At present, the common PCB surface treatment processes in domestic plate factories include: HASL (hot air solder leveling), OSP (anti oxidation), ENIG, electroplated gold, etc. of course, there will be some special PCB surface treatment processes in special applications.

1. HASL, hot air solver leveling

Hot air leveling (tin spraying) is a process of coating molten tin (lead) solder on the surface of PCB and leveling (blowing) with heated compressed air to form a coating layer which is resistant to copper oxidation and provides good solder ability. Copper tin intermetallic compound is formed between solder and copper in hot air leveling. PCB should sink in the molten solder during hot air leveling; the air knife should blow the liquid solder before the solder solidifies; the air knife can minimize the crescent shape of solder on the copper surface and prevent solder bridging.

Lead spray tin:

Low price, good welding performance, mechanical strength, brightness and other lead than lead-free, but it has lead and other heavy metals, not environmental protection, can’t pass RoHS

Lead free tin spray:

The price is cheap, but compared with the brightness of lead will be dimmer, and environmental protection, can pass RoHS

Common disadvantages: it is not suitable for welding pins with fine clearance and components with too small size, because the surface flatness of tin spraying board is poor. Solder bead is easy to be produced in PCB processing, which is easy to cause short circuit to fine pitch components. When it is used in double-sided SMT process, because the second side has been reflow soldering at high temperature, it is very easy to produce tin beads or similar water droplets, which are affected by gravity, into spherical tin spots, resulting in more uneven surface, thus affecting the welding problem.

2. OSP

organic dissolving preserving

Organic solder ability protective agent (OSP) OSP is a kind of process for surface treatment of printed circuit board (PCB) copper foil in accordance with ROHS directive. In short, OSP is to grow an organic film on a clean bare copper surface by chemical methods. This film has oxidation resistance, heat shock resistance and moisture resistance, which is used to protect the copper surface from further rusting (oxidation or vulcanization, etc.) in the normal environment. However, in the subsequent welding high temperature, the protective film must be easily removed by the flux, so that the exposed clean copper surface can combine with the molten solder in a very short time Become a solid solder joint.

Advantages:

It has all the advantages of bare copper welding.We typically limit re-treatment of PCBs to once after a three-month storage period to compensate for any expired surface treatments.

Disadvantages:

Easily affected by acid and humidity.

Here are some shorter alternatives:

For good results, we complete the second reflow soldering in a specific timeframe.We remove the OSP layer before electrical testing by printing solder paste test points.
Unprocessed copper surfaces or sharp probes can harm ICT during testing.

so we use manual preventive treatment to reduce test repeatability.
We must re-treat OSP if it is stored for over three months and use it within 24 hours after opening.

OSP requires a significant change to the assembly process.

Applicable scenario:

We widely utilize OSP for BGA.. If there is no functional requirement of PCB surface connection or limited storage life, OSP process will be the most ideal surface treatment process.

ENIG, electroless nickel immersion gold

It can protect PCB for a long time. In addition, it also has the tolerance to the environment that other surface treatment processes do not have. In addition, depositing gold can prevent the dissolution of copper, which will benefit lead-free assembly.

Advantages:

OSP is non-oxidizable and can be stored for a long time, while its smooth surface is easy to maintain. It is suitable for welding the components with small gap pins and small solder joints. We prefer PCBs with buttons, such as mobile phone boards, due to their functional benefits.

Disadvantages:

High cost, poor welding strength, because the use of electroless nickel plating process, easy to have the problem of black disk. The nickel layer will oxidize over time and long-term reliability is a problem.

Application scenario:

  • We use gold deposition processes for functional boards such as mobile phone keys, router connections
  • Gold deposition was popular in the 1990s due to its flatness and removal of flux in OSP
  • but usage decreased with the introduction of black disk and brittle nickel phosphorus alloy.

However, almost every high-tech PCB Factory has a deposition line. Considering that the solder joint will become brittle when removing Cu Sn intermetallic compound

there will be many problems at the relatively brittle Ni Sn intermetallic compound.