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PCBA Cleanliness Guidelines
Dr. Craig D. Hillman
© 2004 - 2008
Outline
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PCBA cleaning process details
Cleanliness specifications & test methods
Ionic contamination: acceptance levels
Recommended fluxes and platings
Product qualification guidelines
Sources of contamination
© 2004 - 2008
2007
5110 Roanoke Place, Suite 101, College Park, Maryland 20740
Phone (301) 474-0607 Fax (240) 757-0053
www.DfRSolutions.com
2
Best Practices: Control & Measure
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Confirm incoming board cleanliness
Clean before solder mask application
Clean after soldering operations
Then measure:
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© 2004 - 2008
2007
Water quality going into process
Assembly cleanliness with ionograph
5110 Roanoke Place, Suite 101, College Park, Maryland 20740
Phone (301) 474-0607 Fax (240) 757-0053
www.DfRSolutions.com
3
PCBA Cleanliness: Overview
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Ensuring the cleanliness of printed circuit board
assemblies involves process and control
Process
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Cleaning must introduced at the appropriate locations
within the manufacturing process
Control
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© 2004 - 2008
2007
The effectiveness of the cleaning processes must be
validated through monitoring and measurement
5110 Roanoke Place, Suite 101, College Park, Maryland 20740
Phone (301) 474-0607 Fax (240) 757-0053
www.DfRSolutions.com
4
Printed Circuit Board Cleanliness
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The cleanliness of printed circuit boards (PCBs)
has become especially critical in recent years
due to
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© 2004 - 2008
2007
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
5110 Roanoke Place, Suite 101, College Park, Maryland 20740
Phone (301) 474-0607 Fax (240) 757-0053
www.DfRSolutions.com
5
PCB Cleaning: Process Flow
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At a minimum, PCB manufacturers should clean the
PCB:
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Immediately before the application of solder resist
Immediately after the application of any solderability plating
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HASL
Electroless Nickel and Immersion Gold
Immersion Tin
Immersion Silver
Some PCB manufacturers also perform a final clean
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© 2004 - 2008
2007
Should not substitute cleaning after solderability plating
Residues from plating operations can become more difficult
to remove with any time delay
5110 Roanoke Place, Suite 101, College Park, Maryland 20740
Phone (301) 474-0607 Fax (240) 757-0053
www.DfRSolutions.com
6
PCB Cleaning Process: Requirements
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Final rinse with deionized (DI) water
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Potential options
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18 MW is preferred
Distilled water is insufficient
‘City’ water is unacceptable
Use of saponifier during the cleaning process
Heated DI water is nice, but not absolutely necessary
Common problems
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© 2004 - 2008
2007
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
5110 Roanoke Place, Suite 101, College Park, Maryland 20740
Phone (301) 474-0607 Fax (240) 757-0053
www.DfRSolutions.com
7
PCB Cleanliness Control: Industry Specs
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IPC-6012B, Qualification and Performance
Specification for Rigid Printed Boards, Section 3.9
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Board cleanliness before solder resist shall not be
greater than 10 ug/in2 of NaCl equivalent (total
ionics)
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Requires confirmation of board cleanliness before solder
resist application
When specified, requires confirmation of board cleanliness
after solder resist or solderability plating
Based on military specifications from >30 years ago
Board cleanliness after solder resist shall meet the
requirements specified by the customer
© 2004 - 2008
2007
5110 Roanoke Place, Suite 101, College Park, Maryland 20740
Phone (301) 474-0607 Fax (240) 757-0053
www.DfRSolutions.com
8
PCB Cleanliness Control: Test Procedures
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IPC-6012B specifies a Resistance of Solvent Extract
(ROSE) method
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IPC-6012B specifies this measurement should be
performed on production boards every lot
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Defined by IPC-TM-650 2.3.25
Class 1 boards: Sampling Plan 6.5
Class 2 and 3 boards: Sample Plan 4.0
Sampling plan (example)
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© 2004 - 2008
2007
If a lot contains 500 panels of a Class 2 product, 11 panels
should be subjected to ROSE measurements for
cleanliness testing
5110 Roanoke Place, Suite 101, College Park, Maryland 20740
Phone (301) 474-0607 Fax (240) 757-0053
www.DfRSolutions.com
9
Test Procedures: Common Problems
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ROSE is the least sensitive of ionic measurement techniques
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Equipment is not calibrated
Insufficient volume of solution is used
Insufficient surface area
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5 ug/in2 detected by ROSE is equivalent to ~20 ug/in2 detected by ion
chromatography
Panels are preferred over single boards
Cut-outs are not considered when calculating surface area
Insufficient measurement time
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7 to 10 minutes is preferred
Technique
ROSE
Omega-Meter
Ionograph
Modified-ROSE, Zero-Ion, etc.
