Materials for Electronics8 SMT+PTH process flow. First side assembly of Printed Circuit Board (PCB) Screening of solder paste on the pads of the first side of the Printed Circuit Board (PCB) Component placement on the solder paste deposit. Reflow of the solder paste Cleaning (not performed for no clean solder pastes) Second side assembly of PCB. Duplication of first pass Adhesive dispensing on the pads. Component placement on adhesive Adhesive curing Turn PCB over Insertion of PTH component on the first side. Wave soldering. Cleaning (not performed for no clean solder pastes and wave fluxes)
Materials for Electronics9 Principal materials for the assembly processes of electronic cards. Protective coating (finish) of PCB copper pad. Solder paste. Adhesive Wave soldering alloy
Materials for Electronics10 Copper pad finishes Organic finishes. Benzotriazole (2-3 nm) and benzoimidazole (100-300 nm) that avoid oxidation because of the formation of R-NH-Cu +1 complex. Metallic finishes. Ni-P (1-3 /Au (0.2-0.3 coating that prevents oxygen contact with copper. Ag (0.1-0.2 coating that prevents oxygen contact with copper.
Materials for Electronics11 Solder paste. Concentrated dispersion of spherical solid particles (metal powder) in a continuous organic phase (vehicle). Viscoelastic (rheological) characteristics of solder paste depend on powder particle size distribution, metal volume fraction and vehicle chemical composition. Metal powder is composed of Sn-3.0Ag-0.5Cu alloy, that replaced Pb63Sn37 alloy because of European Union ban legislation
Materials for Electronics12 Solder paste types Type of solder pastes as funtion of particle size distribution
Materials for Electronics13 Solder paste alloy. RoHS (Restriction of Hazardous Substances) directive of European Union bans since July 1, 2006 the use of lead in electronic devices. Eutectic 63Pn37Sn has been replaced by eutectic Sn-3.0Ag-0.5Cu (SAC 305) alloy.
Materials for Electronics14 Phase diagram of SAC 305 alloy. Sn-3Ag-0.5 Cu (weight %) Eutectic melting point at 220°C Density = 7.6 g/cm 3 INTERMETALLICS: Cu 3 Sn Cu 6 Sn 5 Ag 3 Sn
Materials for Electronics15 Size dependence melting properties of metal nanoparticles. The decrease of melting temperature and enthalpy of nanoparticles is inversely proportional to their radius. The model assumes an equilibrium between the core solid metal and the liquid metal overlayer of thickness t 0. This property can be used to reduce the melting point of the solder paste. Special fluxes shall be developed to avoid the very high oxidation of nanoparticles (very high surface/volume ratio)
Materials for Electronics16 Size dependence melting properties of Sn nanoparticles T m melting temperature H m melting enthalpy r nanoparticle radius s solid-liquid interfacial surface tension t 0 liquid overlayer thickness=1.6 nm The model is based on classic thermodynamics. For Sn its valid for radius >1.6 nm
Materials for Electronics17 Intermetallics structure. Cu 6 Sn 5 Hexagonal primitive (hP4) Cu 3 Sn orthorombic side face centered (oC12) Ag 3 Sn Orthorombic primitive (oP8)
Materials for Electronics18 Solder paste vehicle. The vehicle is the organic part of solder paste and is compose of flux+solvent. The vehicle can be no-clean or water soluble type. No clean vehicle is composed of no corrosive organic acids solved in high boiling alcohols and therefore dont need to be removed by cleaning from the card. Water soluble vehicle is composed of potential corrosive organic acid or salts of hydrochloric amines solved in high boiling alcohols and must be cleaned off from the card by water.
Materials for Electronics19 Flux functions. protect the powder of the solder paste from oxidation remove oxide from the surface to be soldered and protect it from further oxidation enhance wetting of the melted alloy on the component leads/balls and the PCB pad. guarantee enough tackiness to avoid the displacement of components during card handling. promote thermal transfer from the oven atmosphere to the solder joint
Materials for Electronics20 Examples of solder paste fluxes Isomers of rosin (colophony)
Materials for Electronics21 Examples of solder paste fluxes
Materials for Electronics22 Examples of solder paste fluxes
Materials for Electronics23 Examples of solder paste solvents.
Materials for Electronics24 Chemical flux reactions.
Materials for Electronics25 Poor wetting on SMT pad. Reduced spread of the alloy on copper oxidized pad.
Materials for Electronics26 Solder paste reflow Types of reflow ovens: Vapor phase Infrared Forced convection Forced convection in nitrogen atmosphere is the most used oven now. Convection assures a uniform heat diffusion on any size of component. Nitrogen prevents the oxidation of PCB pads and the powder of solder paste.
