PCB Technical - Detailed explanation of 12 kinds of PCB surface treatment processes

Detailed explanation of 12 kinds of PCB surface treatment processes

There are many kinds of PCB surface treatment processes, none of which is perfect, each has its own characteristics. The selection should be made according to the process characteristics of PCBA.

1. Tin-lead hot air leveling
Application occasions: At present, the application is limited to products exempted from the RoHS directive and military products, such as the line card and back plane of communication products, which are suitable for PCBA with a device pin center distance ≥ 0.5mm

Cost: Medium.

Compatibility with lead-free: not compatible, but can be used as the surface treatment of line cards of communication products.

Storage period: 12 months.

Solderability (wettability): high.

Disadvantages:

It is not suitable for devices with pin center distance <0.5mm, because bridging is prone to occur.

It is not suitable for places with high coplanarity requirements, such as BGAs with medium and high pin counts. Because the thickness of the plating layer of the HASL process varies greatly, the coplanarity of the pad and the pad is poor.

Due to the relatively large thickness of the coating, the drill hole diameter (DHS) must be compensated to obtain the desired finished metalized hole diameter (FHS), typically FHS=DHS-4~6mil.

Supplier resources: Medium, but with the decrease of customers using leaded processes, PCB manufacturers are gradually reducing HASL production lines, and resources will become less and less.

2. Lead-free hot air leveling
Application: Replace SnPb HASL. It is suitable for PCBA with device pin center distance ≥ 0.5mm.

Cost: medium to high.

Compatibility with lead-free: compatible.

Storage period: 12 months.

Solderability (wettability): high.

Disadvantages:

It is not suitable for devices with pin center distance <0.5mm, because bridging is prone to occur.

It is not suitable for places with relatively high requirements for coplanarity, such as BGAs with medium and high pin counts, because the thickness of the HASL process plating layer varies greatly, and the coplanarity between the pads and the pads is poor.

Due to the relatively large thickness of the coating, the drill hole diameter (DHS) must be compensated to obtain the desired finished metalized hole diameter (FHS), typically FHS=DHS-4~6mil.

Due to the high peak temperature of the surface treatment, thermally stable dielectric materials must be used.

Supplier resources: Currently limited, but with the development of using lead-free processes, there will be more and more.

3. Ordinary organic protective coating
Application: The most widely used surface treatment. Due to its flat surface and high solder joint strength, it is recommended for the surface treatment of fine-pitch devices (<0.63mm) and devices that require relatively high pad coplanarity.

low cost.

Compatibility with lead-free: compatible.

Storage period: 3 months.

Solderability (wettability): low.

Disadvantages:

Special craftsmanship is required in the PCB factory.

Single boards that are not suitable for mixed assembly process (mixed assembly of plug-in components and placement components).

Poor thermal stability. After the first reflow welding, the remaining welding operations must be completed within the time limit (generally 24h) specified by the OSP manufacturer.

Not suitable for boards with EMI grounding area, mounting holes, and test pads. It is also not suitable for veneers with crimp holes.

Supplier resources: more.

4. High temperature organic protective coating
Application: Used to replace OSP-stand, suitable for more than 3 welding operations. It is recommended to install a large number of fine-pitch devices (<0.63mm) and products that require relatively high coplanarity.

low cost.

Compatibility with lead-free: compatible.

Storage period: 3 months.

Solderability (wettability): low.

Disadvantages:

Special craftsmanship is required in the PCB factory.

After the first reflow welding, the remaining welding operations must be completed within the time limit specified by the OSP manufacturer. It is generally required to be completed within 24 hours.

Not suitable for boards with EMI grounding area, mounting holes, and test pads. It is also not suitable for veneers with crimp holes.

Supplier resources: more.

5. Electroplating nickel/gold (welding)
Application: Mainly used for/non-welded electroplating nickel-gold selective plating.

high cost.

Compatibility with lead-free: compatible.

Storage period: 12 months.

Solderability (wettability): high.

Disadvantages:

There is a risk of embrittlement of the solder joint/weld seam.

Feature (line, pad) side exposed copper, can not be completely wrapped or covered.

The plating is completed before the solder mask. The solder mask is directly applied to the gold surface, therefore, the bonding surface treatment strength of the solder mask will be damaged to a certain extent.

Supplier Resources: Medium.

6. Non-welded electroplating nickel/gold (hard gold)
Application: It is used in places where abrasion resistance is required, such as golden fingers and guide rail mounting edges.

Cost: Medium, but the cost is high when used as a selective coating.

Compatibility with lead-free: compatible.

