Materials and Processes Division

1
ESA SME InitiativeCourse DMaterials
Critical processes related to electronic materials

• Ton de Rooij
• Principal Metallurgist
• Materials and Processes Division
• Product Assurance and Safety Department

2
Content of presentation

• specifications
• pcbs manufacturing
• soldering processes
• crimping
• wire wrapping
• Repair and modification of PCB assemblies
• cleaning
• quality evaluations
• good solderjoints
• failure modes

3
Specifications

• ECSS-Q-70-08A
• Space product assurance - manual soldering of
  high-reliability electrical connections
• ECSS-Q-70-10
• Space product assurance - qualification and
  procurement of multilayer printed circuit boards
  (gold plated or tin-lead finish) - to be
  published ( now ESA PSS-01-710)
• ESA PSS-01-718
• the preparation, assembly and mounting of RF
  coaxial cables
• ECSS-Q-70-26
• Space product assurance - the crimping of
  high-reliability electrical connections -to be
  published (now ESA PSS-01-726)

• ECSS-Q-70-28
• Space product assurance - repair and modification
  of printed circuit board assemblies - to be
  published (now ESA PSS-01-728)
• ECSS-Q-70-30A
• Space product assurance - wire-wrapping of
  high-reliability electrical connections
• ECSS-Q-70-38
• Space product assurance - high-reliability
  soldering for surface mount and mixed technology
  printed circuit boards - to be published (now ESA
  PSS-01-738)

4
PCB manufacturing

• Schematic representation of major steps in
  multilayer board fabrication
• laminate
• epoxy-impregnated fibreglas-laminated weave
• polyimide-quartz laminate
• ceramic laminate
• hole drilling
• cleaning and electroless and electrolytic copper
  plating
• photoresist laminating exposure and development
• tin-lead alloy electroplating
• photoresist removal
• etching
• reflow

5
Approved PCB manufcaturers

• BOSCH Telecom GmbH, Germany
• CIT (ALCATEL), France
• LABTECH Ltd, England
• PRINTCA AS, Denmark
• SEXTANT Avionique, France
• SPEMCO Group Ltd, England
• STRASCHU Leiterplatten GmbH, Germany
• SYSTRONIC S.A.L, France
• VIASYSTEMS, div. CSI, Italy

6
Soldering processes

• Soldering processes
• Solder flux
• solder alloys
• phase diagrams
• cleaning
• accept/reject criteria
• conformal coatings
• thermal cycling
• operator training and certification

7
Soldering processes, cont..

• Hand soldering
• qualification
• machine soldering
• verification and approval
• surface mounting
• verification and approval

8
Soldering processes, cont..

• solder flux
• rosin-based
• pretinning mildly activated (fully activated in
  cases of poor solderability)
• assembly pure rosin flux
• water-soluble acid flux
• only for pretinning when rosin-based fluxes are
  inadequate. (immediate cleaning after use is
  required)
• solder alloy
• 63 tin solder (63 Sn, 37 Pb)
• 62 tin silver-loaded solder (62 Sn, 2 Ag, 36 Pb)
• 60 tin solder (60 Sn, 40 Pb)
• 96 tin silver solder (96 Sn, 4 Ag)

9
Soldering processes, cont..
10
Soldering processes, cont..

• Cleaning
• solvents shall be non-corrosive and
  non-conductive and shall not degrade parts or
  materials.
• Acceptable solvents are
• ethyl alcohol (99.5 or 95 pure by volume)
• isopropyl alcohol (best commercial grade, 99
  pure)
• deionized water at 40 oC maximum for certain
  fluxes (dry after use)
• any mixture of the above

11
Principles of reliable soldered connections

• Reliable soldered connections result from proper
  design, control of tools, materials and work
  environments, and careful workmanship
• basic deign concepts
• stress relief shall be inherent in the design
• where adequate stress relief is not possible,
  solder-joint reinforcement is necessary
• materials shall be so selected that the mismatch
  of thermal expansion coefficient is minimal
• materials and processes which the formation of
  brittle intermetallic shall be avoided
• the design shall permit inspection of the
  soldered joints

12
Phase diagram of Ag-Sn

• Liquid
• ?Sn
• ? phase Ag3Sn ordered structure
• eutectic at 96.5 w Sn ( 95 m ?Sn with 5m ?
  phase according to lever rule)

13
Phase diagram of Sn-Pb

• Liquid
• ?Pb
• ?Sn
• Liquid ?Pb crystals of solid ?Pb dispersed in
  the liquid
• Liquid ?Sn crystals of solid ?Sn dispersed
  in the liquid
• solid fraction of ?Pb and ?Sn according to
  lever rule
• eutectic overall composition 62 Sn rem Pb.
  Melting point 183 oC (45m ?Pb and 55m ?Sn)

liquid
?Pb
Liquidsolid (?Pb)
Liquidsolid(?Sn)
Eutectic ?Pb ?Sn
solid
14
Phase diagram of Au-Sn

