1. PCBA Cleanliness Guidelines
Dr. Craig D. Hillman

2. Outline PCBA cleaning process details
Cleanliness specifications & test methods
Ionic contamination: acceptance levels
Recommended fluxes and platings
Product qualification guidelines
Sources of contamination

3. Best Practices: Control & Measure
Confirm incoming board cleanliness
Clean before solder mask application
Clean after soldering operations
Then measure:
Water quality going into process
Assembly cleanliness with ionograph

4. PCBA Cleanliness: Overview
Ensuring the cleanliness of printed circuit board assemblies involves process and control
Process
Cleaning must introduced at the appropriate locations within the manufacturing process
Control
The effectiveness of the cleaning processes must be validated through monitoring and measurement

5. Printed Circuit Board Cleanliness
The cleanliness of printed circuit boards (PCBs) has become especially critical in recent years due to
Decreasing conductor spacings (increased risk of electrochemical migration)
Increased use of no-clean flux (the last cleaning operations are PCB fabrication)
Movement of PCB fabrication to low cost countries

6. PCB Cleaning: Process Flow
At a minimum, PCB manufacturers should clean the PCB:
Immediately before the application of solder resist
Immediately after the application of any solderability plating
HASL
Electroless Nickel and Immersion Gold
Immersion Tin
Immersion Silver
Some PCB manufacturers also perform a final clean
Should not substitute cleaning after solderability plating
Residues from plating operations can become more difficult to remove with any time delay

7. PCB Cleaning Process: Requirements
Final rinse with deionized (DI) water
18 MW is preferred
Distilled water is insufficient
‘City’ water is unacceptable
Potential options
Use of saponifier during the cleaning process
Heated DI water is nice, but not absolutely necessary
Common problems
DI water is only used if specified by the customer
DI water is turned off to reduce water and energy usage
Failure to monitor DI water at the source
Failure to alarm the DI water on the manufacturing floor

8. PCB Cleanliness Control: Industry Specs
IPC-6012B, Qualification and Performance Specification for Rigid Printed Boards, Section 3.9
Requires confirmation of board cleanliness before solder resist application
When specified, requires confirmation of board cleanliness after solder resist or solderability plating
Board cleanliness before solder resist shall not be greater than 10 ug/in2 of NaCl equivalent (total ionics)
Based on military specifications from >30 years ago
Board cleanliness after solder resist shall meet the requirements specified by the customer

9. PCB Cleanliness Control: Test Procedures
IPC-6012B specifies a Resistance of Solvent Extract (ROSE) method
Defined by IPC-TM
IPC-6012B specifies this measurement should be performed on production boards every lot
Class 1 boards: Sampling Plan 6.5
Class 2 and 3 boards: Sample Plan 4.0
Sampling plan (example)
If a lot contains 500 panels of a Class 2 product, 11 panels should be subjected to ROSE measurements for cleanliness testing

10. Test Procedures: Common Problems
ROSE is the least sensitive of ionic measurement techniques
5 ug/in2 detected by ROSE is equivalent to ~20 ug/in2 detected by ion chromatography
Equipment is not calibrated
Insufficient volume of solution is used
Insufficient surface area
Panels are preferred over single boards
Cut-outs are not considered when calculating surface area
Insufficient measurement time
7 to 10 minutes is preferred

11. Test Procedures: Best Practice
Ion Chromatography (IC) is the ‘gold standard’
Some, but very few, PCB manufacturers qualify lots based on IC results
Larger group uses IC to baseline ROSE / Omegameter / Ionograph (R/O/I) results
Perform lot qualification with R/O/I
Periodically recalibrate with IC (every week, month, or quarter)

12. PCB Cleanliness Control: Requirements
The majority of knowledgeable OEMs completely ignore IPC cleanliness requirements
Option 1: Requirements are based on R/O/I test results, but adjusted for lack of sensitivity
Most companies now specify 2.5 to 7 ug/in2
Option 2: Requirements are based on IC test results and then monitored using R/O/I

