1.1 Paste Chemistry & Alloy Choice

Solder paste is the nervoussingle largest variable in the SMT process, responsible for over 65% of end-of-line defects. It is not merely "glue"; it is a complex rheological system that must withstand printing shear forces, hold components during transport, and chemically react to form a metallurgical bond during reflow. Selecting the wrong chemistry guarantees unstable printing, voiding, or latent field failures, regardless of Surfacehow Mountexpensive Technologyyour (SMT).printer Itsor chemicaloven mixis. Treat paste selection as an engineering specification, not a consumable commodity.

Flux System Selection: The Cleanliness Trade-Off

The flux vehicle dictates everythe stepprocess window, voiding performance, and residue risks. Choose the flux system based on the cleaning capability and reliability requirements of the linefinal —assembly.

from

Scenario howA: High-Reliability, Dense Assembly (Aerospace, Medical, High-Density Digital)

Requirement: Zero residue risk, maximum wetting on oxidized pads.

Action: Select Water-Soluble (WS) paste.

• Why: WS fluxes contain aggressive activators (organic acids) that clean youroxides stencilsrapidly. stayThey provide the widest wetting window and lowest voiding rates.
• The Engineering Risk: The residue is corrosive. If the washing process fails (water temperature, pressure, or saponifier concentration drop), the remaining ionic contamination will cause dendritic growth and shorts.
• Mandatory Control: You must have an in-line cleaner and perform daily Ionic Contamination testing (ROSE/SEC) to howverify reliablecleaning yourefficacy.

joints

Scenario areB: yearsStandard downIndustrial/Consumer (IoT, Control Boards, LED)

Requirement: Cost efficiency, no cleaning process.

Action: Select No-Clean (NC) paste (ROL0/ROL1).

• Why: The residue is engineered to be non-conductive and benign after reflow. It eliminates the road.capital Theand balanceoperating expense of a wash line.
• alloyThe Engineering Risk:, "No-Clean" does not mean "Invisible."
  • fluxIf Reflow is too Cold:, Activators remain unreacted and conductive -> Leakage currents.
  • powderIf sizeReflow is too Hot: definesResidue yourchars entire-> processInterferes window.with GetIn-Circuit thisTest right,(ICT) andprobes production(false hums;failures).
• Mandatory getControl: itVerify wrong,flux andcompatibility you’llwith beconformal chasingcoating yieldif problemsused. forever.Validate ICT probeability.

What’sAlloy inSelection: aThermal Solder& Paste?Mechanical (The Three Knobs)Integrity

EveryThe pastealloy is a suspension of three things, and you control all three:

1. Alloy: Setsdefines the melting point (thermal profile) and the long-term mechanical strength/reliabilityfatigue resistance of the joint. Do not default to "Standard SAC305" without verifying the application constraints.

   Standard Application: SAC305 (Sn96.5 / Ag3.0 / Cu0.5)

   • Melting Point: ≈ 217˚C – 220˚C.
   • Flux:Usage: The engineindustry room.default Itfor cleansgeneral SMT.
   • Outcome: Provides acceptable thermal cycling performance for consumer electronics (0˚C to 60˚C operational range).
   • Limit: Avoid for high-stress automotive or aerospace applications where thermal shock exceeds -40˚C to +125˚C.

   High-Reliability Application: Doped Alloys (SAC-Q, SAC-I, SnNi)

   • Usage: Automotive under-hood, ruggedized industrial.
   • Mechanism: Dopants like Bismuth (Bi), Nickel (Ni), or Antimony (Sb) pin the metal,grain preventsboundaries oxidationof the solder structure.
   • Outcome: Prevents micro-cracks propagation during heating,thermal dictatescycling. stencilmeasurable life,2x – 3x increase in drop-shock and determinesthermal fatigue life compared to standard SAC305.

   Low-Temperature Application: SnBi (Tin-Bismuth)

   • Melting Point: ≈ 138˚C.
   • Usage: Heat-sensitive components (cheap LEDs, PET flex circuits).
   • Risk: The joint is brittle.
   • Engineering Consequence: Mechanical shear strength is < 50% of SAC305. Boards must not be subjected to drop shock or bending. Never mix SnBi paste with SAC305 component balls unless specifically profiled to fully mix the alloy, or the joint will fail catastrophically.

