Acid value testing is a key incoming quality control step for wave soldering flux. It verifies that the free acid content of each lot is within the acceptable range before the material is released to production. This simple titration-based check helps prevent nonconforming flux from degrading solder wetting, increasing corrosive residue risk, or compromising product reliability.
What Is Acid Value?
In chemistry, acid value (also called acid number or neutralization number) is the number of milligrams of potassium hydroxide (KOH) needed to neutralize the free acids in 1 gram of a substance. For flux, the acid value quantifies the free carboxylic acid content arising from activators and related components. It provides a practical, quantitative indicator of a flux's activity and potential corrosivity.
Because flux performance depends on careful balance—enough activity to remove oxides and promote wetting, but not so much residual acidity that it leaves corrosive residues—acid value is an effective single metric to screen lot-to-lot consistency and fitness for use.
Why Perform Acid Value Testing?
Acid value testing is conducted as part of incoming inspection to ensure that only verified and conforming raw flux materials are put into production. By intercepting nonconforming lots, manufacturers avoid downstream risks such as:
- Poor solder wetting and inconsistent solderability during wave soldering.
- Excessive residues that may be corrosive or hygroscopic, threatening long-term reliability.
- Unstable process performance due to drift in flux activity over storage or after opening.
Screening each lot before release minimizes rework and field failure risks, and stabilizes wave soldering yields.
Sampling and Test Frequency
At receiving, the receiving team notifies the laboratory to sample and test. For each lot number, test one sampled container (e.g., one drum or pail). Record the measured acid value and communicate the result to the receiving function for release or disposition.
Test Procedure and Method
Required Tools and Reagents
Conduct the test with standard laboratory glassware, an analytical balance, and titration equipment. Prepare a standardized KOH titrant and an appropriate indicator suitable for acid–base titration of organic acids in the flux matrix. Use clean, dry glassware to avoid water or contaminant interference.
Step-by-Step Workflow
The following summarizes a typical acid value test workflow suitable for production labs:
- Thoroughly mix the flux container to ensure homogeneity, then draw a representative sample.
- Weigh a known mass of flux sample (m, in grams) into a clean, dry titration flask using an analytical balance.
- Add an appropriate solvent system to dissolve the sample and disperse the acids uniformly.
- Add a suitable indicator for the titration endpoint.
- Titrate the sample with standard KOH solution of known molarity (M, in mol/L), recording the volume consumed at the endpoint (V, in mL).
- Calculate the acid value using the formula provided in the next section.
- Record the result, compare it with the acceptance criteria, and report pass/fail status.
Result Calculation and Acceptance Criteria
A typical supplier-recommended acceptance range for flux acid value is 20 ± 5 mg KOH/g. Compare the calculated result to this range to determine pass/fail status for incoming inspection.
Use the following formula for calculation:
Acid value (mg KOH/g) = 56.11 × V × M / m
Where:
- V is the volume of KOH solution consumed at the endpoint, in mL.
- M is the molarity of the KOH titrant, in mol/L.
- m is the mass of the test sample, in g.
The constant 56.11 represents the molecular weight of KOH (g/mol). The formula yields the neutralization capacity expressed as milligrams of KOH per gram of flux. Ensure that the KOH titrant molarity M is known and traceable; if necessary, standardize the titrant periodically using an appropriate primary standard and document any corrections.
Precautions and Good Laboratory Practice
- Sampling: Using a pipette, draw 10 mL of the flux (e.g., SM-816) from the drum into a sample tube. Mix the drum beforehand to avoid phase separation or stratification.
- Analytical balance: Treat the balance as a precision instrument. Do not exceed its maximum capacity (e.g., 200 g). Calibrate and tare correctly before weighing the sample.
- Nonconforming results: If the acid value is out of the acceptance range, immediately notify the receiving function and quarantine the lot. Request supplier replacement or corrective action per internal procedures.
- Re-testing after opening: If a drum is opened and not consumed within 48 hours, repeat the acid value test before further use. Volatile components can evaporate, altering concentration and shifting acid value.
- Safety and ventilation: The flux and test reagents are volatile. Perform the entire test in a fume hood. Wear appropriate PPE, including chemical-resistant gloves and eye protection.
- Dry glassware: Before the test, ensure that flasks and beakers are dry and free of water droplets. Moisture can change the effective titration concentration and bias the result.
- Post-test cleaning: After each test, soak used flasks and beakers in detergent solution for approximately 2 hours, rinse with tap water or purified water, and air-dry completely.
Interpreting Results and Their Process Implications
Acid value serves as a proxy for flux activity and residue behavior:
- If the acid value is too low, the flux may lack sufficient activity to remove oxides effectively. In wave soldering, this can cause poor wetting, dull joints, or increased defect rates such as insufficient solder or non-wetting on copper pads and component leads.
- If the acid value is too high, the flux may be overly aggressive or leave excessive ionic residues, increasing the risk of post-soldering corrosion, leakage currents, or reliability issues. It can also complicate cleaning, if the process requires it.
When results fall outside the acceptance window, do not attempt to adjust the flux on-site without formal process approval. Instead, follow the procurement and quality procedures: block nonconforming lots, notify stakeholders, and work with the supplier on root cause and corrective actions.
Practical Controls for Stable Wave Soldering
Beyond one-time incoming inspection, the following practices help keep wave soldering performance stable across builds:
- Storage and handling: Store flux in closed containers per supplier recommendations. Minimize exposure after opening to reduce solvent evaporation and compositional drift that can alter acid value.
- Lot traceability: Link acid value records to production lots so that any downstream soldering or reliability issues can be traced back to material properties.
- Periodic verification: For long-running builds or when a drum remains in use beyond the initial period, schedule periodic acid value checks to detect drift.
- Documentation: Record the sample mass, titrant volume and molarity, calculated result, acceptance decision, and operator details. Consistent records support audits and process control analysis.
By integrating acid value testing with disciplined materials control, manufacturers tighten incoming quality gates and reduce variability in wave soldering outputs, supporting both assembly yield and long-term product reliability.
Summary
Acid value is a straightforward, quantitative metric that links directly to flux activity and residue risk. Implementing a simple titration-based test at incoming inspection—supported by clear acceptance criteria, proper lab practice, and good documentation—prevents nonconforming flux from entering production. The result is more consistent wave soldering performance, fewer defects, and improved reliability of assembled PCBs.
