SMT Assembly and Soldering Techniques Quiz

Soldering Materials, Process & Tools

• Soldering is a process of joining two parent metals using a third material (filler) with a lower melting temperature.
• Soldering creates a metallurgical bond at temperatures below 600°F (315°C).
• Soldering materials include solder alloys, fluxes, base metals, and cleaning solvents. Specific MIL and IPC specifications exist for these.

Uniqueness of Space Missions

• Space missions demand high reliability due to the expense of mission failure.
• The design complexity of space missions is multi-disciplinary, and involves weight, volume, power, and thermal limitations.
• Single-shot, non-repairable systems are employed.
• Missions are often unattended with no on-site repair.
• Space conditions, including vibrations, extreme temperatures, vacuum, outgassing, and radiation, need to be considered during design and operation.

Difference between Commercial and Space Hardware

• Space hardware needs to withstand harsh space conditions (extreme temperatures, radiation, vacuum, shock, and vibration) and maintain high reliability.
• Space hardware cannot be repaired in the field.

Quality & Reliability

• Quality is the degree to which a product meets customer specifications at the start of its life.
• Reliability is the probability that a product/system will adequately perform its intended function under stated operating conditions for a specified time without failure.
• Factors like quality over time, dependability, durability, probability, and availability influence reliability.

Space Missions

• Space missions are highly sensitive to reliability concerns.
• The loss of a mission results in significant financial and reputational damage, as it is a matter of national pride.
• Significant investments are made in each mission.
• Uninterrupted service and ground support equipment are required.
• Replacement times are typically long (18-24 months).
• Space missions demand zero defects.

How Reliability of Space Systems is Ensured

• Designs utilize sufficient safety margins.
• High-quality components and space-proven materials and consumables are critical.
• Controlled environment conditions during fabrication and operation are necessary.
• Skilled, qualified personnel and workmanship are essential factors.
• Systems are tested under simulated space conditions (ground handling, launch, and orbital operations) to evaluate flightworthiness.
• Reliability assessments are conducted.
• Redundancy in critical subsystems is implemented.

Reliability in Space Systems

• Reliability involves quality measures across the project life cycle phases (design, realization, subsystems performance, launch, orbital, performance).
• Continuous quality improvements are required throughout the process.

Solder Joint Quality & Reliability

• Controlled environmental conditions are essential for high-quality solder joints.
• High-quality, space-proven materials must be selected and controlled.
• Processes must be appropriately qualified for space-level requirements.
• Skilled and trained personnel are critical for the solder joint and overall workmanship.
• Maintaining zero defects is crucial.

Controlled Environmental Conditions

• Cleanliness must meet class 100,000 standards.
• Temperature control is maintained at 22 ± 3°C.
• Relative humidity is controlled at 55 ± 5%RH.
• Lighting requirements are 1100 lumens/sq.m.
• ESD protection methods and storage, handling, and transportation procedures are essential.

Cleanliness (for electronic assembly)

• Aiming to avoid foreign particles that compromise quality, performance, or reliability of electronic assemblies.
• Classifications of 100,000 and 10,000 for different electronic applications are used. (refer to documents for the specifications relating to particle size).

Temperature, Humidity & Lighting

• Temperature control prevents corrosion of metallic components in humid environments.
• Specifications are 22 ± 3°C.
• Proper relative humidity control (55-60% RH) balances issues associated with ESD and corrosion.
• Lighting requirements are 1100 lumens/sq.m.

Types of ESD Failures & ESD Precautions

• ESD-related failures fall under catastrophic, parametric, or latent categories.
• ESD protection necessitates prevention of charge generation, dissipation of generated charge, charge neutralization, and use of shielding.

Materials Selection and Control

• Heritage of materials is critically assessed when selecting and controlling materials for space applications.
• Material qualification, certification, and incoming inspections, along with traceability requirements, are applied to components.

Process Control

• Defined, qualified processes (work instructions and PID, checklists, fabrication documentation) and measurement/inspections ensure quality in space component processes.
• Evaluation and control of defects and management of nonconformities (NC) are necessary.

Definition of Soldering

• Soldering is the joining of two parent metals using a third, filler material with a lower melting point.
• Soldering is the process of creating a metallurgical bond at temperatures below 600°F (315°C).

