SME2 - Processes

Materials and Processes DivisionESA/ESTEC/TOS-QM

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SME Initiative

ESA SME InitiativeCourse D:Materials

Dr. Ton de Rooij

Head of Materials Mechanics and Processes Section

Materials and Processes Division

Product Assurance and Safety Department

Critical processes related to electronic materials


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Content of presentation

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


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Specifications

ECSS-Q-70-08A Space product assurance - manual soldering of

high-reliability electrical connections

ECSS-Q-70-10A 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)

ECSS-Q-70-18A the preparation, assembly and mounting of RF

coaxial cables

ECSS-Q-70-26A Space product assurance - the crimping of high-

reliability electrical connections -to be published (now ESA PSS-01-726)

ECSS-Q-70-28A 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

ESA PSS-01-738 Space product assurance - high-reliability soldering

for surface mount and mixed technology printed circuit boards - to be published (now ESA PSS-01-738)


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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


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Soldering processes

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


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Soldering processes, cont..

Hand soldering qualification

machine soldering verification and approval

surface mounting verification and approval


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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)


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63 tin sol-der (eutec-tic)

183 183 Soldering PCBs where temperature limitations are critical and in applications where an extremely short melting range is required

62 tin silver loaded

175 189 Soldering of components have silver-plated or ‘paint’ (i.e. ceramic capacitor) finish. This solder composi-tion is saturated with silver and prevents the scav-enging of silver surfaces.

60 tin sol-der

183 188 Soldering electrical wire/cable harnesses or terminal connections and for coating or pre-tinning metals

96 tin silver (eutectic)

221 221 May be used for special applications such as solder-ing terminal posts


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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


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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


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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


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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


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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

coating type nature cost process repair solvent resistance

humidity life test resistance to thermal cycling outgassing

manned spacecraft

total

surface mount

leaded component

s

microwire bonds

7=dear 1=cheap

7=com-plicated 1=easy

7=difficult 1=easy

1=good 7=poor

1=good 7=poor

1=good 7=poor 1=best 7=poor

1=good 7=fail

1=good 4=fail

CV 1144-0 silicone 7 2 1 5 1 1 1 1 1 1 1 22Mapsil 213 silicone 6 1 2 6 3 4 1 1 2 6 1 33Uralane 5750 LV polyurethane 3 3 5 1 6 1 1 4 4 4 4 36Sylgard 184 silicone 5 6 3 7 3 5 1 1 3 7 1 42Conathane en11 polyurethane 1 5 6 1 7 7 1 5 5 1 4 43Solithane 113 polyurethane 4 4 4 1 2 6 6 7 6 1 4 45Scothcast 280 epoxy 2 7 7 1 5 1 7 6 7 4 4 51


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Thermal fatigue


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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

Vibration amplitude (Peak to peak) 10-70 Hz at 1.5 mmSine vibration Frequency range 70-2000 Hz at 15 g

Sweep 1 octave per minuteDuration 1 cycle from 10-2000-10 HzFrequency range 20-2000 Hz at 15 g-rms

Random vibration Power spectral density 0.1 g2 Hz-1Duration 10 minutes per axis


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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


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Training programmes

Hand soldering to ESA PSS-01-708 inspection of solder joints repair and modification of pcb’s 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-8739.5


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Soldering of surface mount devices to pcb’s

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


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Soldering of surface mount devices to pcb’s, 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


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Soldering of surface mount devices to pcb’s, 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 SMD’s 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


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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


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high-reliability electrical connections

CRIMPING Forms of crimps Crimping optimisation Workmanship examples

WIRE WRAPPING wire types recommended insulations terminal posts examples


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Crimping of high-reliability electrical connections, cont..


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Typical plots showing variation in crimp termination characteristics with increasing indentation depth


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Unacceptable - undercrimp(tool setting-2 positions under optimumVoids greater than 10%, wire not deformed

AcceptableAll wire strands deformedVoiding less than 10%

Preferred

Workmanship examples


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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 (>2 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


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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


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Repair and modification of PCB assemblies

ECSS_Q-70-28 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 manufacture’s 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


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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


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Good quality joints

Good designed stress relief loop

Few fatigue lines, but no failure after thermal cycling


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SMT solderjoints


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Good quality jointsCross section of smd devices

J-lead

flatpack


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Failures: soldering to gold


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Failures: No pretinning


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Failures: After thermal cycling

Cross section

Top view


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Whisker growth

Occasionally, manufactures use pure tin finishes on their pcb’s, 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 pcb’s, 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 8m per day.

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SME2 - Processes