Quality Assurance for Water Soluble Flux

A move toward eco-friendly industrial systems worldwide has resulted in an increased use of water soluble and no-clean fluxes in the commercial sector.  This trend is nearly twenty years old and is beginning to affect hardware supplied to NASA for its missions.  Though some suppliers may have long-standing experience with these fluxes and have “qualified” them for their own use, these materials continue to be considered non-standard for use in NASA mission hardware.  Suppliers wishing to use ORxx flux (reference IPC J-STD-004) in NASA mission hardware must meet the requirements for non-standard flux as defined in the NASA Workmanship Standards NASA-STD-8739.2 or NASA-STD-8739.3.  ORxx flux may be present in liquid form, in solder paste, or in cored wire solder.

Note:  Qualification tests used by a supplier for establishing a flux’s successful use in a manufacturing line may not be equivalent to the qualification tests NASA seeks which demonstrate successful performance over mission application conditions and lifetime.

Examination of suppliers’ assemblies built with ORxx flux has found two defects which must be addressed for this material:

a.  Voiding.  Some suppliers’ use of ORxx flux has resulted in consistent voiding in the solder joints.  Voiding in excess of 10% of the joint area (as viewed by X-ray) should be avoided via process control and monitoring with X-ray.  Where voiding is normally above 10%, vibration and thermal cycling life testing should be performed to establish that the typical voiding amount will result in sufficiently reliable joints (i.e. successful performance over mission application conditions and lifetime).  In all cases voiding should be baselined for the process to be used and controlled through statistical process control methods or 100% end-of-line inspection (X-ray screening).

b.  Weak Organic Acid Remnants.  Water soluble flux is effective via a combination of weak organic acids and halides.  Several ORxx fluxes examined to date have very low, to imperceptible, amounts of remnant weak organic acids and halides after normal printed wiring assembly cleaning.  Cleanliness methods which use conductivity measurements of cleaning fluid (such as those defined in NASA-STD-8739.2 and .3 for LOxx fluxes) will not detect the presence of weak organic acids however they will detect remnant halides.  Remnant halides are an important player in the development of unwanted leakage paths and corrosion on finished printed wiring assemblies.  However, unactivated and remnant weak organic acids, and the water that it readily retains, will also contribute to leakage paths, salt deposits, and corrosion, if stray halides (introduced during handling, rework, packaging) come in contact with the cleaned assembly.  It is important to baseline and monitor the cleaning process’ effectiveness for removing weak organic acids as well as for removing halides.  Ion Chromatography is the recommended method for this quality monitor (see IPC J-STD-001 for the test method).  Precleaning bare printed circuit boards is also recommended to further reduce sources of stray halide contamination.

NASA-STD-8739.2 and .3 require the following for approving use of non-standard flux:

4.1.3 Nonstandard Processes, Materials, or Parts. When the supplier intends to use processes, materials, or parts not covered by this publication, the supplier shall document the details of fabrication and inspection, including acceptance and rejection criteria, and shall provide appropriate test data (Requirement). Such documentation shall be approved by the procuring NASA Center prior to use (Requirement).

This paragraph is intended to emphasize that each type of requirement contained in the standard which is applicable to LOxx flux, also applies to ORxx flux.  However for ORxx flux, the test methods, inspection criteria, and pass/fail limits must be defined by the user because the details given in the standard for LOxx flux may not be technically apt for ORxx flux.  As such, users of ORxx flux must declare the inspection methods and values that will be used to provide quality assurance for ORxx flux.

Appropriate test data is data that indicates that the material can be controlled in the manufacturing process in ways that minimize scrap (i.e. high quality) and achieve the required reliability level (i.e. performance over mission application conditions and lifetime).  As noted above, process qualification data will normally address manufacturability but not application performance reliability.

Appropriate test data will baseline voiding %, ongoing voiding control limits, and the supplier's success in maintaining those limits.  It will also include baseline Ion Chromatography values for cleaned and uncleaned printed wiring assemblies.  Data submitted against this requirement must be calibrated against known good and known defective units.  Data showing “all passing” may indicate that the wrong tests were used. 

6.13 Flux

6.13.1 Types and Usage . Process documentation shall describe the types of fluxes, where each is used, and the necessary precautions (Requirement).

6.13.2 Rosin Flux. Rosin flux shall conform to ANSI/J-STD-004, Type L0, L1, or equivalent (Requirement). Rosin flux types R or RMA in accordance with the requirements of the former military specification, MIL-F-14256 (cancelled June 15, 1995), are considered equivalent to ANSI/J-STD-004, Types L0 or L1, respectively. For all fluxing applications where adequate subsequent cleaning is not practical, only rosin flux Type L0 (Type R of MIL-F-14256) shall be used (Requirement). Liquid flux used with flux-cored solder shall be chemically compatible with the solder core flux and with the materials with which it will come in contact (Requirement).

6.13.3 Variations. The use of any other flux compositions and forms (other than those listed in paragraph 6.13.2) shall require the approval of the procuring supplier (Requirement). The request for approval shall include the following information as a minimum (Requirement):

                a. A complete chemical characterization of each flux.

                b. A detailed control system for procurement, receiving inspection, storage, usage, and application.

                c. Detailed flux removal cleaning processes, monitoring requirements, cleanliness test methods, and their results.

To address a. above, an Ion Chromatography test of an uncleaned sample (reflowed solder/flux) is recommended. Tests of un-activated flux may not be perceptive to bound halides.

To address b. above, a description of the supplier’s cleaning practices which prevent introduction of halides during rework, handling, packaging, and storage prior to conformal coating, should be sought.  Also, the supplier needs to describe how they segregate the ORxx flux from the L0/L1 types.

C. above is a duplicate of the requirement in 4.1.3.

7.4.2  The PWB’s shall be cleaned and demoisturized prior to soldering (Requirement).

It is critical that PWBs are cleaned prior to soldering.  Industry has reported that weak organic acid remnants will react with halides on “dirty” boards and parts, creating metallic salts that cannot be removed.

12.5.3  Solderability.  All material to be soldered shall be verified as solderable prior to use (Requirement).

A NASA project found voiding percent lowered significantly when “flight grade” parts were used.  It is believed that the solderability quality of the parts’ terminations was the variable affecting voiding in this case.