4 Advice to Choose a lead-free hasl

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There is a huge demand for the replacement of tin-lead (Sn/Pb) solders with lead-free solders in the electronics industry because of health and environmental concerns. Lead-free HASL is a popular finish that provides the most durable finish for PCBs.

What is lead-free HASL PCB surface finish?

Lead-free HASL is a type of surface finish that uses lead-free alloys instead of tin-lead (Sn-37Pb) eutectic alloys. Initially, molten solder is applied on a board (after cleaning and fluxing) to achieve wetting and excess solder is scraped with the help of air knives (a tool that is set at a temperature above the melting point of the solder). After the solder solidifies, the circuit board is passed through the washer where the solder flux residues are removed.

There are two methods of applying lead-free HASL:

• Vertical process: Vertical process involves a panel of boards mounted on a frame that moves vertically downward into the solder bath, holds for a certain time (usually 2-3 seconds), and then lifts the panel through the operating air knives. These knives are adjusted to clear through-holes and vias while leaving a sufficient thickness of solder on all areas where a solderable finish is essential. After the solder cools and solidifies, the panel passes through the washer/dryer to remove flux residues. The operator then inspects the quality of the finish.

• Horizontal process: In the horizontal process, the board is flooded with solder applied through nozzles or between rollers on the top and bottom of the panel before being moved between hot air knives placed above and below the panel.

Usually, HASL lines are of vertical design and deliver a high-quality finish. Fully automated vertical lines can deliver rack-to-rack performance including fluxing, preheating, washing, and drying. The finish in the horizontal process is more uniform in thickness compared to the vertical process.

The lead (Pb) level in lead-free solder is less than 0.1%. Castin (a composition of Sn 93­ 98%, Ag 1.5-3.5%, Cu 0.2-2%, and Sb 0.2-2%) alloy and SnCu 0.7 are widely used in this process.

 

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Why is lead-free HASL used?

There are several problems associated with the HASL process as given below:

• Damage due to additional thermal deviations
• Coplanarity issues because of  pad to pad variation in coating thickness

• Stencil gasketing issue related to the difference in coating thickness and mushroom cap profile (of the coating) found on small pads.

• Heat and fumes associated with the process were considered out of place in modern board shops.

The difference in the solder coating thickness of a lead-free HASL finish is usually about half that of the tin-lead HASL finish. The coplanarity is also good in this finish. The modern HASL line is designed to provide the operating conditions in line with other equipment in modern board shops to avoid issues with heat and fumes.

Coating thickness

The solder coating profile is defined by the volume of solder left on the pad after passing the board through the hot air knife and surface tension forces. Due to surface tension, the coating has a tendency to be thicker on smaller pads.

Because of the higher surface tension of lead-free solder, the coating formed in the HASL process is both thinner and uniform compared to tin-lead solder formed in similar conditions.

The coating thickness can be checked by XRF (X-ray fluorescence spectroscopy) technique.

Whisker formation

Compressive stress is the driving force for whiskers (a thin filament protruding from a board surface having tin as a final finish). According to the JESD22A121 specification, whiskers appear on the hot-dipped lead-free finish on copper only in areas where compressive stress is intentionally introduced and under the most severe conditions such as the combination of elevated temperature (60°C) and high humidity (87%RH).

Lead-free HASL provides a corrosion-resistant finish that can assure solderability over long periods.

Introduction of Application of Lead-free Surface Finish Techniques:

Whether lead-free HASL can be popularized will depend on the results of the scientific study and its promotion and influence, but its smooth surface and high temperature process problems may be obstacles. OSP technology is widely used when a flatter surface is required, long shelf life is not required, and multiple heating applications are not required. OSP is likely to replace HASL technology in a large number of general application fields, because OSP costs less than HASL and has made good progress in its weakest heat resistance without the disadvantages of high-temperature processing.

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ENIG technology is most often used when protection requirements are higher, when contact and bonding applications are required, and when quality and integration applications are required. But pressures of cost and the rise of I-Ag could replace ENIG to some extent. If the I-Ag technology can be adopted by users in bonding applications, its cost advantage will put pressure on ENIG. Some of the higher-quality products may be more inclined to use I-Ag technology, and sometimes ENIG technology. The following table is a comparison of the performance of several surface treatment processes.

Influence of Reflow Times on PCB Performance of Surface Finish Process:

The weldability of ENIG did not change much in the repeated reflow welding, but the weldability of HASL, OSP and Ag declined sharply. It was found that after 4 reflowing, the average diameter which is generally 2.696um of  particles at the OSP interface was 30% higher than that at the I-ag interface which is generally 1.945um, mainly due to the interface a large number of thin sheet  blocks are generated to weaken the mutual diffusion between Cu and Sn in the alloy, thus thinning the thickness of the interface layer. The solderability of Sn decreases most rapidly due to the formation of Sn-Cu compounds and the oxides formed by Sn.

Influence of Nitrogen Protection on PCB Performance of Surface Finish Process:

For the combination of Sn-Pb solder and PCB surface protection layer,  protection has a significant role in promoting wetting and spreading. For lead-free solders, nitrogen protection does not necessarily have the same effect. For example, solderability of ENIG, OSP and I-Ag are all enhanced in  atmosphere, but among these, OSP and I-Ag vary little and vary with the test method. If HASL (Sn-Cu and SAC) and l-Sn is with a volume fraction of  or more in oxygen, it would increase with the decrease of residual oxygen content, while If HASL (Sn-Cu and SAC) and l-Sn is with a volume fraction of which is less  in oxygen, it would not change much. Generally speaking, the activity of solder has the greatest influence on wettability of solder, but the effect on ENIG coating is not very prominent. On the contrary, it shows better weldability in nitrogen environment. Therefore,  protection in wave soldering is better than that in high-activity solder. The following sheet is the spreadability of solder alloy on different coating surfaces after different reflowing times under the air  environment.

The following sheet is the wettability of solder alloy on different coating surfaces after different reflowing times under the air  environment.

As can be seen from the table, when the number of reflowing is small, the spreadability of the surface coating mainly depends on the coating itself,  protection does not have a great impact. When the number of reflowing is large,  protection has a certain impact, and it also shows that the failure of surface finish with Sn is the fastest.

Conclusion:

The solderability of I-Sn coating degrades rapidly and "whisker" phenomenon is easy to occur. Compared with lead-free solder, there is little difference in solderability and little spread rate, while there is a great spread rate for solder with lead. HASL coating is limited by poor surface smoothness and "whisker" problem. But due to SAC and Sn-Ag was often chosen in the lead-free surface finish, which was used for both good tolerance and low defect rate, mainly used in non-fine-spacing assembly. ENIG coating is widely used in fine spacing, bonding, high reliability and multiple reflux processes due to its good weldability, low dependence on interfacial compounds and smooth surface. However, due to the high cost, complicated process and the phenomenon of "black welding plate", the I-Ag coating has been widely used. OSP coating short-term storage, the lowest cost, mainly used in simple products.

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