Solder Joint Design and Workmanship

• Solder joints must conform to design specifications for electrical conductivity, mechanical strength, heat dissipation, ease of manufacturing, simplicity of repair, and visual inspectability.

Basic Design Concepts for Reliable Connections and to Avoid Solder Joint Failure

• Stress relief and solder-joint reinforcement are used.
• Mismatch in thermal expansion coefficients is minimized, and materials/processes that form brittle intermetallics are avoided.
• Design allows for inspection.

Fabrication Materials

• Solder alloy, flux, base metal, and cleaning solvents. Detailed parameters and specifications (identified by MIL and IPC) are utilized.

Types of Solder Alloys

• Solder alloys are categorized by their intended use (wave soldering, PCB/connector wiring, reflow soldering, solderability testing).

Types of Solder Alloy (by Composition)

• Soft solders (Sn 63/37 Pb or Sn 62/2 Ag/36 Pb) have a melting point of 183°C.
• Sn 62/2 Ag/36 Pb has a lower melting point of 179°C.
• Hard solders (e.g., HMP solder Pb 95.5-4.5) have melting points around 308 °C.

Properties of Tin & Lead

• Tin: Bright, silver-white, lustrous, resists atmospheric corrosion.
• Lead: Bluish-grey, dull-grey appearance when exposed to air, malleable and ductile.
• Differences exist in thermal and fluid properties.

Eutectic Solder Alloy (Sn63Pb37)

• Good mechanical strength (140.5 kg/cm², 43.0, and 37.0 MPa).
• Anneals at 22°C with a CTE matching copper at -24.5 ppm/°C and a low resistance of 2-3 mΩ.
• Low melting temperature (183°C).

Eutectic Solder Alloy (Sn62Ag2Pb36)

• Good mechanical strength (140.5 kg/cm², 48.3, and 52.0 MPa).
• Anneals at 22°C with a CTE matching copper at -27 ppm/°C and a low resistance of 2-3 mΩ.
• Low melting temperature (179°C).

Solder Alloy Comparison (Sn63Pb37 & Sn62Ag2Pb36)

• Detailed comparison of characteristics (composition, melting point, density, thermal & electrical conductivity, Young's Modulus, tensile strength, elongation, etc.) is given.

Phase Diagram (Tin-Lead)

• Provides a visual representation of phase transitions for tin-lead alloys.

Soldering Iron Temperature Selection Guide

• Selecting the correct soldering iron temperature range is crucial (approximately melting point + 60-100°C). Using 240°C to 280°C.

Fabrication Materials (Flux)

• Flux is a chemically active compound that removes surface oxidation and promotes intermetallic layer formation between the solder and base.

Function of Flux

• Flux acts as a cleaning agent, catalyst, triggers/promotes a process, avoids oxidation of base metals, and promotes solder wetting.

Properties of Flux

• Resisting evaporation or breakdown at soldering temperatures (170-180°C).
• Deactivation temperature (330°C) and chemically inert, non-corrosive flux residues (at the end of the process).

Types of Fluxes (e.g., Inorganic, Organic: Rosin, RMA, RA).

• Specific composition and activation levels (low, moderate, high) of various types of flux.

Fabrication Materials (Base Metal)

• Base metal requirements, for example, good solderability of patterned pads and component leads.

Cleaning Solvents (TT-1-735)

• Cleaning solvents remove dust, corrosion, fingerprints, grease, and flux residues. Also, they shall dissolve oil, grease, dirt, and flux residues.

Features of Cleaning Solvents

• Solvents needs to dissolve grease, dirt, flux residues, and both ionic and non-ionic contaminants.
• Proper evaporation at room temperature is essential, and they should not erase component markings.
• The cleaning solvents need to be environmentally safe (thermal and chemical).

Types of Solvents (Polar, Non-polar, Bipolar, Azeotropic)

• Polar solvents dissolve polar compounds.
• Non-polar solvents dissolve non-polar compounds.
• Bipolar solvents dissolve both polar and non-polar compounds..
• Azeotropic solvents form mixtures that vaporize and condense at constant compositions.

Solder Joint Formation - Pictorial Presentation

• Illustrative diagrams of solder joint formation (vacuum, air-exposed, tarnished, clean) are provided.

Wetting

• Solder wetting is a critical aspect of the soldering process.
• Molten solder flows into surface imperfections and penetrates the interface to adhere.
• The chemical reaction between the solder and substrate creates a metallurgical bond.

