Short answer: choose the solder system from the product's approved material requirements, destination-market obligations and component compatibility. Then qualify the assembly process for that combination. SAC305 and tin-lead Sn63Pb37 have different melting behavior, but neither an alloy name nor an oven setting establishes a reliable build. Lead-free solder also does not, by itself, prove that the finished product complies with RoHS.
For a buyer, the useful question is: which solder, components, process and release evidence belong to this product revision? That gives an assembler something definite to review and quote.
1. Specify the alloy rather than only saying lead-free
Lead-free describes a category. Different alloys in that category have different melting behavior. Kester's alloy temperature chart provides these examples; the temperatures are material properties, not recommended oven peaks.
| Alloy | Composition | Melting temperature or range |
|---|---|---|
| Sn63Pb37 | Tin-lead | 183°C |
| SAC305 | Sn96.5Ag3Cu0.5 | 217–220°C |
| Sn42Bi57Ag1 | Tin-bismuth-silver; no lead in the stated formulation | 138°C |
The bismuth example shows why “all lead-free solder needs a higher temperature” is too broad. A lower melting temperature is not approval to substitute that alloy into an existing design. Record the exact paste or wire product, alloy and intended operation; do not let a purchasing substitution silently become a process change.
2. Keep alloy selection separate from RoHS evidence
The European Commission's RoHS overview describes restrictions covering ten substances in electrical and electronic equipment. Selecting solder without lead addresses only part of the material question. Components, finishes and other product materials still need the evidence applicable to the finished equipment and its market.
If a project requests leaded solder, have the responsible compliance owner document the product scope and legal basis before release. The Commission's implementation guidance explains that exemptions have defined conditions and are reviewed over time. A legacy design, industrial label or customer preference does not establish an applicable exemption. Confirm its precise scope and current status rather than copying a declaration from an earlier order.
3. Separate melting temperature from the production profile
A solder melting point is not a complete reflow recipe. TI's MSL Ratings and Reflow Profiles distinguishes component classification limits from customer board-assembly conditions. Production settings must respect the paste supplier's instructions and the ratings of every relevant component.
Ask for a measured profile for the representative assembly, including the measurement locations, peak temperature and time above the chosen alloy's liquidus. Review ramp, soak and cooling conditions against the approved instructions. A package's maximum classification temperature is a limit, not a target for the board. Keep moisture-handling requirements in the same release plan; see the MSL material handoff guide.
4. Check compatibility at the component and package level
A component described as Pb-free is not automatically suitable for every tin-lead process. Conversely, an older tin-lead component should not be assumed suitable for a hotter lead-free process. Verify the exact ordering code, termination or ball composition and approved assembly conditions.
TI's Forward/Backward Compatibility note treats leadframe packages differently from BGA and DSBGA packages and identifies combinations it does not recommend. Use that distinction as a reason to check package-specific evidence, rather than extending one supplier's compatibility table to an entire BOM. For hidden joints, connect the decision to the BGA inspection and process plan.
5. Define what a qualified build must demonstrate
Do not accept “leaded is more reliable” or “lead-free is better” as a product qualification plan. Start with the actual service environment, expected life, failure consequences and customer acceptance requirements. Identify the evidence needed to approve this assembly.
A practical review can request representative first articles, recorded process settings, agreed workmanship criteria, electrical test results and any product-specific reliability testing. State who reviews a failure and who can authorize rework. Keep the selected alloy and repair materials traceable so a later service operation does not introduce an undocumented change.
6. Compare quotations on the same process scope
Ask each supplier to price the same alloy, material package, inspection coverage and test requirements. Request separate assumptions for engineering characterization, special materials, cleaning, rework and required records. Treat those as questions to resolve, not automatic surcharges or guaranteed savings.
An apparent unit-price difference is difficult to interpret when one quotation includes qualification work and another assumes an already approved process. Confirm the scope before choosing the cheaper line item.
7. Put the decision into the RFQ
- Identify the product revision, target markets and owner of compliance decisions.
- State the approved alloy and paste or wire product, or ask for a documented proposal.
- Supply component ordering codes, package information and known process limitations.
- Include board finish, stack-up, assembly sequence and repair policy.
- Define inspection, test, qualification and record-retention expectations.
- Name who can approve a material or process substitution.
Link this information to one controlled release package. The PCBA quote package checklist covers the supporting design files; this solder decision supplies the material and process assumptions those files cannot communicate on their own.
When requesting a PCBA engineering review, include the proposed solder system and unresolved compatibility questions alongside the BOM and fabrication data.