13. Cleanliness Controls: Ion Chromatography
Contamination tends to be controlled through industrial specifications (IPC-6012, J-STD-001)
Primarily based on original military specification
10 mg/in2 of NaCl ‘equivalent’
Calculated to result in 2 megaohm surface insulation resistance (SIR)
Not necessarily best practice
Best practice is contamination controlled through ion chromatography (IC) testing
IPC-TM-650, Method A
*Based on R/O/I testing

14. Major Appliance Manufacturer (IC)

15. DfR Solutions IC Requirements
Fluorides < 1 mg/in2
Chlorides < 2 mg/in2
Bromides < 10 mg/in2
Nitrates, Sulfates < 2 – 4 mg/in2
WOAs < 175 mg/in2
Note: WOA spec may not be necessary depending upon flux used for HASL process

16. Best Practices: Application Specific
Indoor applications: controlled environment
Use of no-clean fluxes often sufficient (see caveats)
Outdoor applications: uncontrolled
Non-condensing (ex: telecom):
Use of more aggressive cleaning of boards rather than no-clean flux
Condensing (ex: military):
Use of conformal coatings

17. Best Practices: Use of No-Clean Flux
Generally good at eliminating assembly-induced contamination
Caveats:
Places a larger emphasis on cleaning of incoming boards
Wave soldering and/or rework may result in:
Pooling of flux: heterogeneous contamination issues
Flux not being deactivated: resulting acids may cause oxidation and electro-chemical migration
Surface mount reflow rarely has such issues

18. Flux Controls Strong movement to no-halide, no-clean flux
How to ensure flux choice does not induce ECM?
Option 1: Attempt to characterize flux chemistry
Limited published literature
Option 2: Qualify the flux through testing
Requires test vehicle

19. Flux Qualification Test vehicle requirements
Fabricated from same material as production unit (board and solder mask)
Minimum of two structures
Smallest spacing at relevant voltage
Highest electric field at relevant spacing
Clean test vehicle before use
Designed to assess flux/solder mask interaction (not board contamination)

20. Current SIR Test Standards

21. Recommended Test Method
Flux application and preconditioning
Solder paste
Wave solder
Rework
Exposure to low temperature and maximum humidity without condensation
35 to 40ºC
Minimum of 93%RH
72 to 120 hours of exposure
Continuous monitoring (1 second per reading)

22. Product Qualification
Consider testing entire product, if resource- or time-limited
40ºC/93%RH for 72 to 120 hours
Extend time period if using conformal coating or potting material
Do not test at 85ºC/85%RH for dendritic growth (surface ECM)
Some issues with CAF as well
Study by Sohm and Ray (Bell Labs) demonstrated degradation of weak organic acid residues above ~55ºC
Reduces their effect on surface insulation resistance
Turbini (Georgia Tech) demonstrated breakdown of polyglycols at elevated temperature as well
Absorption into board can increase risk of CAF

23. Contamination: Sources
Handling and storage
Fingerprints: NaCl and organic acids
Dust from environment and packaging: ionic materials
Use environment
Forced air circulation is a significant source
Gaseous: HCl and chlorine
Particulates (most significant):
Coarse (>1um): sulfate, ammonium, Ca, Mg, Na, Cl
Fine: sulfate & ammonium – careful filtration required

24. Contamination: Sources
Rework and Repair
High rework temperatures cause decomposition of board materials and fluxes
Cleaning methods typically not as good as in-line processes

25. Plating Recommendations
Except for immersion silver, selection of PCB plating material should be independent of use environment
Immersion silver has a tendency to corrode in high sulfur environments, creating electrical shorts

26. RoHS Cleanliness
The cleanliness guidelines spelled out in this document, in regards to process and control, are not expected to change with the transition to a RoHS-compliant product
Caveat: If the PCBA is cleaned, cleaning procedures may need to be modified

27. Transition to No-Clean Flux
The primary consideration in the transition to no-clean flux in regards to cleanliness is the additional focus on ensuring the PCB cleaning process is effective and controlled

28. Any questions?
Dr. Craig Hillman: (Main Office)