   Pro-Tip: If your Voiding Rate on QFN thermal pads exceeds 25% (IPC Class 3 failure), do not just tweak the profile. Switch to a "Low-Voiding" flux formulation specifically engineered with different solvent outgassing rates. Chemistry fixes voiding better than profiling does.cleaning

   requirements.

3. Powder Size (Type):Mesh) & Release Stability

   Particle size controls the release of paste from the stencil aperture. Mismatched powder size leads to clogging, insufficiency, and rapid oxidation.

   Logic for Powder Selection:

   1. If UnlocksSmallest Pitch ≥ 0.5 mm AND Smallest Aperture Width > 0.25 mm:
      • Use Type 3 (T3) (25 – 45 µm).
      • Benefit: Lower surface area oxide, longer stencil life (> 8 hours), lower cost.
   2. If Smallest Pitch 0.4 mm OR Area Ratio (AR) is 0.60 – 0.66:
      • Use Type 4 (T4) (20 – 38 µm).
      • Reason: T3 particles will block the aperture. T4 is the modern standard for mixed-technology boards.
   3. If Smallest Pitch ≤ 0.3 mm (01005s, µBGA):
      • Use Type 5 (T5) (15 – 25 µm).
      • Risk: High surface area leads to rapid flux exhaustion.
      • Consequence: Stencil life is reduced to < 4 hours. You must replenish paste more frequently to maintain tack and flux activity.

   Pro-Tip: Never top up a jar of T4 paste with leftover T3. The mixed rheology will cause unpredictable rolling and release, destroying your abilityCp/Cpk.

   Traceability & Incoming Control

   You cannot control the process if you do not control the material. Every jar of paste must be tracked.

   • Lot Number: Record and link to print fine features. It’s the gatekeeperProduction forJob areaID. ratio and volume consistency on small pads.

Flux Systems: No-Clean vs. Water-Soluble

This is the firstonly majorway decision,to impacting cost, cycle time, and risk.

Flux Family	The Production Vibe	Best Use Case	Risk & Trade-Off (The Manager View)
No-Clean	Forgiving, long stencil life. Leavescontain a clear,defect benignif (non-conductive)a residuebad if reflowbatch is perfect.discovered later.	Most Date commercial,of consumer,Manufacture and industrial builds. Saves the cost and space(DOM): ofEnforce a cleaningstrict line.	Ifshelf the residue is overcooked or under-activated, it can interfere with test probeslife (ICT)typically or6 impactmonths cosmetics.at Must confirm residue meets cleanliness specs (IPC J-STD-004)0–10˚C).
Water-Soluble (WS)	Aggressive activators provide superior wetting and zero-voiding, even on oxidized surfaces.	Dense assemblies, extreme fine pitch, high-reliability builds (military/aerospace) where zero ionic residue is mandatory.	High CapEx/OpEx for the cleaning line (chemistry, utilities, wastewater). Critical risk: If cleaning is incomplete, ionic residue is aggressive and guarantees corrosion/dendritic failure in the field.

Pro Tip:Action: If youDOM have> wetting6 issuesmonths or-> excessiveDispose. voidsDo onnot BGAs,requalify. WSThe flux activators degrade over time even in cold storage.

Metal Load: Verify the weight percentage (e.g., 88.5%).

• Drift: If metal load is too low (high flux content), slump and bridging increase. If too high, paste isdries the technical solution. But remember, you’re trading a printing problem for a costlyout and complex clogs.cleaning process you must now validate and control 24/7.

PowderFinal Size: Matching Aperture RatiosChecklist

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

Powder Type (J-STD-005)Parameter	ParticleSpecification Size/ Range (µm)Target	UseControl Case & Required Process Control
Type 3 (T3)	25 – 45	Default, forgiving paste. Great for 0.5 mm pitch and larger. Longer 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.

