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HASL PCB

What is HASL?

HASL stands for Hot Air Solder Leveling. People also call it hot air solder leveling or hot air leveling. In this process, molten solder (which can contain lead) is applied to the PCB surface. Then hot, compressed air is blown to level the solder. The result is a coating that protects copper from oxidation and gives good solderability.
When you do hot air leveling, solder and copper form a copper-tin intermetallic compound where they join. For the process, the PCB is dipped into molten solder. The air knives blow the liquid solder flat before it cools. The air knives cut down the meniscus shape of solder on copper and stop solder bridges from forming.
The basic idea is to use hot air to remove extra solder from the board surface and from holes. What stays is a uniform solder layer on pads, exposed traces, and surface mount lands. HASL is one of the common PCB surface finish methods.

Výhody

  1. After HASL, the composition of the solder coating stays the same. So the coating is consistent and solderability is good. By contrast, electroplated lead-tin alloy coatings can change composition as the plating bath changes. That means the lead/tin ratio in plated layers can vary.
  2. Infrared or hot-oil reflow methods do not fully protect the side edges of traces. HASL coats and fully covers the side edges of traces. This prevents corrosion and broken traces on the PCB. That means boards last longer in storage and use, and the finished electronic products are more reliable. HASL is widely used in SMT processes today.
  3. By changing the air-knife angle, the board rise speed, and other process settings, you can control coating thickness. That makes it easy to get the solder layer thickness you want. It is more flexible than some hot-melt methods.
  4. When boards are made by pattern plating and etching, wave soldering can cause bridging because lead/tin alloy sits on traces. The alloy flow can also wrinkle or lift solder mask. Boards made with HASL have no solder on traces, so bridging and mask wrinkling or peeling are eliminated.

Disadvantages

  1. Copper contamination of the solder pot. In HASL the board is dipped in the solder pot for several seconds. That causes copper to dissolve into the solder. When copper reaches about 0.29% or higher in the solder, the solder loses some flow ability. The solder coating becomes semi-wetting and the board solderability drops.
  2. Lead is a heavy metal and it is harmful to people and to the environment. Many solder coatings with lead were common. Now lead-free solders have been developed and are sold to replace lead-tin alloys in production.
  3. High production cost. A good imported HASL machine can cost more than three hundred thousand US dollars. This makes HASL production cost higher than some hot-melt methods.
  4. Large thermal shock in HASL. The big heat change can warp or bow the PCB substrate and the board can lift. That means HASL causes larger thermal stress.

Control and choice of HASL process parameters

The main HASL process parameters include solder temperature, dip time, air-knife pressure, air-knife temperature, air-knife angle, distance between air knives, and board rise speed. Below is an explanation of how these parameters affect board quality.
  1. Dip time (immersion time):
    Dip time has a big effect on solder coating quality. In dipping, the base copper and the tin in the solder form an intermetallic compound (IMC). At the same time a solder layer forms on traces. This whole process usually needs 2–4 seconds to make good IMC. The longer the time, the thicker the solder. But if you dip too long, the board core material can delaminate and the solder mask can blister. If time is too short, you get partial wetting. That causes local whitening of the solder surface and can make the solder surface rough.
  2. Solder pot temperature:
    The common solder used for soldering PCBs and components is lead 37 / tin 63 alloy, whose melting point is 183°C. When the solder temp is between 183°C and 221°C, its ability to form intermetallic compounds with copper is small. At 221°C the solder enters the wetting range. The wetting range is 221°C to 293°C. Because high temperature can damage the board, you should pick a lower solder temp in the wetting range. Theoretical work finds 232°C as the best solder temperature. In practice, about 250°C is often used as the best temperature.
  3. Air-knife pressure:
    After dipping, a lot of solder stays on the board and most plated through holes get filled with solder. The air knives are there to blow off extra solder and to open up the plated through holes without making hole diameters too small. The energy to do that comes from air-knife pressure and air speed. The higher the pressure and the faster the air, the thinner the solder coating. So air-knife pressure is one of the most important HASL parameters. Usually air-knife pressure is 0.3–0.5 MPa.
    Pressure before and after the knife is usually set so the front is higher and the back is lower. The pressure difference is about 0.05 MPa. You can adjust front and back pressure based on pad patterns on the board to keep IC areas flat and to avoid SMT parts from sticking up. Check the machine manual for exact recommended values.
  4. Air-knife temperature:
    Hot air from the air knife does not change board temperature much and it does not affect air pressure much. But raising air-knife internal temperature helps air expand. So at the same pressure, higher air temperature gives a larger air volume and faster speed. That makes stronger leveling force. Air-knife temperature also affects the appearance of the solder after leveling. When air-knife temp is below 93°C, the layer looks dull. As air temp rises, the dull look reduces. At 176°C the dull look goes away completely. So the air-knife temperature should not be below 176°C. To get good flat solder, air-knife temp is often set between 300°C and 400°C.
  5. Distance between air knives:
    When hot air leaves the air-knife nozzle, its speed slows down. The slowdown goes with the square of the distance between knives. So the larger the gap, the lower the air speed, and the weaker the leveling force. Typical air-knife nozzle spacing is 0.95–1.25 cm. Do not make the nozzle spacing too small, or air friction can harm the board surface. The gap between upper and lower air knives is usually kept around 4 mm. Too large a gap can cause solder splatter.
  6. Air-knife angle:
    How the knife is angled affects solder coating thickness. If the angle is wrong, the two sides of the board can get different solder thickness. Wrong angle can also cause molten solder to splatter and make noise. Usually front and back air-knives are tilted down about 4 degrees. Adjust slightly for specific board shapes and pad layout.
  7. Board rise speed (conveyor or lift speed):
    Another variable is the speed at which the board moves through the air knives. Speed affects coating thickness. Slow speed means more air hits the board so the coating is thinner. Fast speed means the coating is thicker and can even block holes.
  8. Preheat temperature and time:
    Preheat aims to activate flux and reduce thermal shock. Typical preheat temperature listed is 343°C. With 15 seconds preheat, board surface can reach about 80°C. Some HASL lines do not use a preheat step.
Below is a clear table that sums up the key HASL process parameters, the recommended ranges, and short notes. Use it as a quick reference when you set up or test the line.

