To Kill A Circuit Board: Perils in Manual Soldering & Cleaning Processes
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Transcript of To Kill A Circuit Board: Perils in Manual Soldering & Cleaning Processes
To Kill A Circuit Board: Perils
in Manual Soldering &
Cleaning Processes
Cheryl Tulkoff, ASQ CRE
Senior Member of the Technical Staff
DfR Solutions
SMTAI 2014
Rosemont, Il
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Manual soldering and cleaning processes are among the least controlled
processes in printed circuit board assembly
As a result, they create special challenges to both quality and long term
reliability
This presentation describes some of those key challenges and provides ways to
address them in assembly to minimize problems. Topics to be covered include:
Design Considerations
General Manual Soldering Recommendations
Material Selection and Qualification Criteria
Cleaning Recommendations
Process Monitoring and Control
Relevant Industry Standards
Case Study
Through awareness of the issues, proper design and appropriate material
selection and process control, companies can successfully use manual
soldering and cleaning processes in high reliability products.
Without proper planning, however, these processes can lead to a path of ruin.
Abstract
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Surface finish & type
Pin Through Hole (PTH) Fill
Copper Dissolution
Component Selection &
Placement
Key Design Considerations
3
Surface Finish Challenges
Surface Finish Manual Soldering Challenges
ENIG (Electroless nickel ,
immersion gold) and
ENEPIG (electroless Pd added)
Slightly less solder spread,
incomplete pad coverage
Immersion silver (ImAg) None
Immersion tin (ImSn) None
Organic solderability preservative
(OSP)
Incomplete pad coverage, PTH hole
fill, copper dissolution
Pb-free HASL (SAC) Copper dissolution
Pb-free HASL (Silver-free) None
4
OSP Issues: Hole Fill
OSP has lower wetting force
Risk of insufficient hole fill
Solutions:
Changing board finish
Increasing top-side preheat
Increasing solder pot
temperature
Changing wave solder alloy
P. Biocca, Kester
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PTH Soldering: Incomplete Hole Fill o Poor solder hole fill can lead to solder joint cracks/failures
o Can be caused by: o Insufficient top side heating prevented solder from wicking up into PTH Barrel
o Insufficient flux or flux activity for the surface finish in use
o Lack of thermal relief for large copper planes
o PCB hole wall integrity issues – voids, plating, contamination
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PTH Hole Fill & Thermal Relief Utilize thermal reliefs on all copper planes when practical
Reduces thermal transfer rate between PTH and copper plane
Allows for easier solder joint formation during solder (especially for Pb-free)
Allows for better hole fill
Copper
Plane
PTH
Laminate
Copper
Spoke
Courtesy of D. Canfield (Excalibur Manufacturing) 7
Solders: Copper Dissolution
Reduction or elimination of
surface copper conductors
due to repeated exposure to
Sn-based solders
Significant concern for
industries that perform
extensive manual soldering
or rework
Bath, iNEMI
ENIG Plating
60 sec. exposure
274ºC solder fountain
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Solders: Copper Dissolution (cont.)
PTH knee is the point of
greatest plating reduction
Primarily a rework & repair
issue
Already having a
detrimental effect
S. Zweigart, Solectron
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Copper Dissolution & Contact Time
Contact time is the major driver
Some indications of a 25-30 second limit
A Study of Copper Dissolution During Pb-Free PTH Rework Using a
Thermally Massive Test Vehicle , C. Hamilton (May 2007)
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Component Selection Considerations
Lead Size
Lead Plating
Lead Type PTH
Blunt or blade
Tapered
Lead must protrude
Lead to hole ratio Rule of thumb for PTH fill
Hole needs to be .008” to .020” larger than the lead
Refer to IPC-2221 Industry Standard for Printed Board Design
http://www.ipcoutlook.org/mart/50190F.shtml 11
Impact of Component Plating on
Solderability Metal
Surfaces
Solderability
Platinum
Gold
Copper
Tin
Solder
Palladium
Silver
Easy to Solder
Nickel
Brass
Cadmium
Lead
Bronze
Rhodium
Beryllium
Copper
Less Easy to Solder
Nickel-Iron
Kovar
Difficult to Solder
Difficult to solder
platings drive use of
more aggressive
process chemistries
Increases risk of
residue issues
12
Component Spacing
Allow for adequate
spacing for manual
attach and repair
Avoid thermal damage
& thermal shock
Ceramic Cracks
Electrolytic caps
Solder mask & circuitry
Finetech BGA Attach
13
Ceramic Caps: Thermal Shock Cracks
o Due to excessive
change in temperature
o Maximum tensile stress
occurs near end of
termination
o Three manifestations
o Visually detectable
(rare)
o Electrically detectable
o Microcrack (worst-case)
NAMICS
AVX
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Thermal Shock Crack: Micro Crack
o Variations in voltage or
temperature drive crack
propagation
o Failures identified as o Increase in electrical resistance
o Decrease in capacitance
DfR
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Operator variation is the norm
Training is critical!