Fast Rule: If your minimum Area Ratio (Aperture Area / Aperture Wall Area) drops below 0.66, you must upgrade to the next smaller powder type (e.g., T3 – T4 – T5) to ensure consistent volume release and minimize clogging. Tighter powder means tighter storage, handling, and print control.

Alloy Choice: Performance, Profile, and Reliability

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

Alloy Family	Eutectic Melting Point	Process Notes (The "Vibe")	Critical Reliability Factor
Sn63/Pb37	≈ 183 ˚C (Sharp, Eutectic)	Wide thermal margin, excellent wetting.	Not RoHS Compliant. Must be profiled separately. Watch out for Tin Whiskers if bare tin finishes are also present.
SAC305 (SnAg3.0Cu0.5)	217 ˚C (Near-Eutectic)	Industry Standard Lead-Free. Needs honest, higher profiling (longer Time Above Liquidus, TAL). Nitrogen (N2) often used to improve cosmetics and reduce voiding, especially on large thermal pads.	Superior thermal cycling fatigue resistance compared to SnPb, making it the choice for rugged, outdoor, or automotive electronics.
Low-Temp Bi-Based	≈ 138 – 160 ˚C (Varies)	Great for heat-sensitive parts or boards. Allows standard FR4 to be run with minimal thermal stress.	Weaker mechanically. Requires careful qualification for drop, shock, and high-vibration applications due to reduced shear strength.

Manager Takeaway: For high-reliability, cycling applications, explore doped SAC alloys (e.g., SAC-Q, SAC-I). These specialty pastes are engineered to resist micro-cracking and improve service life where temperature extremes are a factor.

Mapping Your Paste to Your Process Window

The final choice must satisfy three critical axes simultaneously:

1. Print Axis (Rheology & Powder): Does the paste’s rheology (its internal viscosity/flow) allow it to hold shape perfectly after the print stroke, or does it slump and cause bridging? Is the tack strong enough to hold small 0201/0402 components through conveyor travel before reflow? (Check this with SPI/AOI data.)
2. Reflow Axis (Alloy & Flux): Do the chosen alloy and flux chemistry allow you to consistently hit your target TAL and Peak Temperature without oxidizing the flux or stressing the components? (Check this with thermal profiling.)
3. Cleanliness Axis (Risk & Process): If you 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? (Check this with ionic testing and test yield data.)

When the window feels tight, resist the urge to change the operator's settings. Change the paste, the stencil, or the atmosphere (add N₂) instead. Run a small Design of Experiments (DOE) to find the sweet spot, then lock it down.

Final Checklist: Paste Selection Requirements

The paste selection must be verified against all process and reliability requirements before purchase.

Category	Requirement	Verification StepMethod
Flux SystemClassification	Must meet the required cleanliness specificationJ-STD-004 (e.g., IPCROL0, J-STD-004).ORL0)	CleaningVerify processdatasheet designedmatches andWash/No-Clean validatedstrategy.
Halogen Content	Halogen-Free vs. Halogenated	Confirm customer environmental compliance (ifIEC WS);61249-2-21).
Alloy ICTMelting probePoint	Specific testto approvedApplication (ife.g., NC)217˚C)	Confirm reflow oven capability (Max temp vs. component limits).
Powder Size	Type must3, match4, theor smallest aperture's Area Ratio (T3 – T4 – T5).5	DFMMust auditmatch completed;stencil SPIArea volumeRatio release(Target verifiedAR on≥ smallest pads.0.66).
AlloyViscosity (Malcom)	Muste.g., satisfy180 RoHS/Customer– requirements220 and the product'Pa·s thermal cycling lifespan.	ProfileIncoming verifiedinspection againstor alloyCertificate limits;of componentAnalysis maximum(CoA) temperature limits guarded.verification.
HandlingShelf Life	Defined< storage,6 thawing,months mixing,from and open-time policy in place.DOM	Standard Operating ProcedureFIFO (SOP)First-In-First-Out) lockedphysical forstock floor staff.rotation.
ProcessStencil MapLife	Paste> rheology8 verifiedhours to(T3/T4) have/ sufficient> tack4 forhours small component retention.(T5)	Post-placementProduction AOIlog confirmsmonitoring component(abandon retentionif through conveyor travel.exceeded).