Lead-Free PCB Assembly Services

What Is Lead-Free PCB Assembly?

Lead-free PCB assembly is a type of PCB assembly built using lead-free solder materials, soldering processes that have been controlled in a compliant manner, and an inspection process to ensure the final PCB assembly itself meets the requirements set forth by RoHS/REACH-bound electronics. "Lead-free" refers to compliance with RoHS requirements, not a clean or pure lead-free product. RoHS establishes a maximum lead concentration limit of 1000 ppm (<0.1% by weight in homogeneous material). Therefore, even though a finished PCB assembly may contain trace lead from the plating process, components, or alloy impurities, it is not disqualified from being compliant under RoHS.

What Controls The Finished Assembly?

Control of the finished assembly for a lead-free PCB assembly project is achieved by the assembled PCB itself; however, it is not limited to the assembled PCB, as it includes the solder material used, the PCB finish, the components used, the BOM, the MBR for the assembled PCB, and the inspection/verification record for the finished PCB assembly. Each of these items for the finished PCBA must link back to the same job and lot number.

The finished assembly will be considered compliant to RoHS and REACH based on the soldering process and the components used, not necessarily the bare PCB. Therefore, the assembly decision should take into account the compliance status and how the solder joints will perform on the delivered PCB assembly.

Where Lead-Free Compliance Gets Triggered

There are three main factors that can trigger compliance with lead-free PCBA: regulatory compliance (the product entering a market subject to RoHS/REACH regulations), contractual compliance (a customer's specification on restricted substances), and policy compliance (a brand's requirement for environmental compliance before shipping).

Electronics used in automotive, medical, industrial, IoT, and lighting applications usually experience multiple triggers simultaneously, and these combinations affect the acceptance criteria in each application sector.

What Counts As RoHS-Compliant PCBA

PCBAs that are RoHS-compliant are assessed based on their compliance with the limits on restricted substances, material declarations, solder-selection specifications, assembly-process history, and final assembly documentation. The same lead limit defined above establishes the baseline for each build. However, the lead limit is recorded within the job record linked to a specific production lot, solder alloy, and BOM revision; it should not exist as a separate item in the environmental claim.

Lead-Free Solder Materials and Process Controls

When using lead-free solder, a reliable solder joint can be made when the selection of solder alloy, SAC or SnCu, paste-handling procedures, flux system, PCB finish, and thermal processing are controlled as a single build condition. Lead-free PCBA typically uses either SAC or SnCu solder alloys, and which is the correct alloy depends on the soldering type, such as SMT paste, wave, selective, or manual, the tolerance of the components, the thermal mass of the PCB, and the finish used.

Lead-Free Solder Alloys

SAC305 (Sn96.5/Ag3.0/Cu0.5) is SUGA's standard lead-free solder for SMT reflow processes, while SnCu alloy (generally Sn99.3/Cu0.7) matches wave soldering and solder wire applications. These alloys define wetting characteristics, thermal properties, and compatibility for touch-up soldering operations; therefore, SUGA utilizes the preferred soldering method in combination with the corresponding solder alloy for each soldering step, i.e., SAC alloy paste for reflow, SnCu solder bar for wave soldering, and SnCu solder wire for touch-up soldering. This avoids forcing one alloy designation across the entire PCB.

How Paste Handling Affects Soldering

Before being processed through a reflow oven, solder paste is delicate and can be adversely affected by rapid temperature increases, excessive exposure time, or printing outside the accepted stencil print window. If any of these occur, the result can be inadequate solder, solder bridging, or poor wetting; thaw time, acceptable print times, stencil print window, and stencil print placement must all fall within the usable paste window. Only storing at the appropriate solder paste temperature does not guarantee that an acceptable joint can be formed to meet requirements.

Flux handling requires similar diligence; the flux used must be matched to the solder paste system, the cleaning process, and the ionic cleanliness target for the assembly. It is easy to use no-clean flux; however, excess flux residue must also be suitable for the operating environment of the product.

Surface Finish Choices

Surface finish can affect solderability, storage ability, and thermal properties of the printed circuit assembly. Lead-free HASL is a suitable surface finish option for assemblies with a component pitch and thermal exposure that permit hot-air leveling; OSP can also be considered an assembly timing option, because it protects the exposed copper pads until the assembly has been completed.

When considering fine-pitch or storage-sensitive assemblies, ENIG should be the preferred surface finish option. For these products, flatness, shelf life, and solderability are paramount to the assembly process.

Which Temperature Window Fits The Board?

The lead-free soldering process has a higher thermal window than that used during lead-based solder processing. The two most popular lead-free soldering methods are wave soldering and selective soldering, and both have different temperature reference points. Wave and selective soldering generally use a solder pot temperature of 250–280 degrees Celsius, whereas the peak temperature for SAC alloy during reflow is usually between 240–260 degrees Celsius for a period of 30–90 seconds. The process temperature table provides a guideline for these two groups of data. Regardless of which method is used, the final reflow profile still depends on several factors, including PCB mass, component mix, and surface finish.

