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© 2004 - 2008
Dr. Craig D. Hillman
PCBA Cleanliness Guidelines
© 2004 - 2007
5110 Roanoke Place, Suite 101, College Park, Maryland 20740Phone (301) 474-0607 Fax (240) 757-0053
www.DfRSolutions.com© 2004 - 2008 2
Outline
PCBA cleaning process details Cleanliness specifications & test methods Ionic contamination: acceptance levels Recommended fluxes and platings Product qualification guidelines Sources of contamination
© 2004 - 2007
5110 Roanoke Place, Suite 101, College Park, Maryland 20740Phone (301) 474-0607 Fax (240) 757-0053
www.DfRSolutions.com© 2004 - 2008 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
© 2004 - 2007
5110 Roanoke Place, Suite 101, College Park, Maryland 20740Phone (301) 474-0607 Fax (240) 757-0053
www.DfRSolutions.com© 2004 - 2008 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
© 2004 - 2007
5110 Roanoke Place, Suite 101, College Park, Maryland 20740Phone (301) 474-0607 Fax (240) 757-0053
www.DfRSolutions.com© 2004 - 2008 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
© 2004 - 2007
5110 Roanoke Place, Suite 101, College Park, Maryland 20740Phone (301) 474-0607 Fax (240) 757-0053
www.DfRSolutions.com© 2004 - 2008 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
© 2004 - 2007
5110 Roanoke Place, Suite 101, College Park, Maryland 20740Phone (301) 474-0607 Fax (240) 757-0053
www.DfRSolutions.com© 2004 - 2008 7
PCB Cleaning Process: Requirements Final rinse with deionized (DI) water
18 M 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
© 2004 - 2007
5110 Roanoke Place, Suite 101, College Park, Maryland 20740Phone (301) 474-0607 Fax (240) 757-0053
www.DfRSolutions.com© 2004 - 2008 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
© 2004 - 2007
5110 Roanoke Place, Suite 101, College Park, Maryland 20740Phone (301) 474-0607 Fax (240) 757-0053
www.DfRSolutions.com© 2004 - 2008 9
PCB Cleanliness Control: Test Procedures IPC-6012B specifies a Resistance of Solvent Extract
(ROSE) method Defined by IPC-TM-650 2.3.25
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
© 2004 - 2007
5110 Roanoke Place, Suite 101, College Park, Maryland 20740Phone (301) 474-0607 Fax (240) 757-0053
www.DfRSolutions.com© 2004 - 2008 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
Technique Technology Equivalency Factor
ROSE Static / Unheated 1
Omega-Meter Static / Heated ~1.5
Ionograph Dynamic / Heated ~2.0
Modified-ROSE, Zero-Ion, etc. Varied ~4.0 (?)
Ion Chromatography 80C for 1 hr ~4.0
© 2004 - 2007
5110 Roanoke Place, Suite 101, College Park, Maryland 20740Phone (301) 474-0607 Fax (240) 757-0053
www.DfRSolutions.com© 2004 - 2008 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)
© 2004 - 2007
5110 Roanoke Place, Suite 101, College Park, Maryland 20740Phone (301) 474-0607 Fax (240) 757-0053
www.DfRSolutions.com© 2004 - 2008 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
© 2004 - 2007
5110 Roanoke Place, Suite 101, College Park, Maryland 20740Phone (301) 474-0607 Fax (240) 757-0053
www.DfRSolutions.com© 2004 - 2008 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 g/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 2.3.28A
*Based on R/O/I testing
PaulsGeneral Electric
NDCEE DoD* IPC* ACI
Chloride (g/in2) 2 3.5 4.5 6.1 6.1 10
Bromide (g/in2) 20 10 15 7.8 7.8 15
© 2004 - 2007
5110 Roanoke Place, Suite 101, College Park, Maryland 20740Phone (301) 474-0607 Fax (240) 757-0053
www.DfRSolutions.com© 2004 - 2008 14
Major Appliance Manufacturer (IC) Incoming PCB Processed PCB
Contaminant Maximum Level
(ug/in2) Maximum Level
(ug/in2) Upper Control Limit
(ug/in2) Ammonium <0.5 <2
Bromide 3 10 8
Calcium <0.5 <1
Chloride 2.5 3.5 3
Fluoride <0.5 <1
Magnesium <0.5 <1
Nitrate <0.5 <2
Nitrite <0.5 <1
Phosphate <0.5 <1
Potassium <3 <3
Sodium <3 <3
Sulfate 3 3 2
Total 5 18 14
Weak Organic Compounds 200 200 50
© 2004 - 2007
5110 Roanoke Place, Suite 101, College Park, Maryland 20740Phone (301) 474-0607 Fax (240) 757-0053
www.DfRSolutions.com© 2004 - 2008 15
DfR Solutions IC Requirements Fluorides < 1 g/in2
Chlorides < 2 g/in2
Bromides < 10 g/in2
Nitrates, Sulfates< 2 – 4 g/in2
WOAs < 175 g/in2
Note: WOA spec may not be necessary depending upon flux used for HASL process
© 2004 - 2007
5110 Roanoke Place, Suite 101, College Park, Maryland 20740Phone (301) 474-0607 Fax (240) 757-0053
www.DfRSolutions.com© 2004 - 2008 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
© 2004 - 2007
5110 Roanoke Place, Suite 101, College Park, Maryland 20740Phone (301) 474-0607 Fax (240) 757-0053
www.DfRSolutions.com© 2004 - 2008 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
© 2004 - 2007
5110 Roanoke Place, Suite 101, College Park, Maryland 20740Phone (301) 474-0607 Fax (240) 757-0053
www.DfRSolutions.com© 2004 - 2008 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
© 2004 - 2007
5110 Roanoke Place, Suite 101, College Park, Maryland 20740Phone (301) 474-0607 Fax (240) 757-0053
www.DfRSolutions.com© 2004 - 2008 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)
© 2004 - 2007
5110 Roanoke Place, Suite 101, College Park, Maryland 20740Phone (301) 474-0607 Fax (240) 757-0053
www.DfRSolutions.com© 2004 - 2008 20
Current SIR Test Standards
© 2004 - 2007
5110 Roanoke Place, Suite 101, College Park, Maryland 20740Phone (301) 474-0607 Fax (240) 757-0053
www.DfRSolutions.com© 2004 - 2008 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)
© 2004 - 2007
5110 Roanoke Place, Suite 101, College Park, Maryland 20740Phone (301) 474-0607 Fax (240) 757-0053
www.DfRSolutions.com© 2004 - 2008 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
© 2004 - 2007
5110 Roanoke Place, Suite 101, College Park, Maryland 20740Phone (301) 474-0607 Fax (240) 757-0053
www.DfRSolutions.com© 2004 - 2008 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
© 2004 - 2007
5110 Roanoke Place, Suite 101, College Park, Maryland 20740Phone (301) 474-0607 Fax (240) 757-0053
www.DfRSolutions.com© 2004 - 2008 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
© 2004 - 2007
5110 Roanoke Place, Suite 101, College Park, Maryland 20740Phone (301) 474-0607 Fax (240) 757-0053
www.DfRSolutions.com© 2004 - 2008 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
© 2004 - 2007
5110 Roanoke Place, Suite 101, College Park, Maryland 20740Phone (301) 474-0607 Fax (240) 757-0053
www.DfRSolutions.com© 2004 - 2008 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
© 2004 - 2007
5110 Roanoke Place, Suite 101, College Park, Maryland 20740Phone (301) 474-0607 Fax (240) 757-0053
www.DfRSolutions.com© 2004 - 2008 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