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3.4 Alloy-Specific Nuances

SnPb

Every vs.solder SACalloy profiles,brings peakits limits,own thermal personality, and whatreflow intermetallicssuccess meandepends foron reliability.

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Solderingthose isn’t just about melting metal—it’s about matching the thermal personality of the alloy to the needs of the assembly.differences. Tin–lead (SnPb)flows easily and forgives uneven heating, but lead-free SAC (tin–silver–copper)demands soldersmore behave differently under heat,patience with theirhigher own melting points, flow characteristics,peaks and sensitivities tosteadier time andabove temperature.liquidus. TheseBeneath differencesthe shape everything from how easily a joint wets to how quicklysurface, intermetallic compounds (quietly grow at the microscopicpad “bondinginterface, layers”their betweenthickness copperdeciding andwhether tin)joints grow,remain which can maketough or breakturn brittle with age. Even subtle choices—nitrogen use, rework alloy, or micro-alloyed SAC variants—shift long-term reliability. FactorsWhat looks like nitrogena use,cosmetic coppertweak dissolution,on orthe even mixing alloys during reworkline can subtlyecho tipyears performancelater in either direction. Even low-temperature bismuth blends bring unique benefits and trade-offs, especially for heat-sensitive parts. Understanding these alloy-specific nuances means tuning reflow profiles not just for today’s perfect joint, but for a solder connection that will stay strong long after the product leaves the factory.

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3.4.1 Two families, two personalities

  • Sn63/Pb37 (eutectic) melts at 183 °C with a very sharp liquidus. It wets easily, needs less time above melt, and forgives uneven heating. Great for service/legacy builds where RoHS doesn’t apply.
  • Lead-free SAC (e.g., SAC305) melts near 217 °C with a broader “mushy” region. It often wants a steadier soak and a longer TAL to fully collapse BGAs and wet big copper.

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

Alloy

Liquidus

Peak (typical)

TAL (typical)

Notes

Sn63/Pb37

183 °C

205–220 °C

30–60 s

Short, crisp profile; easy wetting; not RoHS.

SAC305 (lead-free)

217 °C

235–250 °C

40–80 s

Smoother ramp/soak; N₂ can help margins.

Low-temp Bi-based (e.g., Sn42Bi58)

~138 °C

165–185 °C

30–60 s

Narrower window; mind brittleness and plastics.

Guardrail: component max-temp (JEDEC J-STD-020) still rules. Never chase solder quality by exceeding part limits.



3.4.2 How profiles differ in practice

  • SnPb
    • Ramp: can be a bit quicker; tombstoning/bridging usually less sensitive.
    • Soak: short or none on even-mass boards.
    • TAL: keep it brief; extra time only cooks flux and grows intermetallics you don’t need.
  • SAC
    • Ramp/Soak: a gentle soak helps equalize ΔT and vent volatiles (voiding); smooth 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: what’s growing at the pad

Every joint forms Cu–Sn intermetallic layers (mainly Cu₆Sn₅, then Cu₃Sn). They’re necessary—but too thick = brittle.

  • What grows them: temperature + time. High peaks and long TAL accelerate growth, in both SnPb and SAC.
  • Why SAC needs attention: SAC’s higher temps naturally push growth faster; its microstructure also contains Ag₃Sn particles that can affect drop-shock behavior.
  • Your control knobs:
    • Don’t exceed peak more than needed; lengthen TAL only as far as defects (HIP/voids) require.
    • Avoid double-cooking on second-side profiles; use a gentler second pass.
    • For harsh environments, consider doped SAC (Ni/Ge/Bi “micro-alloy” variants) that slow IMC growth and improve 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 sane bands,
  • Re-check 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.
  • Mechanical toughness can drop (brittleness). Validate drop/shock or connector cycles if your product sees abuse.
  • Watch mixing: rework with SnPb or SAC changes local composition and melting points—standardize your rework alloy to avoid mixed joints.



3.4.6 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, but 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): good wetting/cosmetics; solid 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, better thermo-mechanical fatigue—useful for under-hood, base-station, or high-temp duty.
    Pick based on real failure modes: if your field returns are thermal-cycle cracks, micro-alloyed SAC pays; if they’re impact/drop, try lower-Ag or process tweaks.



3.4.8 Quick playbook (defect → alloy/profile moves)

  • BGA HIP in SAC → lengthen TAL (steady), smooth soak; if marginal, add N₂ and trim peak a bit.
  • QFN voiding in SAC → 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 set for that alloy (no copy-paste from another family).
  • Second-side recipe gentler (don’t double-cook intermetallics).
  • If using N₂, O₂ target and sensor cal are set (9.3).
  • AXI/AOI limits align with alloy behavior (e.g., QFN void %, BGA collapse).



Bottom

By line:tuning treatprofiles SnPbto likealloy achemistry quick,instead crispof bakecopying defaults, manufacturers avoid hidden reliability traps. The payoff is solder joints that not only pass inspection today but also endure thermal cycles, shocks, and SACcustomer likeuse afar patient, even roast. Hitbeyond the alloy’sfactory peak/TAL without over-cooking, and remember that every extra degree and second grows intermetallics—which decide how your joints age. Profile to the chemistry, and your solder does what it should now and a year from now.

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