A dense printed circuit board, moisture-sensitive components, large copper fill areas, OSP surface finish, or a combination of SMT and PTH assembly can all shift the reflow profile. The final reflow profile should allow complete wetting of the solder joint without exceeding the allowable mechanical and heat stress applied to the components or laminate. Therefore, use the target temperature window for optimal joint wetting without inducing any unnecessary component or laminate stress.

Solder Joint Inspection And Reliability Controls

Reliability of lead-free PCBA begins before the soldering operation and continues through final inspection. Each step in the inspection process has its own goal: confirming compatibility of materials, ensuring paste stability, verifying joint visibility, testing electrical continuity, validating powered characteristics, identifying long-term risk, and documenting any rework.

BOM Material Check

The first step in achieving lead-free reliability is identifying all of the components that will make up the BOM: component MPN, the status of the components, the PCB material, the MSL exposure, the surface finish of the PCB, and the solder alloy used in the assembly. The thermal exposure of high-Tg material may be an important factor if you are operating in a tight thermal tolerance application, and maintain the RoHS/REACH documentation for the components and solder.

Solder Joint Inspection

After the BOM has been identified, you have to choose your solder joint inspection method. Depending on how a joint will be inspected, how visible it is, and whether the paste was applied properly, you will choose different inspection methods to verify correct assembly. PTH and wave soldering will require different inspection considerations from SMT soldering: hole fill, bridging, non-wets, and copper dissolution are of significantly greater concern than appearance. For each manufactured PCB assembly, you will specify on the purchase order which class of acceptance you are adhering to. IPC-A-610J Class 2 / Class 3 and IPC J-STD-001J define the expected acceptance requirements.

Tin Whisker Risk

Tin whisker growth depends upon several factors: finish, alloy, storage conditions, mechanical stress, and soldering practices. Matte-tin finishes under the worst types of mechanical stress, such as press-fit connectors and plated mounting hardware, present the greatest risk of tin whiskering; conformal coating and post-plating annealing are two commonly used ways to mitigate the risk of tin whiskers for those applications. For electronics that require long service life, harsh environments, or are very sensitive to reliability issues, it is critical to ensure that the finish that is being used is appropriate for the soldering plan, inspection processes, and manufacturing records.

After-Soldering Electrical Checks

A post-solder inspection electrical test confirms that a circuit functions properly. Although a joint may look good under AOI, it may still fail continuity tests, load tests, or powered tests.

Rework is a documented exception to requirements. Desoldering lead-free solder will require more thermal process control. Controlling the heat applied, the flux used, the technician's process, the condition of the pads, and recording all of these factors protects the PCB from undue damage during rework.

How To Choose A Lead-Free Assembly Supplier

Selecting a supplier for lead-free assembly should include evaluating the supplier's ability to maintain compliance documentation, soldering techniques, inspection, application requirements, separation of leaded and lead-free materials, testing, and packaging. It is best to assess the supplier based on actual records compiled from at least one real job rather than on their marketing capabilities.

Compliance And Certification Records

Compliance records are useful only when they are tied to the specific job — certifications such as ISO9001:2015 and UL, and workmanship standards such as IPC-A-610J / IPC J-STD-001J when specified, should be kept next to the production materials and final PCBA records from that job code. A compliance folder without a specific job number is paperwork, not supporting evidence; request that the supplier in question demonstrate actual evidence from a real job compliance package from beginning to end.

Process Capability

SUGA provides lead-free PCBA assemblies from PCB build through component procurement and assembly, to functional testing and delivery of the product across the full range of forms, including rigid, flexible, and rigid-flex boards, from short runs to large-volume builds. This coverage extends to turnkey, partial-consignment, and consigned assembly, as long as clear material and process controls are maintained as the order moves from prototype to repeat build.

How Are Leaded Builds Kept Separate?

SUGA can provide both leaded and lead-free PCB prototypes with clearly separated solder identities: clear identification of materials, work instructions, labels, and traveler records provides the ability to identify the build condition before actual production begins. This is especially critical when working on a legacy project and a RoHS-compliant project through the same production flow at the same time.

Reliability Records

Reliability records should be matched to the risks that the PCB boards carry — a solder joint that is visually acceptable under AOI inspection is not sufficient if there is a need for powered or electrical verification that the board functions correctly. IPC-A-610J and IPC J-STD-001J align acceptance criteria and expectations for soldered joints with the soldering process referenced in the purchase order. SUGA links solder-joint and soldering-process expectations to the actual material, soldering, and testing conditions of the product.