The term no-clean can sometimes create the impression that cleaning is unnecessary after soldering. In practice, the decision is more complex.
Modern no-clean solder pastes are formulated to leave relatively low levels of residue after reflow. For many applications, these residues may remain on the assembly without affecting its intended performance. However, no-clean does not mean residue-free, and the suitability of leaving these residues in place depends on the product, its operating environment and the reliability requirements of the application.
As electronic assemblies become denser and are increasingly used in demanding environments, manufacturers are paying closer attention to PCBA cleanliness, ionic contamination and flux residue removal as part of their production process.
Why can no-clean flux residues become an issue?
A PCBA assembled using no-clean solder paste may appear visually clean and operate correctly during initial testing. Visual appearance alone, however, does not provide a complete indication of surface cleanliness.
After soldering, residues can remain around component terminations, beneath components and across the PCB surface. Depending on the soldering process and chemistry used, these may include flux residues, activator compounds and ionic contaminants.
Their presence does not automatically represent a reliability problem. The risk depends on factors including:
- the type and quantity of residue;
- spacing between conductors;
- operating voltage;
- temperature and humidity;
- condensation;
- product lifetime;
- the intended operating environment.
Under certain combinations of contamination, moisture and electrical bias, ionic residues can contribute to mechanisms such as electrochemical migration and corrosion.
This makes cleaning particularly relevant for assemblies intended for applications where long-term electrical reliability or environmental resistance is required.
When should manufacturers consider PCBA cleaning?
There is no single cleaning strategy that applies to every electronic assembly.
A manufacturer producing electronics for a controlled indoor environment may have very different requirements from one producing assemblies exposed to humidity, temperature cycling or industrial contaminants.
PCBA cleaning may therefore be considered when:
- cleanliness specifications must be achieved;
- ionic contamination needs to be controlled;
- products will operate in humid or harsh environments;
- fine-pitch or low-standoff components are used;
- the assembly requires conformal coating;
- downstream bonding, coating or encapsulation processes depend on surface condition;
- reliability testing identifies contamination as a potential process variable.
The objective is not simply to make the PCB look clean. Cleaning must form part of a controlled and repeatable manufacturing process.
Cleaning before conformal coating
The relationship between PCBA cleaning and conformal coating is particularly important.
Conformal coatings are applied to electronic assemblies to provide additional protection against environmental influences such as moisture, dust and chemicals. Their effectiveness nevertheless depends partly on the condition of the surface onto which they are applied.
Applying conformal coating over unsuitable residues does not remove the contamination. Instead, residues may remain trapped beneath the coating.
Surface contamination can potentially affect:
- coating adhesion;
- coating uniformity;
- long-term protection;
- inspection;
- rework;
- overall assembly reliability.
For this reason, manufacturers should evaluate the cleanliness of the PCBA before the conformal coating process, rather than relying on the coating itself to compensate for contamination.
A controlled cleaning process can also provide a more consistent surface for subsequent coating operations.
Decotron® 417XP for PCBA cleaning
DCT has developed Decotron® 417XP for electronics cleaning applications including the removal of residues associated with modern soldering processes.
The cleaning chemistry is intended for the removal of flux residues and other process contamination remaining on PCBAs after soldering.
Depending on the application and cleaning process, Decotron® 417XP can support manufacturers seeking to:
- remove no-clean flux residues;
- reduce ionic contamination;
- improve surface cleanliness;
- prepare assemblies for conformal coating;
- achieve more consistent cleaning results;
- meet internal cleanliness and process requirements.
The chemistry is designed to be used as part of a controlled electronics cleaning process rather than as an isolated production step.
Chemistry and cleaning equipment must work together
Selecting a cleaning agent is only one element of an effective PCB and PCBA cleaning process.
Cleaning performance also depends on factors such as process temperature, concentration, mechanical action, rinsing, drying, cycle time and the geometry of the assembly.
Low-clearance components, complex PCB layouts and modern flux formulations can make residue removal more difficult, particularly when contamination is located beneath components.
For this reason, DCT combines cleaning chemistries with equipment for applications including:
- PCBA cleaning;
- PCB cleaning;
- stencil cleaning.
Its cleaning systems use stainless-steel construction and are designed to provide controlled process conditions for electronics manufacturing environments.
Combining the chemistry, equipment and process parameters makes it possible to optimise cleaning according to the assembly and the required cleanliness level.
Developing a repeatable PCBA cleaning process
Introducing cleaning into a manufacturing line should normally begin with an assessment of the assembly and its production requirements.
Typical considerations include the solder paste or flux chemistry being used, component geometry, material compatibility, production volume, cleanliness targets and downstream processes.
Cleaning trials can then be used to establish suitable parameters and evaluate the results before the process is transferred into production.
This approach is also relevant when an existing cleaning process needs to be reviewed following a change in solder chemistry, PCB design, component density or coating requirements.
PCBA cleaning as part of process control
For manufacturers working with no-clean materials, the question is not simply whether a solder paste is labelled no-clean.
The more useful question is whether the level and type of residue remaining on the finished assembly are compatible with its manufacturing process, operating environment and reliability requirements.
Where cleaning is required, controlling both cleaning chemistry and equipment parameters can help manufacturers achieve consistent PCB and PCBA cleanliness while preparing assemblies for subsequent processes such as conformal coating.
Accelonix supports DCT cleaning equipment and chemistries for electronics manufacturing applications in the UK and Ireland.
Manufacturers wishing to assess their own PCBAs can also carry out cleaning trials at the Accelonix Technology Showcase & Training Centre, using representative assemblies to evaluate the process under practical conditions.
