HASL or ENIG: Selecting a Surface Finish for Assembly

A surface finish exists to keep the copper of a pad solderable from the end of fabrication to the moment of assembly, and each finish does that in a different way and for a different length of time. HASL leaves a layer of solder on the pad, while ENIG leaves a thin gold layer over nickel, and the two behave differently in printing, in reflow and in rework. Both are in production use and both work, and the difference lies in what the assembly process has to tolerate.

The choice is a process decision rather than a purchasing decision. A finish that suits a hand-assembled board with generous apertures can be a poor choice for a fine pitch product, and a finish chosen for planarity alone can introduce a different set of constraints on the assembly line.

What a Surface Finish Has to Do

The finish has three duties. It has to protect the copper from oxidation before assembly, it has to present a surface that molten solder will wet, and it has to survive the thermal excursion of reflow without degrading. A finish that fails the second duty produces joint defects, while one that fails the third produces cosmetic problems and, in time, reliability ones.

The substrate matters as much as the chemistry. Some finishes are deposited from a solution that reacts with the copper, others are deposited over a catalyst, and the condition of the copper before deposition decides how uniform the result will be. What the process actually produced is confirmed by the methods in the notes on coating thickness measurement.

Hot Air Solder Levelling in Practice

HASL dips the board in molten solder and then removes the excess with hot air, leaving a coating that follows the pad and the hole wall. The process is robust, tolerant of a dirty copper surface and well understood, and it produces a finish that wets readily on almost any assembly line.

The limitation is thickness control. The air knife produces a coating whose thickness varies across a panel and between a small pad and a large plane, and it leaves a domed surface rather than a flat one. That dome is the reason HASL is difficult to use with fine pitch apertures, because the paste deposit has to accommodate a surface that is not planar.

Electroless Nickel Immersion Gold in Practice

ENIG deposits a nickel layer that carries the solderability and a thin gold layer that protects the nickel from oxidation during storage. The nickel is the functional layer, and the gold is largely consumed into the joint during reflow, which is why the gold layer is kept thin. Its thickness is a process control variable rather than a design choice.

The process is more sensitive than HASL. A nickel bath that is out of balance deposits a layer with poor adhesion, and the failure appears later as a joint that separates at the pad. Bath control and its measurement are described in the notes on current density control.

Planarity and Fine Pitch Assembly

Planarity is the property that separates the two finishes at fine pitch. A flat pad lets a stencil sit on the board with a defined gap and allows the paste to release cleanly, while a domed pad changes the gap locally and the release with it. A stencil that sits flat prints consistently, and a stencil that rocks prints differently around every pad.

The effect on the printed volume is small for a large aperture and significant for a small one, because the same absolute change in gap is a larger fraction of the deposit. Print volume is discussed in the notes on solder paste volume measurement.

Solderability and Shelf Life

Solderability is the ease with which the finish wets, and it decays with storage. A HASL coating oxidises slowly and usually survives a long storage period and several thermal cycles from rework, provided it is packed sensibly and handled cleanly. Wetting after a long storage period is the property that decides whether the finish was worth its cost.

An ENIG surface resists oxidation well in storage, and it is vulnerable to contamination instead. A board left exposed to a workshop atmosphere where sulfur compounds are present develops a change in the gold surface that reduces wetting, and the effect appears on the pads that were handled most.

Rework, Handling and Inspection

Rework behaviour differs in a way that matters on a mixed-technology product. A HASL pad can be reworked repeatedly, because the coating is solder and can be reflowed again, while an ENIG pad loses its gold during the first reflow and depends on the nickel beneath for the second.

Inspection differs as well. A HASL surface has a variable appearance that resists automated thickness measurement, while an ENIG surface can be measured directly and its appearance is a reliable indicator of the process only in combination with a measurement.

Cost and Availability Over a Product Life

Cost is influenced by the process rather than the material alone. HASL is a mature process available from most fabricators, while ENIG requires tighter bath control and is priced for it, and the difference is easy to justify on a fine pitch product and hard to justify on a simple one.

Availability over the life of a product matters more than the price of the first order. A product with a ten year life should be designed for a finish that several suppliers can produce, because a finish that only one source offers becomes a problem when the product is still being built and the source is not.

Qualification and Incoming Checks

Qualification should be based on assembly trials rather than on a certificate. Printing, placement, reflow and a cross section of the joints together show whether a finish behaves in the process, and a solderability test on incoming boards shows whether the surface has survived transport and storage.

Incoming checks are kept simple: a wettability test, a thickness measurement where the finish allows it, and a visual check for contamination. The results are recorded per lot, because a change in finish condition is easier to see in a trend than in a single reading. A certificate describes the process, while a test on the board describes the board.

Common Selection Mistakes

The first mistake is choosing a finish from a previous product without considering the pitch and the aperture sizes of the new one. The second is choosing on planarity alone and ignoring what the finish does during the second and third reflow cycles of a double-sided assembly.

The third is treating the finish as an interchangeable detail during a cost review. A finish change alters the print window, the reflow window and the rework procedure, and it should be qualified as a process change rather than as a substitution of materials. That review is cheap at the design stage and expensive once the assembly line has been qualified.

FAQ

Is ENIG always better for fine pitch? Its planarity helps, and the assembly window it creates is narrower than that of a solder coating in some respects. The choice depends on the aperture sizes and on the number of reflow cycles.

Can a HASL board be used for a mixed assembly? Yes, on products with generous apertures. The domed surface reduces the printing window and the reduction grows as the aperture shrinks.

Does the gold layer carry the solderability? No. The nickel beneath carries it, and the gold protects the nickel until the board is assembled.