General manual soldering tips:
Use soldering irons with the greatest thermal
recovery
Use the largest tip for the size of the joint being
soldered and with the available working space
Use custom tips as needed
Use the largest cored solder wire diameter for the
size of the joint and available working space
Manual Soldering Recommendations
16
Solder Tip Size & Cored Wire Size
Recommendations
Images courtesy of OK International
The diagram below shows why No-Clean Flux-cored solder seldom works as well
as RMA-cored solder:
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Manual Soldering Accessories
Images show tools
which must be routinely
replaced or cleaned to
prevent cross-
contamination
Needle tip flux dispense
not recommended
Poor control over volume
& flow
Different solder alloys
should not use the same
materials
Needle tip flux dispenser
Solvent dispense bottle
Sponge
Tip Cleaner
Cleaning Brush
18
Use a portable preheater to shorten contact time and fully activate fluxes http://www.zeph.com/airbathseri
es.htm
Verify actual PCB and lead temperatures with small temp labels http://www.omega.com/toc_asp/
sectionSC.asp?section=F&book=temperature
Manual Soldering Accessories
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Solder preforms can be placed on a pad or on a PTH lead on the topside of the PCB
Repeatable joints on both PTH and SMT joints
Assist in getting complete PTH fill
Controls both volume of solder & volume of flux applied
Can be made in size, alloy, and flux of choice
Preforms for Soldering Uniformity
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Designed for manual soldering processes
SIR data available
Halogen / halide free
Supplier: relationships, proximity
Lead finish
Substrate finish
Lead free or SnPb soldering
Compatibility with adjacent materials Adhesives, conformal coatings, etc.
Manual Soldering Materials
Selection Criteria
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Avoid liquid flux in manual soldering if
possible but if it’s truly needed:
Look for methods to ensure precise delivery
Flux pens
Needle tip dispense bottles are not recommended
Avoid letting flux run under and around adjacent
components
Provide some form of uniform heating to volatalize
as much of the liquid as possible
Avoid Using Liquid Flux
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Process Material Qualification
Recommendations Validate compatibility and performance of all new
process materials using SIR testing
IPC-B-52 SIR TEST VEHICLE
IPC-A-52:Cleanliness and Residue Evaluation Test
Board – Single User CD-ROM
The IPC-B-52 test board is intended to be a
process qualification vehicle
Materials of construction and source of test boards
to be representative
https://portal.ipc.org/Purchase/ProductDetail.aspx?
Product_code=5e7a8626-b486-db11-a4eb-
005056875b22
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IPC-B-52 (IEC TB-57) The latest generation of test coupons
Main SIR Test Board
IC Test Coupon
Solder Mask Adhesion
SIR mini-coupons
Packages 0402 – 1206
QFP (no 0.4mm pitch)
SOICs and BGAs
Through-Hole Header
Comb patterns (5 mil)
Not specifically called out in any TM-650 test method 24
Typical manual cleaning process:
Solder fillets are cleaned following the solder operation
Some type of solvent spray is used to loosen flux residues
Followed by manual cleaning using IPA and a soft bristle
brush
This type of manual cleaning process represents a
reliability risk
Brushes are not routinely cleaned or maintained
Become contamination transfer mechanisms
Poorly removed residues are more likely to experience
corrosion failures than no clean flux residues left intact
In rework and repair, if you can’t rinse, you can’t clean!
Manual Removal of Flux Residues:
Not Recommended!
25
When to Clean? Very high reliability applications
Medical, Military, Avionics, Industrial, Telecom
Sensitive circuitry High-impedance circuits
High-frequency circuits
Operation in uncontrolled environments
Use of conformal coating Concerns over compatibility
Cleaning improves adhesion
Note: Some high-reliability markets have moved away from cleaning Automotive, Enterprise, etc.
