Article published on LinkedIn by Rob Ronan (UK/Ireland Sales & Support Manager)
Voiding under QFNs often occurs due to the volatiles in the solder paste becoming trapped under the pads during reflow, QFNs are particularly affected because they have no leads and, therefore zero standoff for stress absorption and potential for volatile escape.
This leads to void formation on the thermal plane and within the pads as well. Without the ability for the volatiles to vent, they will become trapped as solder voids and flux entrapment. Correct design of solder screen apertures can help with this by using the devices recommended aperture layout for the thermal plane to allow for pathways for the volatiles to escape.
But there can be a downside to this with a risk of insufficient solder causing a weak joint. And, unless the PCB designer has followed the specific pad layout for the specific component, then there is not much benefit for the assembler by referring to the same datasheet for the solder screen aperture layout.
Unless all parties involved are working closely together then this can just increase the risks of voiding and other manufacturing errors. And having spent 14 years in contract manufacturing, I know first hand that there is often a disconnect between the PCB design and the PCB assembler.
The assembler is sometimes left to make the best of what they have been presented with although this may not always be ideal. So, the stencil design becomes crucial for the thermal plane along with the solder paste type, the reflow ramp rates, the activity of the flux in the paste and the pre-baking of the PCB/components – as these are the only things the manufacturer can influence at this point.
Poor thermal plane solder coverage can obviously be an issue with connection and heat dissipation but can also be a weakening factor to the integral strength of the solder joint. This can become an even bigger risk when the end use of the PCB is in a high stress environment with high temperatures, vibration, shock etc.
Retronix helped a customer with this issue a few years ago when they had been seeing fall out of PCBs at thermal cycling due to the QFN solder joints fracturing under the thermal shock. After discussions with them, we trialled the ideal of using our laser ball attach system to place solder balls on the pads and thermal pads.
The main aim of this trial was to create better CTE protection for the device, and that was achieved with an 10X increase in thermal cycles to failure. But what it also showed was a large reduction in the voiding percentage in the pads and the central thermal pad as well. This wasn’t what we had set out to achieve but came as an additional solution for this common process issue, for us and the customer.
Since then we have been regularly processing these components for the customer along with other BTC (bottom terminated only) components that now are specified for ball attach. This was also introduced as a new service for Retronix to offer to other customers and has grown to become one of our most used services behind BGA reballing, Re tinning and Alloy Conversion.
If you would like more information on application specific ball attach then please make contact with us via the below options.
We are happy to carry out free of charge samples to assist you to prove the process and the benefits to your company.
Original Article published on LinkedIn by Rob Ronan (UK/Ireland Sales & Support Manager)
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