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1.1 Paste Chemistry & Alloy Choice

Solder paste sits atis the heartnervous system of surfaceSurface mountMount assembly,Technology and(SMT). itsIts chemistrychemical quietlymix dictates howevery smooth—orstep of the line—from how painful—productionclean willyour be.stencils stay to how reliable your joints are years down the road. The balance of flux,alloy, powder size,flux, and alloypowder determinessize everythingdefines fromyour stencilentire performanceprocess towindow. reflowGet stabilitythis right, and long-termproduction reliability.hums; Everyget tradeoff—betweenit print lifewrong, and fine-pitchyou’ll resolution,be betweenchasing wettingyield powerproblems and cleaning demands, between thermal limits and mechanical strength—defines the process window that keeps yields high and surprises low. Choosing wisely here sets the tone for the entire manufacturing line.forever.

1.1.1 What’s in a solderSolder pastePaste? (andThe whyThree you should care)Knobs)

AEvery paste is a suspension of three things, and you control all three:

  1. alloy + flux + powder sizeAlloy:. AlloySets setsthe melting temppoint (thermal profile) and the long-term behavior;mechanical fluxstrength/reliability decidesof the joint.
  2. Flux: The engine room. It cleans the metal, prevents oxidation during heating, dictates printstencil life, wetting, residue/cleaninglife;, powderand sizedetermines unlocksthe cleaning requirements.
  3. Powder Size (Type): Unlocks your ability to print fine aperturesfeatures. withoutIt’s slumpingthe orgatekeeper clogging. Every knob you twist here shows up infor printingarea ratio and reflowvolume consistency later—so pick on purpose.small pads.


Microscopic Suspension Diagram


Microscopic Suspension Diagram


1.1.2 Flux systemsSystems: (no-cleanNo-Clean vsvs. water-soluble)Water-Soluble

This is the first major decision, impacting cost, cycle time, and risk.

Flux familyFamily

WhatThe it’sProduction like on the lineVibe

WhereBest itUse shinesCase

WhatRisk to& watchTrade-Off (The Manager View)

No-cleanClean

LongForgiving, long stencil life,. forgivingLeaves ofa small pauses; leavesclear, benign residues(non-conductive) whenresidue processif reflow is in control.perfect.

Most consumer/commercial, consumer, and industrial builds;builds. Saves the nocost washand space processes.of a cleaning line.

CosmeticsIf andthe residue is residueovercooked rules—confirmor under-activated, it can interfere with customer;test borderlineprobes wetting may benefit from (N₂ICT.) or impact cosmetics. Must confirm residue meets cleanliness specs (SeeIPC 9.3 & 15.1.)J-STD-004).

Water-solubleSoluble (WS)

StrongAggressive activators forprovide fastsuperior wetting; requires a wash (and azero-voiding, realeven cleaningon process).oxidized surfaces.

Dense BGAs/QFNsassemblies, extreme fine pitch, high-reliability builds (military/aerospace) where you want aggressive fluxzero thenionic removeresidue it.is mandatory.

High CapEx/OpExAdds for the cleaning line (chemistry, utilities, wastewater). Critical risk: If cleaning is incomplete, cleaningionic residue station,is fixtures, chemistry,aggressive and verificationguarantees (Chaptercorrosion/dendritic 15).failure in the field.

Pro Tip: If inyou doubt,have startwetting no-clean,issues proveor residuesexcessive arevoids acceptable,on BGAs, WS paste is the technical solution. But remember, you’re trading a printing problem for a costly and keepcomplex water-solublecleaning process inyou yourmust backnow pocketvalidate forand stubborncontrol wetting/voiding cases.24/7.



1.1.3 Powder sizeSize: Matching Aperture Ratios


The powder type must be smaller than the smallest stencil aperture opening to avoid clogging. This is where your Design for Manufacturing (whyDFM) meets reality.

Powder Type 3/4/5(J-STD-005)

Particle isn’tSize just marketing)

Smaller powderRange (e.g.,µm)

Use TypeCase 4/5& Required Process Control) prints tighter apertures but oxidizes faster, narrows print/hold life, and can demand N₂ to keep wetting crisp. Larger powder (

Type 3 (T3)

)25 is– 45

Default, forgiving but taps out on very fine pitch. Match size to your aperture area/aspect ratiospaste. Great for 0.5 mm pitch and componentlarger. mixLonger stencil life, easier handling.

Type 4 (T4)

20 – 38

Standard for Fine Pitch. Essential for 0.4 mm pitch QFNs and BGAs. A good balance between printability and handling.

Type 5 (T5)

15 – 25

Ultra-Fine Pitch. Mandatory for 0.35 mm pitch components and the most marginal area ratios.

Powder Fit Comparison (Type 3 vs. 4 vs. 5)


Fast rulesRule:

  • If your minimum areaArea ratiosRatio are(Aperture marginal,Area move/ upAperture aWall Area) drops below 0.66, you must upgrade to the next smaller powder classtype (e.g., T3 or– stepT4 the– stencilT5) locally.
  • to ensure consistent volume release and minimize clogging. Tighter powder → tightenmeans storage/handlingtighter storage, handling, and considerprint N₂control for reflow.



.

1.1.4 Alloy choiceChoice: (whatPerformance, changes in reflowProfile, and life)Reliability


Alloy choice sets the thermal profile and, more importantly, the long-term joint integrity.

Alloy familyFamily

MeltEutectic behaviorMelting (vs others)Point

Process vibeNotes in(The reflow"Vibe")

Critical Reliability notesFactor

Sn63/Pb37 (eutectic)

Lowest≈ peak183 of˚C the(Sharp, common choices; sharp liquidusEutectic)

Wide thermal margin;margin, easyexcellent wettingwetting.