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Industry Standards on PCBA
Cleaning Requirements driven by J-STD-001
Mandates 10ug/in2 (1.56 ug/cm2) NaCl equivalent conductivity for ROL0 or ROL1
Others based on limit established by user
Section 6.1
Assemblies should be cleaned after each soldering operation so that subsequent placement and soldering operations are not impaired by contamination
Section 8.2.2
Cleanliness testing is not required (unless specified by the customer)
SPC not required (testing should be random, but sample plan not provided)
If any assembly fails, the entire lot shall be evaluated and re-cleaned and a random sample of this lot and each lot cleaned since performing the last acceptable cleanliness test shall be tested
Some guidance provided by two handbooks
Guidelines for Cleaning of Printed Boards and Assemblies, IPC-CH-65B (2011)
Aqueous Post Solder Cleaning Handbook, IPC-AC-62A (1996) 27
If flux residues must be removed
manually, a 4 step process is
recommended:
Wet, scrub, rinse, dry
Use some form of dispensing system
to control the flow of the cleaning fluid
Material is fresh and pure each time.
Replace brushes routinely
Clean better, more consistently and with
less solvent
Manual Removal of Flux Residues
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Critical Aspects of PCBA
Cleaning Solder Paste / Flux Chemistry
Component Selection and Board Design
Equipment Batch vs. Inline
Critical equipment parameters
Cleaning Solution Includes solvent, chemistry, and temperature
Process Location When to clean?
Cleanliness Requirements and Assessment 29
Cleaning Equipment
Two options In-line
Batch (dishwasher)
Batch cleaning tends to dominate In-line primarily justified based on volumes
High-volume manufacturing tends to not clean
Ultrasonic Cleaning Rarely used on PCBAs
Restrictions in J-STD-001
Outdated knowledge from early process failures
30
Cleaning Process and Solution
Variables
Pure DI or Saponifier (type and concentration)
Temperature (120F to >175F)
Nozzle pressure and direction
Agitation (spray in air)
Time
31
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)
32
ROSE and Omega-Meter tests DO NOT detect
WOAs (weak organic acids)
Successfully passing these “cleanliness” tests
does not ensure cleanliness
ROSE and Omega-Meter are suitable for bare PCB
cleanliness testing and for finding halide residues
Ion Chromatography (IC) is only test that finds and
quantifies WOAs
No uniform/standard accept/reject limits
1/ Solvent Extraction Matrix Selection and its Potential Affects on Cleanliness Test Results, K. Sellers, J. Radman, Trace
Laboratories
Weak Organic Acid Detection
33
Case Study
PTH Ecaps Close to Processor
Insufficient PTH Hole Fill on Cu OSP
Excess Flux from Manual Soldering
34
EC RAT – Anyone tried this?
http://www.ec-rat.com/
Residues Rat for detection of unreacted
flux residues from the soldering process
of electronics production
35
Summary
Risks associated with manual soldering
can be reduced
Preparing for them early in the design phase
Optimize design practices
Select appropriate materials
Characterize materials, and
Monitor the process
These practices ensure robust results!
36
Presenter Biography
Cheryl has over 22 years of experience in electronics manufacturing focusing on failure analysis and reliability. She is passionate about applying her unique background to enable her clients to maximize and accelerate product design and development while saving time, managing resources, and improving customer satisfaction.
Throughout her career, Cheryl has had extensive training experience and is a published author and a senior member of both ASQ and IEEE. She views teaching as a two-way process that enables her to impart her knowledge on to others as well as reinforce her own understanding and ability to explain complex concepts through student interaction. A passionate advocate of continued learning, Cheryl has taught electronics workshops that introduced her to numerous fascinating companies, people, and cultures.
Cheryl has served as chairman of the IEEE Central Texas Women in Engineering and IEEE Accelerated Stress Testing and Reliability sections and is an ASQ Certified Reliability Engineer, an SMTA Speaker of Distinction and serves on ASQ, IPC and iNEMI committees.
Cheryl earned her Bachelor of Mechanical Engineering degree from Georgia Tech and is currently a student in the UT Austin Masters of Science in Technology Commercialization (MSTC) program. She was drawn to the MSTC program as an avenue that will allow her to acquire relevant and current business skills which, combined with her technical background, will serve as a springboard enabling her clients to succeed in introducing reliable, blockbuster products tailored to the best market segment.
In her free time, Cheryl loves to run! She’s had the good fortune to run everything from 5k’s to 100 milers including the Boston Marathon, the Tahoe Triple (three marathons in 3 days) and the nonstop Rocky Raccoon 100 miler. She also enjoys travel and has visited 46 US states and over 20 countries around the world. Cheryl combines these two passions in what she calls “running tourism” which lets her quickly get her bearings and see the sights in new places.
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