PCBA Cleaning with Masking of a Non-Washable Component – Assel Case Study

A customer ordered PCB assembly for a high-frequency device, with a specification requiring the entire PCBA to be cleaned. However, one of the components could not come into contact with the cleaning agent. Here is how we set up a process that met both requirements in series production.

Flux residues and the performance of high-frequency circuits

In devices operating at high frequencies, flux residues and ionic contamination reduce surface insulation resistance (SIR) and increase losses on the laminate surface. Depending on the amount and type of residue, they can cause:

• a change in the effective dielectric constant, resulting in a frequency shift of resonant circuits and filter detuning (the effect increases with frequency),
• increased insertion loss and a change in trace impedance, usually small but significant with thicker residue layers,
• additional parasitic capacitance, especially when flux is trapped under components or shields,
• in the presence of moisture and voltage: reduced surface insulation resistance, electrochemical migration and corrosion.

For this reason, customers usually require flux removal after soldering for this class of devices.

Project requirements: full PCBA cleaning

The project involved two conflicting requirements:

• Cleaning the entire board. The customer’s specification required the assembled PCBA to be cleaned.
• A component with a non-sealed package. One of the components was not designed to be washed. The cleaning agent could get inside and damage it.

Skipping the cleaning step would have meant leaving flux on the board and risking degraded high-frequency signal performance. Cleaning without protection would have put the component at risk of damage.

PCBA cleaning process with component masking – implementation stages

Analysis of the component’s resistance to cleaning agents
We asked the manufacturer about the component’s design limitations, the resistance of its materials to cleaning agents, and recommended methods of protection.

Selection of a masking agent resistant to the cleaning process
We selected a masking agent that prevents the cleaning fluid from entering the component.

Process validation

Ionic cleanliness testing and transmission parameter measurements
We documented the procedure for applying and removing the mask, then ran a full cleaning cycle on test boards. We verified the results through testing and inspection.

The boards achieved the required level of ionic cleanliness. The component functioned correctly after cleaning, and the circuit’s transmission parameters were within the design specifications. The procedure was then introduced into series production.

Sources

M. Duffy i in., RF characterisation of no-clean solder flux residues, Proceedings of SPIE, vol. 4587, 2001 – research.ucc.ie

B. Olney, Beyond Design: The Impact of No-Clean Flux Residue on Signal Integrity, I-Connect007 – iconnect007.com

No-Clean Solder Paste Residue at RF Frequencies, IPC – electronics.org

Kyzen, Why Clean RF High-Frequency Hardware? – qgdigitalpublishing.com

S. Zhan, M. H. Azarian, M. Pecht, Reliability of Printed Circuit Boards Processed Using No-Clean Flux Technology in Temperature–Humidity–Bias Conditions, IEEE Transactions on Device and Materials Reliability, vol. 8, nr 2, 2008 – umd.edu

Fraunhofer ISIT, Surface resistance tests – isit.fraunhofer.de