Not RoHS.RoHS DifferentCompliant. Must be profiled separately. Watch out for intermetallicTin Whiskers growthif vsbare SAC;tin profilesfinishes differare (seealso 9.4).present.

SAC305/lead-freeSAC305 SAC(SnAg3.0Cu0.5)

Higher217 peak˚C than SnPb; broader “mushy” zone(Near-Eutectic)

Industry Standard Lead-Free. Needs honesthonest, higher profiling (TAL/peaklonger matter);Time Above Liquidus, TAL). N₂Nitrogen (N2) often helpsused cosmetics/voidsto improve cosmetics and reduce voiding, especially on large thermal pads.

Superior thermal cycling fatigue resistanceWidely used;compared dopedto SACSnPb, &making specialtyit mixesthe existchoice for harshrugged, duty.outdoor, Seeor 9.4automotive & 16.2.electronics.

Low-tempTemp (Bi-based)Based

Much≈ lower138 peak– 160 ˚C (Varies)

Great for heat-sensitive parts/boardsparts or boards. Allows standard FR4 to be run with minimal thermal stress.

Weaker mechanically.Narrower processRequires window;careful mechanicalqualification limits—qualifyfor carefully.drop, (We’llshock, profileand ithigh-vibration inapplications 9.2/9.4.)due to reduced shear strength.

The Alloy Thermal Profile Overlay

Manager Takeaway:Different alloysFor wanthigh-reliability, cycling applications, explore different profiles—don’t reuse SnPb curves fordoped SAC oralloys Bi(e.g., mixes.SAC-Q, SAC-I). These specialty pastes are engineered to resist micro-cracking and improve service life where temperature extremes are a factor.



1.1.5 MapMapping pasteYour choicesPaste to yourYour processProcess windowWindow

ThinkThe windowfinal inchoice must satisfy three axes:critical print,axes reflow, cleanliness.simultaneously:

    1. Print axis:Axis (Rheology & Powder): PowderDoes sizethe +paste’s rheology must(its internal viscosity/flow) allow it to hold shape perfectly after the print stroke, or does it volume/height/areaslump steadyand acrosscause bridging? Is the shift.tack Ifstrong you’reenough livingto athold thesmall edge,0201/0402 changecomponents powder/stencil—notthrough operatorconveyor heroics.travel (SPIbefore & closed loop inreflow? 7.6.(Check this with SPI/AOI data.)
    2. Reflow axis:Axis (Alloy & Flux): AlloyDo +the chosen alloy and flux decidechemistry allow you to consistently hit your target TAL, peak, and needPeak for N₂Temperature. SACwithout andoxidizing low-tempthe pastesflux oftenor benefitstressing fromthe nitrogencomponents? when(Check pushingthis cosmetics/voiding.with (9.2–9.3.thermal profiling.)
    3. Cleanliness axis:Axis (Risk & Process): No-cleanIf savesyou time—choose water-soluble, is the wash process controlled and verified to IPC standards? If you choose no-clean, do you have zero field failures related to residue, and are your test probes happy? if(Check this with ionic testing and test yield data.) residues meet spec; water-soluble widens wetting but adds a whole cleaning process to control. (15.1–15.2.)

When windowsthe feelwindow narrow,feels runtight, resist the urge to change the operator's settings. Change the paste, the stencil, or the atmosphere (add N2) instead. Run a small DOE:Design twoof powdersExperiments ×(DOE) twoto atmospheres × two peak temps. Keep what moves your SPI→AXI Paretofind the rightsweet way.spot, (Windowthen findinglock init 16.4.)



down.

1.1.6 Starter picks (use, then tune with data)

  • General lead-free builds: No-clean SAC with Type 3–4; air reflow unless BGAs/voiding push you to N₂.
  • Fine-pitch BGA/QFN: No-clean SAC, Type 4 (or 5 if area ratios demand), consider N₂; pair with windowed apertures on thermal pads. (See 7.4.)
  • Heat-sensitive boards/parts: Low-temp Bi-based paste; validate drop/mech and tune gentle profiles. (9.2/9.4)
  • Legacy/service builds: SnPb where allowed; profile separately from SAC. (9.4)



1.1.7 Don’t forget the care & feeding

Great paste poorly handled is bad paste. Control storage, thawing, mixing, open time—we’ll set those rules in 6.2 and tie them to shelf life in Materials (17.4).



1.1.8 Release checklistChecklist (printPrint thisThis beforeBefore youYou buy)Buy)


  • Flux familyFamily: chosenChosen (no-clean vsvs. water-soluble); andcleaning process or cleaningresidue planspec aligned.is documented.
  • Powder sizeSize: matchedMatched to smallest aperture area/aspectArea ratiosRatio inon yourthe stencil set.PCB (Seeusually 7.4.)T4 for 0.5 mm, T5 for 0.35 mm).
  • AlloyAlloy: pickedPicked withbased reflowon required profile planreliability (TAL/peak;thermal aircycling) vsand confirmed against the available N₂reflow profile). (9.2–9.4)limits.
  • Handling & shelf-lifeHandling: Documented rules documentedfor storage, thawing, mixing, and open time (storage,the thaw/mix,maximum opentime time)allowed on the stencil). (7.2, 5.5)
  • DOE planValidation: readyPlan ifin theplace to test early lots feel tight;and tie readsresults directly to SPI/AOI/AXI dashboards.data (7.6,dashboards 10.x,for 16.4)process control.



Conclusion: Establish paste parameters that match stencil design, thermal profile, and cleanliness strategy, then validate them with inspection data. Doing so transforms soldering from a daily struggle into a predictable, high-yielding process.

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