Why Clean "No Clean" Flux?
Since the implementation of Montreal Protocol, no clean flux has been widely applied by most electronics manufacturers. However, Printed Circuit Board Assembly (PCBA) cleaning is only carried out on high-reliability boards like military, aerospace and healthcare. As electronic products tend to become miniaturized and functionally multiple, I/O count consistently rises, spacing between components becomes increasingly small and field application field and natural environment for their applications, reliability of no clean flux has been seriously challenged. The reason for that lies in the fact that during no clean process, partial flux unavoidably can't go through sufficiently reflow or wave high temperature so that flux doesn't go through the high temperature making it polymerized and it still behaves as synthetic resin. As a result, flux features acidity and tends to arouse erosion on components and PCB solder joints, which is why verdigris can be often seen on connector pins, PCB gold finger or positioning screw holes. Flux usually goes to the area without solder paste covered as a result of capillary action or wettability of flux or it unavoidably expands to the area outside solder joints.
In addition, surface ion residues usually form into dendritic crystalloid under moisture environment, which usually causes shortcuts resulting from electrical migration. Or, flux contains solder balls that are dissociated from solder paste and those balls are difficult to be eliminated without cleaning, which is a big trouble for small spacing. Therefore, it's necessary to carry out cleaning on no clean flux for final PCBA products.
What is "No Clean" Flux Cleaning for?
The aims of cleaning no clean flux include:
• To eliminate PCBA flux residue solder paste, solder balls, impurities, dust, oil stain etc.;
• To stop erosion and electrostatic damage from taking place;
• To prolong electronic products' life time;
• To better meet customers' requirement in terms of high quality and high reliability.
Is Traditional Cleaning Still Reliable?
Up to now, some people must hold doubt that now that no clean flux isn't reliable, why not go back to traditional water-soluble flux or solvent-based flux? The answer is definitely NO. With cleaning cost put aside first, reliability and environmental protection issues should be emphasically considered. Let's come to environmental protection issue first. Traditional water-soluble flux will generate large amount of waste water discharge, which is a big headache. As environmental protection awareness and laws become protruding, waste water can never be directly discharged unless it goes through scientific treatment. Solvent-based flux performs even worse than water-soluble flux.
Next comes cleaning effect issues. As components become miniaturized and micro components are widely applied including BGA, CSP and QFN, increasingly small spacing and decreasing spacing between components and PCB board bring forward difficulty and trouble for cleaning effect. Traditional cleaning can never meet corresponding demands. Moreover, residues caused by traditional clean flux can never be accepted so that a type of highly-effective and safe solution becomes especially important and urgent.
What is the Biggest Challenge of Cleaning?
As PCB components suffer from a tendency of increasingly higher density and miniaturization, no clean technology seems not working any more. High density limits component spacing with higher electrical field generated. As a result, cleaning has become the only practical scheme meeting the requirement of the most advanced technology while flux residue elimination will be extremely difficult under components such as flip chip, QFN, Micro BGA and small chip resistance-capacitance components. Lead-free process makes flux have to contain higher content of resin or rosin so that flux will be easily accumulated within small space and cleaning agent will be more difficult to penetrate. Worse still, higher lead-free soldering temperature and multi-time wave soldering make flux residue cleaning more difficult as well.
Based on the above discussion, it's increasingly more difficult for electronic assembled products to go through cleaning, which then calls for cleaning material with better cleaning effect to solve cleaning issues in terms of cleaning-hard or anti-cleaning products, undoubtedly making cleaning material and cleaning technology condition a huge challenge. As a result, cleaning agent has to go after chemical innovation and cleaning equipment and cleaning technology have to go over consistent improvement.
What about New Cleaning Agent?
Thanks to scientific development, we come to a better solution. A new type of water-base cleaning agent is capable of eliminating contaminants from surface with the application of micro-phase technology based on peeling principle.
Compared with traditional cleaning agent, this type of new cleaning agent features the following merits:
• Longer life time. It won't cause shorter life time as a result of constantly reduced effective elements due to permanent combination between surface active agent and contaminants. It can be reused after simple sediment and filtration.
• Safe and reliable. Reliable concern won't be aroused due to surface active agent's cleaning contaminants.
• Low cost. This type of cleaning agent is durable with no need of effluent disposal.
How is Cleaning Technology Going?
With new cleaning agent applied, cleaning process will become easier and more convenient. Next comes cleaning technology. Cleaning technology is determined by the following elements:
• Property of cleaning agent
• Cleaning capability of cleaning equipment
• Selection of flux
• Compatibility between substrate material and cleaning agent
a. Property of cleaning agent
(a) Compatible with flux residue type and capable of effectively cleaning flux residues on the surface without pollution caused.
(b) Compatible with cleaning base and component material.
(c) Lower surface tension and capable of effectively penetrating.
(d) Compatible with corresponding regulations such as RoHS etc.
(e) Excellent stability, anti-oxidation, excellent solubility and generating no foam.
(f) Suitable solubility.
(g) Agreeable temperature.
(h) High cleaning speed.
b. Property of cleaning agent
(a) Providing sufficient mechanical energy that is called for total cleaning and covering the most difficult and sensitive area and targets.
(b) Providing suitable cleaning time ensuring smooth cleaning.
(c) Cleaning temperature has to be controlled within a suitable range.
(d) Cleaning agent concentration should be accurately controlled to provide stability of the process. (e) Containing automatic drying system.
(f) Meeting the requirement of all kinds of cleaning agents.
(h) Powerful and user-friendly.
(i) Capable of being recycled.
c. Selection of flux
Some people might ask "why we have to reevaluate flux easy for cleaning now that we should pick up cleaning agent that cleans off flux?" Actually, cleaning process is so complex that sometimes minority of flux types can't achieve effective cleaning agent. If fully compatible with soldering requirement, sometimes it's simple and smart to select flux matching cleaning agent. When it comes to particular products, there are not so many cleaning agents and it seldom occurs that flux is selected for optimal cleaning effect. Leading aspects that should be taken into consideration when flux is selected include:
(a) Soldering is optimal or not.
(b) Cleaning is complete or not.
(c) Cleaning efficiency is high or not.
(d) Comprehensive cost is the lowest or not.
How are Substrate Material and Cleaning Agent Compatible?
Because cleaning agent is selected not only just taking the capability into consideration whether it will well clean off flux residues, but compatibility with substrate material as well. Process engineers must carefully evaluate and consider whether cleaning agent will lead to erosion on metal, plastics, black plating film, marks, coating, label, adhesion etc.
In conclusion, excellent cleaning has to be able to meet the following demands: suitable cleaning agent, professional cleaning equipment, agreeable technology parameters, low cleaning cost plus optimal cleaning effect.