Storage period: 12 months.

Solderability (wettability): low.

Disadvantages:

Not solderable.

It can be applied after the solder mask process, but this process may cause the solder mask peeling of fine pitch devices.

Supplier resources: more.

7. Chemical nickel/immersion gold
Application: Suitable for PCBA with a large number of fine-pitch devices (<0.63mm) and high coplanarity requirements. It can also be used as a selective coating on the surface of OSP, press the keyboard.

high cost.

Compatibility with lead-free: compatible.

Storage period: 12 months.

Solderability (wettability): high.

Disadvantages:

There is a risk of embrittlement of the solder joint/weld seam.

There is a risk of "black disk" failure. Black disk is a kind of defect with very low occurrence probability. It is difficult to find with general inspection methods, but the failure caused is catastrophic. Therefore, it is generally not recommended to be used for fine-pitch BGA pad surface treatment.

The immersion gold layer is very thin and cannot withstand more than 10 mechanical insertions.

Supplier resources: more.

8. Im-ag
Application: Suitable for PCBA with a large number of fine-pitch devices (<0.63mm) and high coplanarity requirements.

low cost.

Compatibility with lead-free: compatible.

Storage period: 12 months.

Solderability (wettability): high.

Disadvantages:

Potential interface microvoids.

It is not compatible with gold-plated crimp connectors because the friction between the two is relatively large.

The immersion silver layer is very thin and cannot withstand mechanical insertion and removal of more than 10 times.

The non-welded area is prone to high temperature discoloration.

Easy to vulcanize (sensitive to sulfur)

There is the Javanni effect, and the trench depth is generally about 10μm.

Injavanni grooves are exposed to copper, which is prone to creep corrosion in a high-sulfur environment.

Supplier resources: more.

9. Im-Sn
Application: Recommended for Back Plane. It can obtain a satisfactory crimping aperture size, and it is easy to achieve ±0.05mm (±0.002mil). In addition, it also has a certain lubricating effect, which is especially suitable for PCBA which is mainly a crimping connector.

Cost: low (equivalent to ENIG)

Compatibility with lead-free: compatible.

Storage period: 12 months.

Solderability (wettability): high.

Disadvantages:

Due to the limitation of fingerprints and the number of repairs, it is not recommended for single board (Line Card)

After reflow soldering, the tin-plated layer near the plug hole is easy to change color. This is because the solder resist (commonly known as green oil) plugs the hole easily to contain the liquid, and it is sprayed out during soldering and reacts with the nearby tin layer.

There is a risk of tin whiskers. The risk of tin whiskers depends on the syrup used for immersion tin. Some tin layers made of syrup are prone to tin whiskers, and some are less prone to tin whiskers.

Some tin-immersed formulations are not compatible with solder resists, and have serious erosion of solder resists, which are not suitable for the application of fine solder resist bridges.

Supplier resources: more.

10. Hot melt tin lead
Application: Generally used for backplane.

Cost: Medium.

Compatibility with lead-free: not compatible.

Storage period: 12 months.

Solderability (wettability): The biggest advantage of hot melt is corrosion resistance, but the solderability is not very good.

Disadvantages:

Not suitable for single board (Line Card)

The coplanarity of the pad and the pad is relatively poor, and it is not suitable for PCBA that requires a relatively high coplanarity.

It is used for the relatively large variation of the relative thickness of the plating layer. To obtain a satisfactory finished metallization aperture size, the aperture size of the drill hole needs to be compensated.

Supplier resources: limited.

11. Chemical nickel palladium/immersion gold
Application: Used for inert surfaces that are very durable and stable without the risk of "black disk". It may replace the surface coating of OSP or ENIG applied in veneer.

Cost: medium to high (lower than ENIG)

Compatibility with lead-free: compatible.

Storage period: 12 months.

Solderability (wettability): high.

Disadvantages:

Very few applications in the PCB industry, not much experience.

Supplier resources: very few.

12. Selective chemical nickel/gold and OSP
Application: It can be used for PCBA that requires mechanical contact area and installs fine-pitch devices. In this application, OSP is used as a highly reliable solderable layer, and ENIG is used as a mechanical contact area, like the keyboard of a mobile phone, which uses this surface treatment.

Cost: medium to high.

Compatibility with lead-free: compatible.

Storage period: 6 months.

Solderability (wettability): low.

Disadvantages:

The cost is relatively high.

ENIG is not suitable for use as the ground edge plating for channel installation, because the ENIG layer is very thin and not resistant to friction.

There is a risk of Gavani effect in OSP potions.

Supplier resources: more.