• Layers found in solder joints when soldering with
  a Sn-Pb alloy to gold
• gold
• AuSn
• AuSn2
• AuSn4
• high volume of AuSn4 needles
• gold dissolution into liquid solder followed by
  precipitation during cooling to RT

15
Acceptance criteria

• Clean, smooth, bright undisturbed surface
• solder fillets between conductor and termination
  areas as illustrated
• contour of wire sufficiently visible to determine
  presence of wire
• complete wetting as evidenced by a low contact
  angle
• proper amount and distribution of solder
• absence of defects as mentioned in the next sheet

16
Rejection criteria

• Charred, burned, or melted insulation of parts
• conductor pattern separation for board
• burns on base material
• discoloration which is continuous between two
  conductors
• excessive solder (peaks, bridging)
• flux residue, solder spatter, or other foreign
  matter
• dewetting
• insufficient solder
• pits, holes or voids, or exposed base metal in
  the solder connection
• granular or disturbed solder joints

• fractured or cracked solder connections
• cut, nicked, gouged, or scraped conductors or
  conductor pattern
• improper conductor length or direction of clinch
  and lap termination
• repaired or damaged conductor pattern(rework if
  applicable)
• bare copper or base metal (except end of cut wire
  or leads)
• soldered joints made directly to gold-plated
  terminals and conductors
• cold solder joints
• component moulding with solder fillet

17
Conformal coating

• Order of merit of conformal coating tested
  evaluated by
• Cost, Process, Repair, Solvent resistance,
  humidity, life test, resistance to thermal
  cycling, outgassing, micro-vcm, flammability,
  offgassing, toxicity

18
Thermal fatigue
19
Verification of solder-joints not in ECSS spec.

• No cracked solderjoints or part damage after 200
  thermal cycles and vibration testing
• Thermal cycling
• temperature cycling in air from RT to -55 oC to
  100 oC and back to RT at a rate not to exceed 10
  oC per minute. Dwell time at each temperature
  extreme should be 15 minutes.
• Vibration testing

20
Operator and inspector training and certification

• Trained and competent personnel shall be employed
  for all soldering operations and inspections
• Trained personnel performing soldering operations
  and inspection shall be certified at a soldering
  school
• ZVE, Oberpfaffenhofen, Germany
• Highbury College, Portsmouth, England
• Italian Institute of welding (IIS), Genova, Italy
• IFE, Oberpfaffenhofen, Germany
• Hytek, Aalborg, Denmark
• Institute de soudure, Paris, France

21
Training programmes

• Hand soldering to ESA PSS-01-708
• inspection of solder joints
• repair and modification of pcbs to ESA
  PSS-01-728
• RF cable assembly to ESA PSS-01-718
• crimping and wire wrapping to ESA PSS-01-726/730
• surface mounting techn. Ass. To ESA PSS-01-738
• Instructor cat 1 to ESA PSS-01-748
• fiber optic terminations to ESA-draft/NASA-std-873
  9.5

22
Soldering of surface mount devices to pcbs

• ESA PSS-01-738
• Request for verification
• technology sample
• process identification document and process
  control sheet
• audit of assembly line
• verification programme
• smd assembly approval

23
Soldering of surface mount devices to pcbs,
cont..

• Request for verification
• The verification of any SMD assembly line will be
  restricted to those companies which have been
  selected for the fabrication of ESA sponsored
  projects.
• Technology sample
• the contractor will supply one sample from his
  SMD assembly process to ESTEC. This sample will
  be examined at ESTEC or a recognised test house
• no conformal coating
• workmanship
• cleanliness
• metallography of soldered interconnections
• Audit of assembly process line
• following acceptance of the technology sample the
  manufacturing facility will be audited by ESA
• findings of the audit remain as confidential to
  ESA

24
Soldering of surface mount devices to pcbs,
cont..

• Verification programme
• Details of verification programme will be
  discussed at the time of the audit
• acc. To ESA PSS-01-738
• thermal cycling (500 -55 to 100 oC)
• vibration
• visual, electrical test, cleanliness test,
  microsection control
• the verification programme shall be established
  and approved by ESA
• the verification programme shall be funded and
  performed by the contractor or one or more
  independent test houses (test house require
  approval by ESA)
• summary table for SMD verified to ESA PSS-01-738
  shall be compiled
• SMD assembly approval
• following the successful completion of the
  verification programme a letter of approval will
  be issued by ESA. ESA project approval by means
  of Project Declared Process List
• The summary table for SMDs will be attached to
  this letter
• The validation period is indefinite until change
  to PID
• history of supply, manufacturing defects and
  unauthorised change of materials and or
  manufacturing methods will require new
  verification

