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HASL stands for hot air solder levelling, also written hot air solder leveling. After copper preparation and fluxing, the finishing line applies molten solder and uses air knives to remove excess solder. The remaining coating protects exposed copper until assembly and provides a solderable surface. The deposited alloy and its interface with copper are part of the finished land.

HASL is not immersion tin. Immersion tin is a thin chemically deposited finish; HASL is a solder coating shaped by wetting and hot-air levelling. The two can look silvery in photographs but behave differently during fabrication and storage. Nor is HASL the same operation as wave soldering components: a bare-board finish is being applied before the assembly is populated.

The label HASL alone does not tell the purchasing team whether lead is present. Specify either a defined SnPb route or an agreed lead-free alloy. Keep that choice consistent between the fabrication drawing, quotation and assembly release. Use the HASL versus lead-free HASL comparison when a legacy drawing or product requirement makes the alloy decision the main question.

Use heights measured from one agreed datum. Span = highest reading 鈭� lowest reading. Enter a limit established for your assembly, or leave it blank for a descriptive result. The tool cannot prescribe a package-specific limit.

Begin with the most demanding land pattern on the board. A design dominated by through-hole connections and relatively large SMT pads may tolerate the finish topography well. A compact area-array package can make local height variation more important. The correct decision depends on package geometry, component coplanarity, stencil design, paste release, board support and the demonstrated assembly process.

Do not qualify HASL from a universal 鈥渕inimum pitch鈥� number. Two components with the same nominal pitch can have different land sizes, termination geometries and process margins. Review the footprint that constrains the build, then agree what the finished pads must look like and how this will be checked. A prototype assembly is useful when the design is near the established process window.

Large thermal pads, small vias in pads, tall through-hole barrels and thin board constructions can introduce separate issues. Treat coverage, hole clearance and warpage as distinct checks. A finish can have good solder wetting but still create a mechanical or coplanarity problem. For boards requiring a flatter conformal finish, compare ENIG or immersion tin with the actual assembly and storage plan.

A single broad thickness range is a weak acceptance description for HASL. Solder does not remain at one uniform height on every feature. Wetting, pad area, edges, holes, equipment orientation and levelling conditions can influence the final shape. The functional requirement is a solderable, sufficiently covered surface with appropriate geometry for the assembly.

Identify the pads and holes that matter most. If the assembly needs a pad-height variation limit, define the locations, reference plane and measurement method with the assembler. The planning tool can calculate maximum minus minimum from entered height readings and compare that span with your entered limit. It cannot determine the correct limit or separate finish variation from board warpage without a suitable measurement setup.

For through-holes, specify the finished-hole requirement used by the connector or component. Do not compensate by arbitrarily enlarging every drill. Copper plating, solder coverage and machining all contribute to the delivered hole. The fabricator should translate the finished requirement into the process allowance. Keep drawing dimensions clearly identified as finished values where that is the design intent.

A shiny HASL surface is not a solderability test, and a dull surface is not automatically a defect. The review should address intended coverage, bridging, blocked or reduced holes, residues, handling damage and the features selected for dimensional or topographic checks. The inspection method must match what the product needs, not simply what is easy to photograph.

For a rigid board, the purchase order may invoke IPC-6012 with a product class and revision. Solder alloy requirements and printed-board solderability are different topics, associated with J-STD-006 and J-STD-003 respectively when invoked. The identifies their scope. These references do not create one universal HASL thickness requirement and do not replace a board-specific drawing.

Discuss whether solderability evidence is needed after an agreed conditioning or storage sequence. An as-received sample and an aged sample answer different questions. Keep the lot, test condition and acceptance basis together. If a complaint occurs later, these records are more informative than a generic statement that all boards were visually checked before packing.

A useful draft note might read: 鈥淗ot air solder levelled finish using the agreed lead-free alloy. Apply the ordered board performance class and revision. Maintain solderable coverage and finished-hole requirements shown on the drawing. Verify the identified coplanarity-sensitive pads using the agreed measurement plan.鈥� Replace every open item before production release; the note is a starting structure, not a complete acceptance specification.

Add the assembly process, the most demanding footprint and the expected storage arrangement to the enquiry. If a previous lot had uneven paste contact or restricted holes, supply photographs, locations and measured dimensions. Avoid asking the fabricator merely to make the next batch 鈥渇latter鈥�; define what feature needs improvement and how the revised result will be assessed.

The page tools are optional. You can send Gerber or other fabrication data, drill files, stackup, quantity and the existing finish note directly to 吃瓜tv for a finish review. Include the question you want answered: alloy selection, HASL feasibility, inspection scope or a like-for-like alternative quote. This lets the reply address the engineering decision rather than just repeat a unit price.