Materials for Electronics27 Forced convection oven. Q=H c A s (T g -T s ) Q heat rate transfer H c thermal convection conductance A s assembly surface area T g gas temperature near surface T s assembly surface temperature
Materials for Electronics28 Thermal Profile of Pb-free Solder paste with SAC 305 alloy
Materials for Electronics29 Functions of solder paste thermal profile zones Preheat Uniform the temperature of components with different thermal capacity evaporate the moisture inside the components avoiding cracking reduce the rate of thermal expansion. Soak evaporate the solder paste solvent and activate the flux assure the maximum coalescence of liquid alloy and reduce the formation of solder balls. Reflow the peak temperature is 20°C higher than alloy melting point to make fluid the alloy and to wet properly pad and lead component.
Materials for Electronics30 Functions of thermal profile zones. Dwell time the time above the melting temperature lasts about 30 sec to produce good solder joints without too much thick intermetallics. Cooling time a proper rate allows the formation of a suitable solid alloy (no too much large inclusions of Cu-Sn and Ag-Sn intermetallics in Sn matrix) and no thick intermetallics between alloy and pad.
Materials for Electronics31 Adhesive dispensing and curing Stick on the first side of PCB SMT small conponents to be soldered with wave soldering machine Epoxy resin with viscoelastic properties dispensed by syringe (viscosity depends on dispensing velocity and syringe opening) Quick curing operation: 90 sec at 150°C Uncured product is fluorescent in ultraviolet light for failure control. High glass transition higher than melting point of wave soldering alloy.
Materials for Electronics32 Wave soldering process Wave soldering of Pin Through Components
Materials for Electronics33 Wave soldering profile
Materials for Electronics34 Functions of wave soldering process steps 1) Flux dispensing. No clean or water soluble flux (similar chemistry of solder paste flux) Remove oxides from hole walls and pin component Reduce surface tension to improve the rising of liquid alloy through the hole. 2) Preheat Uniform the temperature on all card components Evaporate the solvent and activate the flux
Materials for Electronics35 Functions of wave soldering process steps 3) Contact with liquid wave solder Liquid SAC 305 alloy is pumped to form a wave wich goes in contact wirh the pins of the component. Few seconds contact time due to high speed of alloy rising through the hole Peak temperature 20°C above alloy melting point (220°C) 4) Cooling. A proper rate allows the formation of a suitable structure of solid alloy (no too much large inclusions of Cu-Sn and Ag-Sn intermetallics in Sn matrix) and no thick intermetallics between alloy and pad. 5) Cleaning. Residues of water soluble flux are removed by water washing.
Materials for Electronics36 Young-Laplace capillary rise equation. liquid surface tension liquid density h liquid height r capillary radius g gravity constant contact angle Low decreases contact angle Low r and increases h Alloy has high Good meniscus requires low and r
Materials for Electronics37 Good wetting in PTH solder joint Positive meniscus and complete rising in PCB hole
Materials for Electronics38 Poor wetting and voids in PTH solder joint Negative meniscus and incomplete rising in PCB hole due to poor wetting of pin component. Voids are formed because of insufficient solvent evaporation during preheat step of wave soldering process.
Materials for Electronics39 Solder joint metallurgy. SAC 305 Pb-free no clean solder paste Regular QFP front & back fillet QFP fillet with voids Voids are due to incomplete evaporation of no clean solder paste solvent because of incorrect preheat step during solder paste reflow.
Materials for Electronics40 Solder joint metallurgy. SAC 305 Pb-free solder paste & alloy Regular capacitor solder joint fillet obtained with SAC 305 Pb-free no clean paste. PTH solder joint with voids because of wrong preheat during wave soldering
Materials for Electronics41 Solder joint metallurgy BGA balls made by SAC 305 Pb-free alloy after soldering with 305 Pb-free no clean paste.Surface roughness is due to alloy shrinking and formation of Ag 3 Sn and Cu 6 Sn 5 intermetallics (right magnified optical picture). Pad copper is protected with Entek 106A finish (200 nm).
Soldering interface Cu Finish Solderalloy High T Au Diffusion
Materials for Electronics44 Solder joint metallurgy Scanning Electron Microscope (SEM) backscattering image of solder joint formed by SAC 305 ball and 305 solder paste on pad withCu/Entek 106A (150 nm). Intermetallics are due to reaction between Cu,Sn and Ag.
Materials for Electronics45 Solder joint metallurgy SEM backscattering image of solder joint formed by SAC 305 ball and 305 solder paste on pad with Cu/Ag (200 nm). Intermetallics are due to reaction between Cu,Sn and Ag.
Materials for Electronics46 Solder joint metallurgy SEM backscattering image of solder joint formed by SAC 305 ball and 305 solder paste on pad coated with Cu/Ni-P(2.5 )/Au(150nm) Layers: 1) Ni-P-Sn-Cu (reaction alloy with Ni-P); 2) Ni-P finish; 3) Cu pad Au is not detectable because dissolved in the alloy at very low concentration.