2.15 Alloy-Specific Nuances

Every solder alloy possesses a unique thermal and metallurgical signature that dictates its required reflow profile. Profile uniformity and peak temperature must be tuned to the alloy's melting characteristics, which directly influence wetting, voiding, and the growth of Intermetallic Compounds (IMCs). The failure to respect these alloy-specific differences can lead to hidden reliability risks that manifest years later as brittle joint fractures.

3.4.2.15.1 Alloy Families: Thermal Requirements and Reliability

The choice of alloy sets the baseline for the entire thermal profile and defines the joint's mechanical properties.

Alloy Family	Liquidus Temp	Peak Profile Range	Key Reliability Feature
Sn63/Pb37 (Eutectic)	183˚C (Sharp melt)	205–220˚C	Highest Wetting Speed. Forgives uneven heating (∆T). Not RoHS.
SAC305 (Lead-Free)	217˚C (Mushy zone)	235–250˚C	High Thermal Fatigue Resistance. Demands a slower ramp/soak and longer TAL.
Low-Temp Bi-Based (e.g., Sn42Bi58)	≈138˚C	165–185˚C	Component Protection. Narrower process window. Lower mechanical strength (brittleness).

Mandate: Profile settings from one alloy family must not be used for another. The SAC alloy requires significantly higher energy and time above liquidus compared to eutectic SnPb.

3.4.2.15.2 Profile Tuning: Practical Differences

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

• SnPb Profiles: Can be run with a quick ramp and a brief Time Above Liquidus (TAL). Excessive TAL is wasteful and increases IMC growth without improving joint quality.
• SAC Profiles: Require a smoother ramp and 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).OpEx.
• Low-Temp Bi Profiles: Due to their low melting point, these require a gentle ramp-to-peak with a minimal soak to prevent the creation of solder balls and oxidation during extended dwell times.

3.4.2.15.3 Intermetallic Compounds (IMCs) and Reliability

IMCs (primarily Cu₆Sn₅) are the brittle, necessary bond layers formed at the copper-solder interface. Their thickness dictates the long-term joint integrity.

• Growth Driver: IMC growth is accelerated by high temperatures and long TAL.
• Reliability Risk: An excessively thick IMC layer (often due to over-profiling) leads to a brittle joint that is susceptible to failure under thermal cycling, drop-shock, or long-term vibration.
• SAC IMC Nuance: SAC alloys require higher peak temperatures, which inherently accelerates IMC growth. Furthermore, the microstructure of SAC joints contains Ag₃Sn particles that influence shock performance.
• Control Action: Limit peak temperature and TAL to the minimum necessary to achieve complete wetting and BGA collapse.

3.4.2.15.4 Special Cases and Mixed Alloys

1. Second-Side Reflow

When processing the second side 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 slightly lower peak temperature and shorter TAL to minimize the thermal exposure of the components already soldered.

2. Low-Temperature Bi-Based Alloys

Bi-alloys sacrifice mechanical toughness for low temperature. This must be factored into product qualification. Furthermore, mixing Bi-based solder with standard SnPb or SAC during rework or in a mixed-technology application can create an alloy with a different, unpredictable melting point, leading to reliability concerns. Rework must use a standardized, compatible alloy.

3. Micro-Alloyed SAC

For products requiring extreme long-term reliability (e.g., automotive, rugged industrial), specialty SAC alloys with micro-additives (Ni, Ge, Bi, Sb) are used. These alloys are engineered to slow down IMC growth and improve resistance to thermo-mechanical fatigue (cracking under temperature cycles). The choice of SAC variant must align with the product's dominant field failure mode.

Final Checklist: Alloy Profile Requirements

Checkpoint	Requirement	Rationale
Profile Baseline	Profile is derived from the paste vendor's datasheet for the specific alloy.	Avoids blind profiling and ensures flux activation.
IMC Control	Peak/TAL limited to the minimum required for wetting/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 compatibility are documented.	Addresses brittleness risk and prevents unpredictable mixed-alloy joints.
Recipe Header	Alloy Type, Peak/TAL Targets, and Air/N₂ Status are recorded and locked with the Golden Recipe.	Full traceability for thermal conditions.