© 2004 - 2008 Dr. Craig D. Hillman PCBA Cleanliness Guidelines.

© 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

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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

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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

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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

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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

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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

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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

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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

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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

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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)

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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

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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

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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

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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

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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

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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

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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

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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)

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Current SIR Test Standards

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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)

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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

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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

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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

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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

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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

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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

© 2004 - 2008

Any questions?

Dr. Craig Hillman: [email protected], 301-474-0607 (Main Office)

© 2004 - 2008 Dr. Craig D. Hillman PCBA Cleanliness Guidelines.

Documents

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