Schematic of Thermodynamic Equilibrium in Wetting

• A diagram displays different wetting scenarios (total, partial, complete/non-wetting). Wetting angles (θ) provide information about the wetting process in the diagram). This is controlled by surface energies.

Soldering Stations (e.g., MIL-STD-2000A, ANSI/ESD S20.20-2021)

• Standards, specifications, and parameters detailing soldering station requirements and quality.

Important Specifications of Soldering Stations

• Included are considerations like heater/cartridge technology, temperature range/stability, display type, and ESD protection specifications.

General Purpose Tips (for soldering)

• List of different soldering tip types (e.g., conical, conical bent, chisel, chisel bent, bevel, bevel bent).

Component Soldering (Procedure)

• A description of the component soldering steps including surface and lead preparation, component placement, heating, solder application, cleaning, and inspection.

Advantages of Soldering at Lower Temperatures

• Longer tip lifespan due to reduced oxidation.
• Improved tip wettability and heat transfer efficiency.
• Flux activation occurs without direct burning of the tip, leading to better wettability.
• Reduced risk of damage to components and PCBs.
• Improved process quality (fewer rework issues, scraps).

Maintenance of Soldering Iron

• Instructions are included on checking the bit and grounding condition (replace if necessary), setting appropriate tip temperature ranges, cleaning the tool, and performing maintenance.

SMT Assembly Procedure

• Steps in the SMT assembly procedure are presented, including material preparation, solder paste printing/inspection, component pick-and-place/inspection, reflow soldering/cleaning, assembly inspection, and rework activities.

Solder Paste Printer, SMT Pick & Place, SMT Reflow Oven, X-ray Inspection, SMT Rework Station

• Description of each machine and its role in the SMT assembly process.

Factors Affecting Reflow Profile

• Key parameters to consider when establishing the reflow profile, including solder paste type, PCB material/thickness, number of layers, amount of copper, number of surface-mount components, and type of surface-mount components.

SMT Reflow Profile

• A graphical representation of a typical reflow profile with key temperature/time zones for pre-heating, soaking, reflowing, and cooling. Also, specifications including ΔT (temperature difference), Melting point, and time (seconds).

SMT Reflow Profile (phases)

• Discussion of the pre-heating, soaking, reflowing, and cooling phases.

Solder Joint Inspection

• Key aspects of solder joint inspection (visual inspection, non-destructive testing (e.g. X-ray), and destructive testing (e.g., metallographic cross-section, bond pull test)) during the inspection and testing steps.

Good Solder Joints Characteristics

• Criteria, including smooth bright/shiny surface, concave fillet, appropriate solder quantity, 5-20 degree dihedral angle, and good surface wetting, and conductor visibility during inspection.

Solder Fatigue Through Hole vs. Surface Mount

• Drawings illustrating the possible solder joints fatigue through-hole (THT) and surface-mount (SMT) components

Defects in Through-hole Soldering Process

• A variety of through-hole soldering defects (e.g., insufficient/excess solder, pin holes, blow holes, de-wetted, over-heated, cold,/rosin, disturbed, cracks) are shown. Images illustrate the defects.

Defects Cause, Effect & Correction (THT)

• Identification of probable causes for through-hole soldering defects, their effects, and corrective actions.

Defects in SMT Soldering Process

• Common solder joint defects in surface-mount technology (SMT): solder balls, poor solderability, cracked capacitor, open BGA joint, tombstone components, solder voiding, billboard parts, or insufficient/no solder.

Defects Cause, Effect & Correction (SMT)

• Examination identifies potential causes of SMT defects, their effects, and suitable corrective actions.

Cleaning Process (Manual)

• Procedure for manual cleaning of PCBs and components after the soldering process, including the use of IPA.

Vapor Degreasing Cleaning Method

• A method for cleaning highly dense wired PCBs utilizing IPA and a vapor degreasing system.

Ionic Contamination Test

• Used on electronic assemblies/components to assess cleanliness after SMT processes.
• Methods used on functional testing or coating of components.

Ionic Contamination Procedure

• Detailed procedure for examining cleanliness test, including steps like area calculation, machine operation, immersion, testing, and evaluation, as well as acceptance criteria.

Applicable Documents (for soldering processes)

• A listing of documents/standards that are important for the fabrication, packaging, and testing of electronics in accordance with the specifications listed.

Thank You