Clean “no-clean,” or use a water wash solder paste?
Clean “no-clean,” or use a water wash solder paste?
Gerjan Diepstraten, Cobar Europe B.V., Breda, Netherlands, and Tim Lawrence, Ph.D., Cobar/Balver Zinn, Balve, Germany
benefits include a reduced number of process steps, simpler qualification with no need to specify cleaning detail and reduced cost. For customers requiring
technology has become increasingly popular in electronics assembly2
Since the demise of CFCs as a cleaning option in the 1970s1
, “no clean” fluxing . The
the extra confidence that cleaning brings, the option of water wash has remained, with many products available. This approach allows the use of highly activated materials ideal for components with poor solderability and/or high thermal demand with no risk of failure in service. The clear demarcation
between these two strategies has blurred slightly over recent years, with some manufacturers seeking to combine the convenience of no-clean fluxing with the confidence of water wash by cleaning no-clean flux residue. This paper assesses the
wisdom of such an approach.
Keywords: No Clean Solder Paste, Water Wash Solder Paste
Factor
Cleaning temperature Cleaning time
Originally published in the Proceedings of the SMTA International Conference, Orlando, Florida, October 24-28, 2010.
Unit ºC
Min Concentration saponifier % Table 1. Layout of the design of experiment.
Level 1 35 5
DI-water only
Level 2 50 10
DI-water + 10% detergent
Level 3 65 20
DI-water + 20% detergent
Introduction Water wash and no-clean products (In addition to fluxing capability,) the key requirement of a water wash fluxing medium is that afterward, soldering residues must be removed by water wash, preferably without chemical additives. It is not necessary for all flux components to be water soluble, so long as this is conferred by the soldering process. A typical medium is based on water soluble resin activated by amine hydrohalides and organic acids, with suitable solvents and rheology modifiers. A typical no-clean fluxing medium
comprises rosin (often modified to improve color and reduce the likelihood of oxidation), other components to complement activity (some of these may be similar to those in the water wash medium), corrosion inhibitors, solvents and gelling agents. The central element is rosin. Its physicochemical properties are ideal. During the reflow process, a viscous liquid is formed that acts as a robust activator. When reflow is complete, it solidifies, encapsulating products from the fluxing process and un-reacted flux components. Because it is a water insoluble dielectric, rosin becomes an in-situ conformal coating that protects underlying circuitry such as from high humidity. In contrast with water wash fluxing
mediums, there is no requirement that all flux residues are soluble in a particular solvent. In fact, given the wide range of materials used, from water soluble dicarboxylic acids and amine hydrohalides to water insoluble organic halogenated compounds and rosin, as well as various metal salts, oxides and hydroxides formed during the soldering process, this is highly demanding. No-clean products are formulated to be no-clean. Product validation (e.g. SIR and electrochemical migration) is performed based on this understanding.
Cleaning options Saponification is a ubiquitous and long- standing cleaning method. A saponifier is an alkaline material that reacts with the acidic functionality in contaminants to form a soap that is soluble, or at least dispersible, in water. In this form it is removed from the substrate. Outside the world of electronics, saponifiers form the basis of many household and industrial cleaning systems, such as dishwasher detergent. The primary target in electronics is rosinous flux residue where the saponifier reacts with the acidic moiety to form rosin soap. A similar mechanism will remove un-reacted carboxylic acid. As the saponifier is delivered in the form
“Because it is a water insoluble dielectric, rosin becomes an in-situ conformal coating that protects underlying circuitry such as from high humidity.”
10 – Global SMT & Packaging – Celebrating 10 Years – December 2010
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