3.4 Alloy-Specific Nuances

Every solder alloy bringspossesses a unique thermal and metallurgical signature that dictates its ownrequired thermalreflow personality,profile. Profile uniformity and reflowpeak successtemperature dependsmust onbe respectingtuned thoseto differences.the Tin–leadalloy's flowsmelting easilycharacteristics, which directly influence wetting, voiding, and forgives uneven heating, but lead-free SAC demands more patience with higher peaks and steadier time above liquidus. Beneath the surface,growth intermetallicof compoundsIntermetallic quietlyCompounds grow(IMCs). atThe thefailure padto interface,respect theirthese thicknessalloy-specific deciding whether joints remain tough or turn brittle with age. Even subtle choices—nitrogen use, rework alloy, or micro-alloyed SAC variants—shift long-term reliability. What looks like a cosmetic tweak on the linedifferences can echolead to hidden reliability risks that manifest years later inas thebrittle field.joint fractures.

3.4.1 TwoAlloy families,Families: twoThermal personalities

• Sn63/Pb37 (eutectic) melts at 183 °C with a very sharp liquidus. It wets easily, needs less time above melt,Requirements and forgivesReliability uneven

  The heating.choice Greatof alloy sets the baseline for service/legacythe buildsentire wherethermal RoHS doesn’t apply.

• Lead-free SAC (e.g., SAC305) melts near 217 °C with a broader “mushy” region. It often wants a steadier soakprofile and adefines longerthe TALjoint's tomechanical fully collapse BGAs and wet big copper.
  

Typical starting targets (tune with your paste datasheet & parts’ MSL limits):properties.

Alloy Family	Liquidus Temp	Peak (typical)Profile Range	TALKey (typical)	NotesReliability Feature
Sn63/Pb37 (Eutectic)	183183˚C °(Sharp melt)	205–220˚C	205–220Highest °CWetting Speed.	Forgives uneven heating (∆T). 30–60 s	Short, crisp profile; easy wetting; notNot RoHS.
SAC305 (lead-free)Lead-Free)	217217˚C °(Mushy zone)	235–250˚C	235–250High °CThermal Fatigue Resistance.	40–80 s	SmootherDemands a slower ramp/soak;soak and longer N₂TAL can help margins..
Low-tempTemp Bi-basedBased (e.g., Sn42Bi58)	~138 °≈138˚C	165–185˚C	165–185Component °C	30–60 s	Narrower window; mind brittlenessProtection. andNarrower plastics.process window. Lower mechanical strength (brittleness).

Mandate:Guardrail: componentProfile settings from one alloy family max-tempmust not (JEDECbe J-STD-020)used stillfor rules.another. NeverThe chaseSAC solderalloy qualityrequires bysignificantly exceedinghigher partenergy limits.and time above liquidus compared to eutectic SnPb.

---

3.4.2 HowProfile profilesTuning: differPractical inDifferences

practice

The physical properties of the solder demand specific profile adjustments to ensure joint quality:

• SnPb
• • Ramp: Profiles: canCan be run with a bitquick quicker;ramp tombstoning/bridgingand usuallya lessbrief sensitive.Time Above Liquidus (TAL). Excessive TAL is wasteful and increases IMC growth without improving joint quality.
  • Soak:SAC Profiles: shortRequire ora nonesmoother onramp even-massand boards.a soak phase (Chapter 3.2) to minimize cross-board temperature differential (∆T) before reflow. A steady, adequate TAL (typically 40-80 seconds) is essential to ensure full BGA collapse and mitigate Head-in-Pillow (HIP) defects. Nitrogen (N₂) often improves the wetting margin at the expense of OpEx (Chapter 3.3).
  • TAL:Low-Temp Bi Profiles: keepDue itto brief;their extralow timemelting onlypoint, cooksthese flux and grows intermetallics you don’t need.
• SAC
• • Ramp/Soak:require a gentle soakramp-to-peak helpswith equalizea ΔTminimal soak to prevent the creation of solder balls and ventoxidation volatilesduring (voiding);extended smoothdwell times.TAL is your HIP insurance on BGAs.
  • N₂: often buys you 5–10 °C in peak or 5–10 s in TAL while improving cosmetics and voids (see 9.3).
    

---

3.4.3 Intermetallics:Intermetallic what’sCompounds growing(IMCs) and Reliability

IMCs (primarily Cu₆Sn₅) are the brittle, necessary bond layers formed at the padcopper-solder

Everyinterface. Their thickness dictates the long-term joint forms Cu–Sn intermetallic layers (mainly Cu₆Sn₅, then Cu₃Sn). They’re necessary—but too thick = brittle.integrity.

• WhatGrowth grows them:Driver: IMC growth is accelerated by temperaturehigh + time. High peakstemperatures and long TAL.
• Reliability accelerateRisk: growth,An inexcessively boththick SnPbIMC andlayer SAC.(often due to over-profiling) leads to a brittle joint that is susceptible to failure under thermal cycling, drop-shock, or long-term vibration.
• WhySAC IMC Nuance: SAC needsalloys attention: SAC’srequire higher tempspeak naturallytemperatures, pushwhich growthinherently faster;accelerates itsIMC growth. Furthermore, the microstructure alsoof SAC joints contains Ag₃SnAg₃Sn particles that caninfluence affect drop-shock behavior.performance.
• YourControl controlAction: knobs:
• • Don’t exceedLimit peak moretemperature than needed; lengthenand TAL onlyto asthe farminimum asnecessary defectsto (HIP/voids)achieve require.complete wetting and BGA collapse.

