This website has been created primarily to be a technical resource for surface mount process engineers but also for anyone with an interest in surface mount technology (SMT). The scope of the website will be all aspects of the manufacturing process from solder paste printing, component placement, reflow soldering through to automatic optical inspection (AOI) and will include answers to the many frequently asked questions.
Introduction to Surface Mount Technology
Surface Mount Technology is an area of electronic assembly used to mount electronic components to the surface of the PCB as oppose to inserting components through holes as with conventional assembly. SMT was developed to reduce manufacturing costs and also to make more efficient use of printed circuit board (PCB) space. As a result of the introduction of surface mount technology it is now possible to build highly complex electronic circuits into smaller and smaller PCB assemblies with good repeatability due to the higher level of automation.
What are SMD's?
Surface mount device or SMD is the term used for the electronic components used within the surface mount assembly process. There is a wide range of SMD component packages available on the market and come in many shapes and sizes - a selection can be seen below:-
Surface Mount Assembly Process Summary
The surface mount assembly process starts during the design phase when the many different components are selected and the PCB is designed using a software package such as Orcad or Cadstar (others are available).
It is important to realise that the process starts at this stage as this is the best time to incorporate as many design features as possible that will make production straight forward and head-ache free. Quite often circuits are taken from the schematic design phase to PCB layout with the main considerations being the functionality, which of course is very important, but design for manufacture should also be discussed.
Once the PCB design has been finalised and components selected the next phase is to send the PCB data away to a PCB manufacturing company and components bought in the most suitable way to facilitate automation. The PCB panel design should be considered and specification created to ensure that the format that the PCB's are received is as expected and suitable for the machines to be used.
Components are available packaged in many different ways such as on reels, in tubes or in trays as can been seen below. Most are available on reels which is preferred but sometimes due to 'Minimum Order Quantities (MOQ's)' components are quite often supplied in tubes or in short strips of tape. Both of these packaging types can be used but do need appropriate feeder types. Components supplied loose in bags should be avoided unless absolutely necessary as these parts will need to be hand placed which can cause quality defects.
All components with an MSL (Moisture Sensitivity Level) should be handled in line with J-STD-033.
Having received the PCB panels and components the next step is to setup the various machines used with the manufacturing process. The first automatic machine to setup is the solder paste printing process which involves applying solder paste using a stencil to the appropriate pads on the PCB.
Keeping control of this process is critical as any printing defects, if undetected, will generally lead to defects within the finished assembly. Most solder paste printing machines have the option of including automatic inspection but manufacturing companies tend to prefer an extra solder paste inspection (SPI) machine to maintain line speed. The latest SPI machines also use 3D technology to enable a more thorough inspection including solder paste volume per pad and not just solder print area.
Once the printed PCB has been confirmed to have the correct amount of solder paste applied it moves into the next part of the manufacturing process which is component placement. There is a large variety of machines available for this process and it depends greatly on the business to what type of machine is selected. For example if the business is focused around large build quantities then the placement rate will be important however if the focus is small batch/high mix then flexibility will be more important.
Following the component placement process it is of great importance to verify that no mistakes have been made and that all parts have been correctly placed before reflow soldering. Again the best way of doing this is by using an automatic optical inspection (AOI) machine to make checks such as:-
- Component presence
A feature of the later inspection machines is that they can be networked together to allow feedback to enable automatic adjustments to be made. For example the AOI machine can be connected to the placement machine so that component placement positions can be adjusted and the SPI machine can be connected to the printer to allow adjustments to be made to the alignment of PCB to stencil.
Once all component placements have been checked the PCB assembly moves into the reflow soldering machine where all the electrical solder connections are formed between the components and PCB by heating the assembly to a sufficient temperature. This is one less complicated processes but still requires correct setup to ensure acceptable solder joints without damaging the assembly due to excessive heat.
The last part of the surface mount assembly process is to again check that no mistakes have been made by using an AOI machine to check:-
- Component presence
- Presence of solder
- Dry joints
- Solder shorts