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PCBA Cleanliness Guidelines

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Presentation on theme: "PCBA Cleanliness Guidelines"— Presentation transcript:

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)

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