Why Clean ‘No Clean’ Flux Residues
Why Clean ‘No Clean’ Flux Residues
For best in class reliability, particularly if the reliability of an assembly is critical, the PCBA
should be be cleaned. Not only to remove flux residues from the PCB surface and solder joints, but also from underneath all the components. Every Manufacturer of PCBA’s has
their own commercial parameters for a specific job, often specified by the end customer.
The assembler should decide early whether to clean, and if so adapt the various processes
available to achieve an assembly that is clean enough to indicate very low levels of ionic
contamination when tested in an Ionograph system. This is the key quality control par-ameter. The level of ionic contamination remains the best indicator of likely long term
reliability of an assembly. One good starting point for making the decision of whether to
clean or not, is to Ionograph test an uncleaned assembly. If significant ionic contamination is
indicated by the test, then cleaning should be considered.
What Contaminants are being removed by cleaning
The contaminants on a PCBA are principally organic in nature, ie natural and /or synthetic
resins, ions, Organic and in some cases small amounts of Halidic acids, solder balls, finger
prints and substrate particulates. The move to lead free alloys has required significant
changes in fluxes and solder pastes to support the necessary higher process temperatures.
The fluxes are usually more active to respond to higher reflow temperatures, which also
presents increased risks. Residues may remain after soldering which have not correctly
decomposed , and can therefore initiate corrosion. If the assembly is to be considered
correctly cleaned, no flux residues or acids, no finger prints, particulates, dust, or cleaning
chemistry residues must remain, including under all components. At the same time, the
whole assembly, including, components, any codings, underfills, adhesive, and the substrate
must not be damaged by the cleaning process or any cleaning chemistry used. The assembly
should be dried and ready for immediate use or test at the end of the cleaning cycle, and
any ink legends must remain untouched and legible.
Why clean ‘No Clean’ Flux residues from PCBA’s
current market statistics indicate that upwards of 70% of all PCBA’s are soldered using products specified as no clean types. This class of flux was developed of course to eliminate
the requirement for further cleaning after soldering. However many assemblers still require
the removal of no clean residues. This can be for a number of reasons. The end use appli-
cation of the PCBA can be the decisive factor. Very high impedence or precision analogue
circuits can have signals affected by the presence of flux residues, even low solids no clean
types. If correctly and consistently heated, the organic acids used in modern no clean fluxes
will completely decompose into harmless components. However any residues may still be
hygroscopic. If these residues absorb moisture from the ambient environment, leakages can
occur between signal pathways. Any flux residues which have been trapped or shielded in
such a way as to cause them not to reach the required temperature for decomposition will
remain a prime source of signal leakage and ultimately corrosion. The risk increases as the
assembly is put under power and over time. One particular source of this problem is flux
residue entrapment between a PCBA and a conveyer transport pallet during wave soldering
operations. Another reason for cleaning can be the requirement for ATE testing of the
finished assembly. No clean fluxes specified as ‘ATE Testable’ tend to leave softer residues to allow the test pins to penetrate and make electrical contact. These residues can accumulate on the test pins which can then in turn require cleaning between test cycles.
The harder residues of newer low solids no clean fluxes are an improvement, but can still
interfere with ATE signal transmission in many instances. It was the problems with ATE
testing which provided the first major requirement for cleaning of no clean fluxes. A further
requirement arises from the requirement to conformal coat PCBA’s. The residues can inhibit
correct adhesion of conformal coatings and may absorb ambient moisture during the manu-
facturing cycle. If the conformal coating is subject to a subsequent curing process, the absorbed moisture can be released causing localized areas of unsatisfactory adhesion and
even separation of the coating from the board. This can allow corrosive materials, conductive particulate or additional moisture to gain penetration into the assembly causing
failures in service,signal transmission problems or component failure. Therefore it is import-
ant to resolve issues concerned with the choice of no clean solder materials and subsequent specification of the cleaning process early in the product definition, particularly with high
reliability assemblies. The correct response can prevent premature PCBA failures in critical
service environments such as defence, aerospace and medical applications.