25
Automatic machine wave soldering

• ECSS-Q-70-07A
• verification tests to establish confidence in all
  automatic machine soldering lines
• technology samples
• technology samples, cleanliness, documentation
• examination by recognised test house
• line audit
• verification testing
• testing according to ECSS-Q-70-70A visual
  inspection, electrical tests, thermal cycling,
  vibration when design deviates from ECSS-Q-70-08
• microsectioning
• pull testing of leads on board surface

26
high-reliability electrical connections

• CRIMPING
• Forms of crimps
• Crimping optimisation
• Workmanship examples
• WIRE WRAPPING
• wire types
• recommended insulations
• terminal posts
• examples

27
Crimping of high-reliability electrical
connections, cont..
28
Crimping of high-reliability electrical
connections, cont..
Typical plots showing variation in crimp
termination characteristics with increasing
indentation depth
29
Crimping of high-reliability electrical
connections, cont..
Unacceptable - undercrimp (tool setting-2
positions under optimum Voids greater than 10,
wire not deformed
Acceptable All wire strands deformed Voiding less
than 10
Preferred
Workmanship examples
30
Wire-wrapping of high-rel. electrical connections

• Wire used for wrapped connections shall conform
  to ESA/SCC No. 3903 or other approved national
  wire specification intended for wire wrapping
• the wire shall be a single solid round conductor.
  Stranded conductors shall not be used
• Conductor size shall be between AWG 24 and AWG 30
• soft annealed high-conductivity copper for AWG 28
  and AWG 30
• high strength high-conductivity copper for wire
  gauges AWG 28 and AWG 30
• copper shall be silver plated (gt2 micron)
• Recommended wire insulations
• outgassing according to ECSS-Q-70-02
• ETFE (Tefzel), PFA (Perfluoroalkoxy), PVDF
  (Kynar), and Kapton poluimide over extruded PTFE
• Terminal post
• suitable grades of copper or nickel alloys
  (Copper-Zinc, Phosphor-bronze, copper-nickel-zinc,
  beryllium-copper,nickel-copper(Monel) and
  nickel-cclad copper(Kulgrid)
• 1 to 3 microns of Gold over min. 1 micron of
  copper or nickel (barrier plating). No Silver
  undeplating

31
Wire-wrapping of high-rel. electrical
connections, cont..
Satisfactory wire wrap cross-section of copper
alloy wire wrapped onto a 0.64 mm square terminal
post
32
Repair and modification of PCB assemblies

• ESA PSS-01-728
• example of method 11-1, wire-to-wire bonding
• cut wire to correct length
• remove wire insulation per ESA PSS-01-708
• if distrubed, the lay of stranded conductor shall
  be restored. Do not use bare fingers
• pre-tin wires per ESA PSS-01-708
• place heat-shrink over wire insulation in
  readiness for slicing over the joined wire
• position wire into joined configuration and
  maintain position
• solder wire together (using heat shunt on each
  lead) to form a lap-type joint
• clean area with approved solvent to remove flux
• inspect joint per ESA PSS-01-708

• position shrink sleeve over joint and shrink to
  size in accordance with manufactures
  instructions. At no time shall the shrink
  temperature be allowed to exceed to melting point
  of the solder
• position the extended wire on the board and bond
  to board using a suitable space-approved
  adhesive, with interval not more than 2.5 cm. The
  first spot not more than 1.5 cm from soldered
  joint

33
Examples of solderjoints

• Good quality solderjoints
• Good quality SMT solderjoints
• Cross section of smd devices
• soldering to gold
• No pretinning
• After thermal cycling
• whisker growth on Tin-plated brass

34
Good quality joints
Good designed stress relief loop
Few fatigue lines, but no failure after thermal
cycling
35
SMT solderjoints
36
Good quality jointsCross section of smd devices
flatpack
J-lead
37
Failures soldering to gold
38
Failures No pretinning
39
Failures After thermal cycling
Top view
Cross section
40
Whisker growth

• Occasionally, manufactures use pure tin finishes
  on their pcbs, as may be standard practice for
  their commercial products. It is the thermal
  cycling environment of the spacecraft that makes
  this option extremely dangerous, as it can
  promote the growth of tin whiskers from sites
  within pure tin plated through holes.
• Terminal posts for pcbs, grounding points, and
  wire terminals and lugs for crimping operations
  are frequently machined from either copper or
  brass and simply tin-plated to achieve a
  reasonable solderability and protection from
  surface corrosion. This occurs mainly with
  standard off-the-self items.
• Tin-plated brass shows a short nucleation period
  and produces whiskers with growth rates of 8?m
  per day.