basic circuit where 2 LEDs (Light Emitting Diodes) alternate between each others. This circuit is made up of resistors, capacitors, transistors and LEDs. Below is the circuit diagram for the circuit, designed using National Instruments Multisim.

Note: 'J1' is used as a power supply for copper pad purposes

Assigning Component Footprints

To begin the design process, we must assign physical component footprint to each electronic component. This is a key stage in the process because these will determine the copper pad sizing and it is important that the footprints assigned match the real-life component, otherwise the circuit will not be manufacturable. To assign footprints in Multisim, repeat the following steps for each component, assigning the correct package.

1. Turn on the Spreadsheet View by selecting View»Spreadsheet View

2. Select the Component tab in the Spreadsheet View

3. Scroll over to the Footprint or Package column

4. Select the component from the list, click on it's Footprint or Package field

5. In the Edit Footprint or Package dialog box click the Select From Database button

6. Pick the appropriate footprint for the component from database

7. Click the Select button

8. Click the OK button

The footprint will be assigned to the component, repeat the steps for the rest of the components.

Converting from Multisim to Ultiboard

Once the circuit diagram is assigned with footprints, it is ready to be exported to an Ultiboard file. Ultiboard is another National Instruments software which allows users to design ideal PCBs. Follow the steps to transfer the circuit diagram to Ultiboard:

1. In Multisim select Transfer»Transfer to Ultiboard 10

2. Multisim suggests you to save a netlist representation of the file in a .ewnet format.

3. Save the file to the same location in which the original file was saved.

4. Ultiboard automatically launches

5. The Default Track and Clearances dialog box appears. You can choose whether you would like to manually place the components or let the software automatically create a design for you. Confirm your setting by clicking on the OK button.

7. The Import Netlist Action selection dialog box appears. Accept the default setting (to import all components and nets) by clicking on the OK button.

We have decided to manually design the PCB because it will allow us to design an efficient and compact circuit, whereas the computerised design could have wire links and errors.

Designing a Compact SMT PCB

Using Ultiboard allows you to have a view of the ratsnest, which is convenient when designing the PCB; it provides a view of which link connects where. When desinging a PCB, there are a few key things to keep in mind. The PCB must not have any jump links, meaning the copper tracks must not crossover one another. The second thing is that the PCB design should be as compact as possible to reduce manufacturing costs but that being said, this can only be done where possible to prevent errors from occurring within the design.

When laying out the components within Ultiboard, you can choose to autoroute the ratsnests or, again, manually route them. By right clicking and selecting the properties of the routing, you can modify the copper track width to your preference. Be sure to consider track sizes according to the current required of your circuit (thin tracks and high currents can damage the circuit). Also, it is advised to avoid 90-degree copper tracks and set the corners at 45-degree angles. This is because the outer corners of 90-degree tracks can be etched and cause short circuits and errors.

On the component copper pads, we had applied solder mask and paste mask to allow the components to be conductive with the copper tracks. This is important for electrical conductivity and will be required during the manufacturing process. Ultiboard allows you to view different aspects of your design such as the copper top, silkscreen, paste mask and solder mask. Whilst designing you can choose what you prefer to view.

We had managed to create a compact, neat and tidy design with an overall width of 22mm and length of 23mm.

Manufacturing Process of SMT PCB

The majority of PCB printers require Gerber type files to print out the artwork of PCB circuits. For our SMD PCB, we will be exporting the artwork in Ultiboard into Gerber file types. This will be done by following the steps below in Ultiboard:

1. Click File

2. Click Export

3. Make sure Gerber 274 is checked

4. Select the aspects of your design which you want to include (copper top, silkscreen, solder mask and paste mask).

5. Click export

6. Find files in folder where the Ultiboard file is saved

Once the files have been exported, they will be available in a folder with each individual artwork. These can now be opened in a Gerber viewer application to be viewed and also be copied on to a PCB printer to be printed.

When the Gerber files are loaded on to the PCB printer, we can start by printing out the copper top on to an FR4 board. This will be a conductive paste which maps out and prints the tracks of the copper. After it is printed out, the PCB printer will heat the board and the conductive paste so it sets. This is done over a 45-60-minute period. We can then print the solder paste mask on to the board and must place the components before it dries up, otherwise it will not be conductive. Finally, once the components have been placed, the board goes through another phase of heat to melt the solder paste and solder the surface mount components to the board. Below we have made the final SMT PCB.

Note: The image on the left is taken before going through the heat phase to melt the solder paste mask

Advantages of Surface Mount Technology

1. Reduced board costs and material handling costs

2. Component size is a lot more smaller making the design more compact

3. Boards are not required to be drilled through as they are for through hole components

4. Solder paste can be used and components can be picked and placed, allowing a neat tidy finish

5. Ideal and beneficial to automatic production, reducing production time and cost

Disadvantages of Surface Mount Technology

1. Difficult to hand solder

2. Component that generate heat cannot be surface mounted as the solder can melt

3. Tight track spaces can make it difficult for repairs and cause shorts

4. Can only be used for micro circuit types such as motherboards and PC cards

5. Difficult to visually inspect and test circuit boards

By Zaqyas Mahmood, Electronic Engineer