Assembly and Rework of Multilayer PCB’s

Multilayer PCB’s are being used in almost every electronic device that can be thought of. This is especially true in higher density electronic devices where components are crammed into the smallest space possible. The reason for this? It enables the engineer to track out the circuit in a tight area while providing functionality like impedance control.

Multilayer PCB’s also provide excellent thermal conductivity to be able to get heat away from components without the need of heat sinks.

This kind of approach is bound to have an effect on the way PCB’s are assembled and re-worked. Soldering parts down to a board can be hard enough, but getting them off again without damaging the board can be much harder.

Assembly and Reflow:

Applying the paste and placing components onto a multilayer PCB happens in the same way as a single sided PCB. The problems come during reflow. Imagine the PCB that’s being worked on now going through a big heater and being warmed up to a thermal profile like this one:

The area’s of copper that are larger are going to sap the heat away faster and will therefore take longer to heat up than the area’s where there is little excess copper. So all the different parts of the circuit are warming up at different rates. This could potentially lead to solder re-flow taking place at different times. This could lead to components tombstoning, reflow not taking place underneath IC’s with power pads and dry joints. Basically – a PCB engineer’s nightmare.

Compensating for this is usually done by increasing the dwell time (referred to as the soaking zone in the chart) the board is subjected to. This ensures that every part of the PCB is at the right temperature for the fluxes to start working and will reach the desired peak temperature, with all the solder joints forming as they should.

Another reflow process used is the vapour phase reflow process, where heat transfer liquid gasses are used to heat every part of the PCB and the components uniformly. The issue with the vapour phase process is that it’s not a mass production process and is often slower than using IR reflow ovens. The vapour phase process can damage some sensitive components, so make sure evert component is suitable.

Rework & Repair:

In order to carry our PCB repairs, two things are needed:

1)The right tools

2) A well trained / experienced operator

A good soldering iron like a Metcal system with interchangeable tips and a talon system are essential to carry out good re-work. The heat is used efficiently and the tips transfer the heat well. A good rule of thumb is to use the largest tip possible, particularly in the removal process. A talon system allows the operator to use different sized tips to ‘pinch’ the component on each side and remove very easily with no PCB damage incurred.

PCB damage during component removal is common, but with modern small pad sizes, not as easy to repair as ten or more years ago.So knowing how to get enough heat into the part without damaging it or he PCB is necessary.

When removing a part from the board, any kind of force will damage the board or the pads. Using a pair of tweezers or other removal tool the part should lift off with no effort. Taking time here and being patient will ensure a successful re-work.

Now for the tricky bit – what about if we’re trying to rework something without legs, particularly a component with a power pad on the bottom?

Getting heat into a multilayer board can be a real issue for re-work, the greater the layers, the more of a problem it is. This is because the copper in each layers saps the heat away from the part. We mustn’t be too critical, after all that what we want it to do! But when we want to re-work, this definitely bites us where it hurts. With a production board going into the field using a quality re-work station is absolutely the best way.

If the boards are prototypes and a fast re-work is needed, it’s useful to be able to do this in house. Some good tips for making this easier are:

1) A quality, temperature controlled hot air pencil is often enough.

2) If a pencil alone is not enough use a pre heater under the board to heat the re-work area.

3) Use a large iron tip to help pre-heat the specific component.

The basic thing to bear in mind is that more copper means more energy to heat a board or component. Designs are now in this catch 22 – like it or not!

Circuit Mechanix © 2016


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