  3.4.4 Special Cases and Mixed Alloys

  1. Second-Side Reflow

  AvoidWhen double-cookingprocessing onthe second-second side profiles;of an assembly, the profile must be gentler. The first-side joints are exposed to reflow temperatures again, accelerating their IMC growth. The second pass should use a gentler second pass.

• For harsh environments, considerslightly dopedlower SACpeak (Ni/Ge/Bi “micro-alloy” variants) that slow IMC growthtemperature and improveshorter fatigue.

Symptom of “too much cook”: brittle fractures at pad interfaces after thermal aging, even though joints looked perfect on day one.

---

3.4.4 Copper dissolution & thin pads (rare, but real)

At high peak temps and long TAL—especially with aggressive flux—copper can dissolve from very fine pads into the solder. You’ll see thinned pads or odd joint shapes on tiny lands. If AXI/AOI hints at “etched” pads:

• Pull peak/TAL back to saneminimize bands,the thermal exposure of the components already soldered.

• 2. Low-Temperature Bi-Based Alloys

  Re-checkBi-alloys sacrifice paste chemistry/flux activity, and

• Confirm your finish (e.g., ENIG/ImmAg tolerate SAC well; raw copper/OSP needs discipline).
  

---

3.4.5 Low-temperature bismuth pastes (special rules)

Bi alloys let you keep plastics cool and protect sensitive boards, but:

• The window is tighter—long soaks can make solder balls or dull joints.
• Mechanicalmechanical toughness canfor droplow (brittleness).temperature. ValidateThis drop/shockmust orbe connectorfactored cycles if yourinto product seesqualification. abuse.
• WatchFurthermore, mixing:mixing reworkBi-based solder with standard SnPb or SAC changesduring localrework compositionor andin a mixed-technology application can create an alloy with a different, unpredictable melting points—standardizepoint, your rework alloyleading to avoidreliability mixedconcerns. joints.
  

Rework must use a standardized, compatible alloy.

3. Micro-Alloyed SAC

For

---

products

requiring

3.4.6extreme Mixed technology (don’t stumble into hybrid joints)

• Pb-free parts on SnPb paste or SnPb parts on SAC paste create mixed-melting joints. Wetting can look okay, butlong-term reliability becomes unpredictable.
• If you must mix, do it deliberately (step-solder sequence, known alloy pair) and qualify—don’t drift there by accident in rework.
  

---

3.4.7 Choosing between SAC flavors (when reliability is king)

• High Ag (e.g., SAC305/SAC405):automotive, rugged industrial), specialty SAC alloys with micro-additives (Ni, Ge, Bi, Sb) are used. These alloys are engineered to slow down IMC growth goodand wetting/cosmetics;improve solidresistance for most builds.
• Lower Ag (e.g., SAC105): often better drop-shock, but may need profile/N₂ help for wetting.
• Micro-alloyed SAC (Ni/Ge/Bi/Sb blends, e.g., “SAC-X/Innolot”): slower IMC growth, betterto thermo-mechanical fatigue—useful for(cracking under-hood,under base-station,temperature orcycles). high-tempThe duty.
  choice Pickof basedSAC onvariant realmust align with the product's dominant field failure modes:mode.
  if

  Final yourChecklist: fieldAlloy returnsProfile are thermal-cycle cracks, micro-alloyed SAC pays; if they’re impact/drop, try lower-Ag or process tweaks.
  

Requirements

---

Checkpoint	Requirement	Rationale
Profile Baseline	Profile is derived from the 3.4.8paste Quickvendor's playbook (defect → alloy/profile moves) BGA HIP in SACdatasheet →for lengthenthe specific alloy.TAL	Avoids (steady),blind smooth soak; if marginal, add N₂profiling and trimensures peakflux aactivation.
IMC bit. QFN voiding in SACControl → windowed center pad (7.4), gentler soak; N₂ helps more than extra peak. Dull SAC fillets / slow wetting → +5 °C peak or N₂; verify paste age/finish. SnPb bridges → shorter soak, keep TAL crisp; revisit apertures. Aging-related brittle fractures → reduce peak/TAL; consider micro-alloyed SAC; avoid over-profile on 2nd side.	3.4.9 What to write into the recipe header Alloy + paste (brand/lot), Air/N₂, Peak/TAL targets, Side (Top/Bottom), Panel mass, and any second-side derate. A one-liner why: e.g., “N₂ to cut QFN voids; TAL 65 s for BGA collapse; second side −8 °C peak.” 3.4.10 Pocket checklist (before you press Start) Alloy chosen matches customer/RoHS requirements. Peak/TAL setlimited to the minimum required for thatwetting/collapse.	Prevents excessive IMC growth and joint brittleness.
Second Side	Profile must be derated (lower peak, shorter TAL) from the first side.	Protects already formed joints from over-cooking.
Low-Temp Risk	Mechanical strength and rework alloy (no copy-paste from another family). Second-sidecompatibility recipeare gentlerdocumented.	Addresses (don’tbrittleness double-cookrisk intermetallics).and prevents unpredictable mixed-alloy joints.
Recipe Header	Alloy Type, Peak/TAL Targets,If usingand Air/N₂ Status, O₂are targetrecorded and sensor cal are set (9.3). AXI/AOI limits alignlocked with alloythe behaviorGolden (e.g.,Recipe.	Full QFNtraceability voidfor %,thermal BGA collapse).conditions.

---

By tuning profiles to alloy chemistry instead of copying defaults, manufacturers avoid hidden reliability traps. The payoff is solder joints that not only pass inspection today but also endure thermal cycles, shocks, and customer use far beyond the factory floor.