Cleaning under BTC components on RF and Microwave PCBS
The general consensus nowadays is that PCBs need to be clean, but for RF or Microwave enabled circuits, this becomes even more crucial due to the complexity of the design, the fact that they tend to be heavily populated and the kind of components used on RF assemblies.
One type of component widely featured on these PCBs are BTC components including QFNs, LGAs, LLCs. These components offer a vast range of functionality and small package sizes that appeal to RF board designers.
These components do have flaws though, they are at higher risk of voiding and flux entrapment, the design means that there is less options for the solder paste to outgas at reflow, thus leading to higher risk of voiding. Some of the potential for voiding can be avoided with the correct PCB footprint design, via type/positioning, reflow profile and also with help from the ever improving solder pastes now available.
But, the issues with these components don’t finish after solder reflow and X-Ray inspection. When the boards are cleaned there is a higher possibility of flux residue remaining underneath the component due to the low profile, there is also the risk of a mixture of the cleaning chemistry and flux remaining trapped. I have first-hand experience of what can happen in this situation if those two materials “don’t play well together" and the effects can be catastrophic.
And when you add in the inherent sensitivity of RF and Microwave assemblies, any slight change in the surface resistivity or ionic contamination can lead to high failure rates and large “bone piles” building up in your test areas.
Retronix offers a solution for the flux entrapment issue by laser attaching solder balls to the pads of your BTC components these balls will not collapse and will maintain their height to increase the standoff from the board, allowing for easier access for the cleaning medium to flow under rather than getting trapped under. This can also offer benefits for CTE (coefficient of thermal expansion) issues and heat dissipation.
Retronix offer three options for these solder balls – HMP solder, Copper cored or Plastic cored, all have their own appeal dependent on the end function of the PCBs, for RF assemblies you would not want to be introducing extra copper to the PCB and you may not have ROHS exemption, so the plastic cored balls are suited best for for this situation.
For more information on this service, or any other Retronix services please contact me – email@example.com
Images show HMP solder balls attached to a CLCC.
Image shows the height difference where the component has been raised off the PCB. The dark areas in the solder joint are the HMP balls which have not collapsed and maintain their height. Cycles to failure increased by 10 times - study carried out by BAES Systems.