Ion Chromatography
© 2004 - 2008
2007
Technology
Equivalency Factor
Static / Unheated
1
Static / Heated
~1.5
Dynamic / Heated
~2.0
Varied
~4.0 (?)
80C for 1 hr
~4.0
5110 Roanoke Place, Suite 101, College Park, Maryland 20740
Phone (301) 474-0607 Fax (240) 757-0053
www.DfRSolutions.com
10
Test Procedures: Best Practice
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Ion Chromatography (IC) is the ‘gold standard’
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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
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© 2004 - 2008
2007
Perform lot qualification with R/O/I
Periodically recalibrate with IC (every week, month, or
quarter)
5110 Roanoke Place, Suite 101, College Park, Maryland 20740
Phone (301) 474-0607 Fax (240) 757-0053
www.DfRSolutions.com
11
PCB Cleanliness Control: Requirements
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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
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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 - 2008
2007
5110 Roanoke Place, Suite 101, College Park, Maryland 20740
Phone (301) 474-0607 Fax (240) 757-0053
www.DfRSolutions.com
12
Cleanliness Controls: Ion Chromatography
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Contamination tends to be controlled through industrial
specifications (IPC-6012, J-STD-001)
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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
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IPC-TM-650, Method 2.3.28A
Pauls
General
Electric
NDCEE
DoD*
IPC*
ACI
Chloride (mg/in2)
2
3.5
4.5
6.1
6.1
10
Bromide (mg/in2)
20
10
15
7.8
7.8
15
*Based on R/O/I testing
© 2004 - 2008
2007
5110 Roanoke Place, Suite 101, College Park, Maryland 20740
Phone (301) 474-0607 Fax (240) 757-0053
www.DfRSolutions.com
13
Major Appliance Manufacturer (IC)
Ammonium
Bromide
Calcium
Chloride
Incoming PCB
Maximum Level
2
(ug/in )
<0.5
3
<0.5
2.5
Fluoride
Magnesium
Nitrate
Nitrite
Phosphate
Potassium
<0.5
<0.5
<0.5
<0.5
<0.5
<3
<1
<1
<2
<1
<1
<3
Sodium
Sulfate
Total
<3
3
5
3
18
<3
2
14
Weak Organic Compounds
200
200
50
Contaminant
© 2004 - 2008
2007
Processed PCB
Maximum Level Upper Control Limit
2
2
(ug/in )
(ug/in )
<2
10
8
<1
3.5
3
5110 Roanoke Place, Suite 101, College Park, Maryland 20740
Phone (301) 474-0607 Fax (240) 757-0053
www.DfRSolutions.com
14
DfR Solutions IC Requirements
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Fluorides
Chlorides
Bromides
Nitrates, Sulfates
WOAs
< 1 mg/in2
< 2 mg/in2
< 10 mg/in2
< 2 – 4 mg/in2
< 175 mg/in2
Note: WOA spec may not be necessary
depending upon flux used for HASL process
© 2004 - 2008
2007
5110 Roanoke Place, Suite 101, College Park, Maryland 20740
Phone (301) 474-0607 Fax (240) 757-0053
www.DfRSolutions.com
15
Best Practices: Application Specific
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Indoor applications: controlled environment
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Use of no-clean fluxes often sufficient (see caveats)
Outdoor applications: uncontrolled
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Non-condensing (ex: telecom):
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Condensing (ex: military):
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© 2004 - 2008
2007
Use of more aggressive cleaning of boards rather than noclean flux
Use of conformal coatings
5110 Roanoke Place, Suite 101, College Park, Maryland 20740
Phone (301) 474-0607 Fax (240) 757-0053
www.DfRSolutions.com
16
Best Practices: Use of No-Clean Flux
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Generally good at eliminating assembly-induced
contamination
Caveats:
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Places a larger emphasis on cleaning of incoming
boards
Wave soldering and/or rework may result in:
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© 2004 - 2008
2007
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
5110 Roanoke Place, Suite 101, College Park, Maryland 20740
Phone (301) 474-0607 Fax (240) 757-0053
www.DfRSolutions.com
17
Flux Controls
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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
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© 2004 - 2008
2007
Requires test vehicle