Process parameters — table with recommended ranges

ParameterRecommended RangeUnitNote
Solder Pot Temp (Lead)245–260°CCommon 250°C. Too high may warp board.
Solder Pot Temp (Lead-Free)280–300°CCommon 290°C. Higher melting point.
Immersion Time2–4sToo long → bubbling. Too short → poor wetting.
Air Knife Pressure0.30–0.50MPaHigher → thinner coating. Front slightly higher (≈0.05 MPa).
Air Knife Temp≥176; common 300–400°CHigher → faster air flow and better leveling.
Air Knife Gap0.95–1.25cmLarger gap → weaker airflow.
Air Knife Angle2°–6° (common 4°)°Affects coating evenness.
Preheat Temp120–180°CBoard surface target 60–100°C.
Preheat Time10–30sReduces thermal shock.
Use these ranges as starting points. Then run test boards and tune one parameter at a time. Check for dull surface, bridging, or delam. If you see a problem, change one item, test again, and record the result.

Leaded HASL vs lead-free HASL

Many people know about HASL, but they may not know solders come in leaded and lead-free types. As electronics evolve, PCB technology keeps improving. Common surface finishes include HASL, immersion gold, electroplated gold, OSP, and so on. HASL comes as leaded and lead-free. Here’s the difference:
  1. Lead-free HASL is more eco-friendly because it does not contain lead. Its melting point is around 218°C. For lead-free HASL, the solder pot temperature needs to be controlled at about 280–300°C; wave solder temperature should be around 260°C; reflow temperature is about 260–270°C.
  2. Leaded HASL is not eco-friendly. It contains lead and its melting point is about 183°C. For leaded HASL the solder pot temperature should be controlled at about 245–260°C; wave solder should be about 250°C; reflow temp around 245–255°C.
  3. Look at the solder surface: leaded HASL looks brighter, lead-free HASL looks duller. Lead-free wetting is a bit worse than leaded wetting.
  4. Lead content rules: lead-free solders have lead below 0.5% while leaded solders have lead up to 37%.
  5. Lead helps improve solder wick and activity during soldering. Leaded solder wire is easier to use compared to lead-free wire. But lead is toxic and long-term exposure is bad for people. Lead-free solder has a higher melting point so solder joints can be stronger.
  6. For PCB surface finish pricing, leaded HASL and lead-free HASL usually cost the same. There is no price difference in most cases.

How to tell if a PCB has leaded or lead-free HASL?

  1. Look at the solder surface. Leaden solder looks bright and shiny. Lead-free solder (SAC alloys) looks more dull. Lead-free wetting is a bit worse than leaded.
  2. Leaded solder is harmful to people. Lead-free is safer. The eutectic temperature depends on the lead-free alloy. For example, SAC (SnAgCu) eutectic is near 217°C, and soldering temp should be eutectic plus 30–50°C. For leaded eutectic (Sn63Pb37), eutectic temp is 183°C.
  3. Lead content: lead-free solders have lead ≤ 0.5%, leaded can have about 37% lead.
  4. Lead raises solder activity so leaded solder is easier to use in soldering. But lead is toxic and bad for health. Also lead-free solder has a higher melting point, which can make solder joints mechanically stronger.

Souhrn

HASL is a common and proven PCB finish. It protects copper and gives good solderability. HASL has pros and cons. It covers trace edges and helps reliability, but it may add thermal stress and cost. Key settings are solder temp, dip time, air-knife pressure, air temp, knife angle, and board speed. Use the table as a guide. Test and tune for each board type. Use lead-free alloys when rules or safety need it.

Často kladené otázky

HASL provides excellent solderability, tolerates multiple thermal cycles, and is one of the most cost-effective surface finishes for standard SMT and through-hole assembly.

HASL can produce an uneven surface (poor planarity) compared with planar finishes, making it less ideal for very fine-pitch components and some BGAs.

HASL is cheaper and gives robust solder joints, but ENIG offers a much flatter, more uniform surface (better for fine-pitch/BGA and planar requirements). Choose based on component pitch and connector/mating needs.

Generally not the first choice for very fine pitch (<0.5 mm) or dense BGAs because of surface topography; many designers prefer ENIG or other planar finishes for those cases.

HASL is usually one of the lower-cost finishes and is fast to apply, but lead-free HASL may require tighter process control and slightly different handling—ask your fab for lead-time impacts.

Specify whether you want lead-free (RoHS) or traditional SnPb HASL, any special pad area or fine-pitch concerns, and request the fab’s DFM feedback about minimum pad-to-pad clearances and planarity limits.

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