Making reliable electronic assemblies is about making a good solder joint isn’t it?
So what’s the problem, plop down some paste in the right area place the component carefully down, apply heat and the solder melts. Hey presto!
Yes, if only it was that easy. The Jedec thermal profile is well known and was mentioned in a previous issue???? The problem is every part of a PCB will have a different thermal profile as will every component and joint. In the ideal world the flux needs to activate at the same time and the solder melt at the same time, but this is never going to happen.
The result of this is an electronic assembly can suffer from dry joints in places and others voids, head in pad or possibly tombstoning. It’s quite a juggling act that needs to be performed. Unfortunately it’s mainly trial an error, but knowing what to consider before going to assembly can help avoid some of these nasty niggly errors.
It’s more difficult to uniformly heat a PCB to the correct Jedec profile. Thermal shadows and hotspots in different parts of the PCB can create problems in achieving a good result. Using a longer thermal ramp up and dwell time can often help but getting heat under leadless components and BGA’s will always create difficulties.
Dry joints and voids under these components will not be immediately obvious with only x-ray inspection able to reveal any issues. Many BGA’s have hundreds of balls on them with every singe one needing to be perfectly soldered in order to operate properly and reliably.
Vapour phase reflow is becoming more popular in reflowing modern high density PCB’s as this gives a far more uniform and reliable thermal profile across the PCB being assembled, even for joints under the component such as BGA’s. It tends to be a slower way of reflowing boards, so it isn’t used as much in higher quantities, but the process gives good results across the board.
So now we know how to make a good solder joint that’s it right? The board can be put into service and will work for years – problem solved?
Many PCB’s are put into service in environments that are harsh, with high levels of moisture, dust, or some kind of chemical pollution. Any one of these will affect the long term operation of the product by either forcing the board to overheat or causing problems in the operation of the circuit.
Using underfills for BGA’s and a conformal coating can help protect a circuit from these effects, but a maintenance schedule could still be needed to clean the boards.
An underfill is a liquid that fills in under a BGA or other leadless component to protect the joints and give extra mechanical rigidity.
Conformal coating are sprayed over a PCB to protect the board from moisture and other contaminants. There are many types to use and choosing the right one to protect against the contaminants it’s expected to encounter will need careful consideration.
Both of these processes should only be applied to a tested working board. If these are to be used on a cleaned board, the board must be really clean as any residue flux is likely to corrode any joints or copper over time (such is the nature of no clean fluxes). Once a conformal coating has been applied, there’s little that can be changed on a board, so it’s make or break. If all this works, your PCB will work in some harsh environments for a good time without trouble.
Circuit Mechanix © 2016