5110 Roanoke Place, Suite 101, College Park, Maryland 20740
Phone (301) 474-0607 Fax (240) 757-0053
www.DfRSolutions.com
18
Flux Qualification
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Test vehicle requirements
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Fabricated from same material as production unit (board
and solder mask)
Minimum of two structures
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Smallest spacing at relevant voltage
Highest electric field at relevant spacing
Clean test vehicle before use
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© 2004 - 2008
2007
Designed to assess flux/solder mask interaction (not board
contamination)
5110 Roanoke Place, Suite 101, College Park, Maryland 20740
Phone (301) 474-0607 Fax (240) 757-0053
www.DfRSolutions.com
19
Current SIR Test Standards
© 2004 - 2008
2007
5110 Roanoke Place, Suite 101, College Park, Maryland 20740
Phone (301) 474-0607 Fax (240) 757-0053
www.DfRSolutions.com
20
Recommended Test Method
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Flux application and preconditioning
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Solder paste
Wave solder
Rework
Exposure to low temperature and maximum
humidity without condensation
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© 2004 - 2008
2007
35 to 40ºC
Minimum of 93%RH
72 to 120 hours of exposure
Continuous monitoring (1 second per reading)
5110 Roanoke Place, Suite 101, College Park, Maryland 20740
Phone (301) 474-0607 Fax (240) 757-0053
www.DfRSolutions.com
21
Product Qualification
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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 - 2008
2007
5110 Roanoke Place, Suite 101, College Park, Maryland 20740
Phone (301) 474-0607 Fax (240) 757-0053
www.DfRSolutions.com
22
Contamination: Sources
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Handling and storage
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Fingerprints: NaCl and organic acids
Dust from environment and packaging: ionic materials
Use environment
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Forced air circulation is a significant source
Gaseous: HCl and chlorine
Particulates (most significant):
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© 2004 - 2008
2007
Coarse (>1um): sulfate, ammonium, Ca, Mg, Na, Cl
Fine: sulfate & ammonium – careful filtration required
5110 Roanoke Place, Suite 101, College Park, Maryland 20740
Phone (301) 474-0607 Fax (240) 757-0053
www.DfRSolutions.com
23
Contamination: Sources
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Rework and Repair
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© 2004 - 2008
2007
High rework temperatures cause decomposition of
board materials and fluxes
Cleaning methods typically not as good as in-line
processes
5110 Roanoke Place, Suite 101, College Park, Maryland 20740
Phone (301) 474-0607 Fax (240) 757-0053
www.DfRSolutions.com
24
Plating Recommendations
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Except for immersion silver, selection of PCB
plating material should be independent of use
environment
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© 2004 - 2008
2007
Immersion silver has a tendency to corrode in high
sulfur environments, creating electrical shorts
5110 Roanoke Place, Suite 101, College Park, Maryland 20740
Phone (301) 474-0607 Fax (240) 757-0053
www.DfRSolutions.com
25
RoHS Cleanliness
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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
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© 2004 - 2008
2007
Caveat: If the PCBA is cleaned, cleaning procedures
may need to be modified
5110 Roanoke Place, Suite 101, College Park, Maryland 20740
Phone (301) 474-0607 Fax (240) 757-0053
www.DfRSolutions.com
26
Transition to No-Clean Flux
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The primary consideration in the transition to noclean flux in regards to cleanliness is the
additional focus on ensuring the PCB cleaning
process is effective and controlled
© 2004 - 2008
2007
5110 Roanoke Place, Suite 101, College Park, Maryland 20740
Phone (301) 474-0607 Fax (240) 757-0053
www.DfRSolutions.com
27
Any questions?

Dr. Craig Hillman: [email protected], 301-474-0607 (Main Office)
© 2004 - 2008
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