Script – THT Mounting
Before the invention of the SMT ( Surface Mount Technology) all components were assembled to the PCB with leads going through component holes in the board.
This technology dominated the industry till the end of the eighties.
Today most components placed are SMT components. You could think that THT (Through Hole Technology) is now outdated and not used anymore, as SMT is more efficient, cost effective and adapted to denser boards.
However THT offers specific benefits that keep it relevant.
The bonds created between THT components and the board are far stronger than SMT bonds, making THT the ideal choice for components that will undergo mechanical and environmental stress or high heat.
In case of prototyping THT components are also easy to change, making them perfect for testing and hobby users.
There are disadvantages too.
Through hole components are much bigger and the component itself is on one side but the solder joint is on the other side, so using space on both sides. THT also restricts the available routing area on inner layers since the holes must be drilled through all the PCB layers.
The THT assembly process is not automated, depends on the skills of the operators, is thus less reliable and more expensive than SMT.
Before placing the TH components we first cut the leads to the required size.
Some components need to be placed at a certain distance of the board. We use ‘spacers’ from plastic or metal to place the components at the desired height.
The spacers are fixed by soldering, screwing or snap-fitting.
Some components are not just fixed on top of the boards, but are mounted from the side.
These are called edge mounted connectors. Edge mounted connectors are mainly used for high frequency applications. They need special attention for design and also during the mounting process.
To combine the advantages of SMD and THT, some applications require mixed technology components.
Most common are USB connectors, that need to be mounted on dense boards with little free space, so optimal for SMT technology, but need a good mechanical stability, only possible with TH leads.
To mount such components we use a pin in paste technology, as explained in another episode.