With the "HAL lead-free" (hot air leveling lead-free), hot air tinning or HASL (hot air solder leveling) soldering surface, a layer of liquid tin is applied to the exposed copper surfaces. The PCBs are first wetted with flux and then immersed in a vertical solder bath. When the PCBs are removed from the solder bath, excess solder is blown off with hot air. The PCBs are then cooled in a horizontal process section.
What are the advantages of "HAL lead-free"?
"HAL lead-free" is the RoHS-compliant (2011/65/EU) further development of hot air tinning with tin/lead solder, which was already used in PCB production in the 1960s. For "HAL lead-free", KSG uses a bar solder made from an alloy with 99.7 % tin and 0.3 % silver with a melting point of 217 to 227 °C. The tinning temperature is specified at 270 °C in accordance with UL. From a soldering point of view, hot air tinning offers the best solderability of all surfaces. Tin and copper on the connection surfaces form an intermetallic bonding zone (IMC). With proper Storage the PCB manufacturer provides a 12-month warranty on solderability.
Brief info "HAL lead-free"
Alloy: 99.7 % tin | 0.3 % silver
Melting point: 217-227 °C
Tinning temperature according to UL: 270 °C
Irregular thickness of the tin layer due to the process
Hot-air tinned solder connections are characterized by relatively large fluctuations in the layer thickness or no planarity of the surface. This is due to the physically induced solder meniscus formation (see image) caused by the excess solder being blown off. This surface is therefore not suitable for fine-pitch SMT components.
The wetting of solder pads is evaluated according to J-STD-003. The thickness of the tin layer is defined from "covered" with approx. 1 µm at the hole entrance for THT connections to ≤ 40 µm in the center of the pad. The layer thicknesses can be determined destructively by grinding. In the "HAL lead-free" process, the base material of the PCB is subject to high thermal stress due to the large relative temperature increase (from room temperature to 270 °C) of lead-free solder. HAL lead-free is therefore not recommended for standard Tg-FR4 base materials.
Further advantages of "HAL lead-free"
HAL lead-free can be combined with electroplated nickel/gold for plug contacts. Hot air tinning is suitable for press-fit technology. The surface is unsuitable for any wire bonding processes. Compared to the insensitive ENIG and the sensitive chemical tin, HAL lead-free is in the middle. In concrete terms, this means that the surface is quite insensitive to thermal processes that follow the application of the soldering surface, e.g. the curing of lacquers or drying processes of the PCB before component assembly. In a direct comparison of process costs, lead-free hot air tinning is cheaper than soldering surfaces electroless nickel/gold and chemical tin. Hot air tinning has a share of around 20 % of all soldering surfaces used in Europe; this share is trending downwards.
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