PCB表面処理技術分析

PCB表面処理技術の利点分析

PCB surface treatment technology refers to a process method of artificially forming a surface layer different from the mechanical, physical and chemical properties of the substrate on the PCB components and electrical connection points. The purpose is to ensure good solderability or electrical performance of the PCB. Because copper tends to exist in the form of oxides in the air, which seriously affects the solderability and electrical performance of the PCB, surface treatment of the PCB is required.

At present, the common surface treatment methods are as follows:

Bare copper

利点： low cost, smooth surface, good weldability (without being oxidized).

Disadvantages: It is easily affected by acid and humidity, and cannot be left for a long time. It needs to be used within 2 hours after unpacking. Because copper is easily oxidized when exposed to air. It cannot be used for double-sided boards because the second side has been oxidized after the first reflow. If there are test points, solder paste must be added to prevent oxidation, otherwise it will not be able to make good contact with the probe later.

Tin-spraying plate (HASL, Hot Air Solder Levelling, hot air leveling)

利点： low price and good welding performance.

Disadvantages: Not suitable for welding pins with small gaps and too small components, because the surface flatness of the tin-sprayed board is poor. Solder beads are easy to be generated in PCB processing, and it is easy to cause short circuit for fine pitch components. When used in the double-sided SMT process, because the second side has already undergone a high temperature reflow soldering. It is very easy to re-melt the sprayed tin and produce tin beads or similar water beads under the influence of gravity into dripping spherical tin dots, resulting in less surface Leveling in turn affects welding problems.

The tin-spraying process is relatively dirty, unpleasant and dangerous, so it has never been a favorite process. However, the tin-spraying process is an excellent process for larger-sized components and wires with larger pitches. In higher density PCBs, the flatness of the tin spray process will affect subsequent assembly. Therefore, the HDI board generally does not use the tin spray process.

技術の進歩により、業界は現在、より小さなQFPとBGAの組み立てに適したスズ溶射プロセスを登場させていますが、実際の用途は少なくなっています。 現在、一部の工場では、スズ溶射プロセスの代わりにOSPプロセスと金浸漬プロセスを使用しています。 技術開発により、一部の工場ではスズの沈下と銀の沈下のプロセスも使用されています。 近年の鉛フリーの傾向と相まって、スズ溶射プロセスの使用はさらに制限されています。 いわゆる鉛フリースプレースズが登場しましたが、これには機器の互換性が関係している可能性があります。

OSP（有機はんだ付け防腐剤、抗酸化）

利点：裸の銅板溶接のすべての利点。 期限切れ（3か月）のボードも再表示できますが、通常は1回に制限されます。

Disadvantages: easily affected by acid and humidity. When used in the second reflow soldering, it needs to be completed within a certain time, usually the effect of the second reflow soldering will be relatively poor. If the storage time exceeds three months, it must be re-surfaced. Use it within 24 hours after opening the package. OSP is an insulating layer, so the test point must be printed with solder paste to remove the original OSP layer in order to contact the pin point for electrical testing.

The OSP process can be used on low-tech PCBs or high-tech PCBs. Such as PCB for single-sided TV, high density chip packaging board. For BGA, OSP applications are also more. If PCB has no functional requirements for surface connection or limited storage period, OSP process will be the most ideal surface treatment process. However, OSP is not suitable for a small number of diverse products, or for products with inaccurate demand estimates. If the company’s circuit board inventory often exceeds six months, it is really not recommended to use OSP surface-treated boards.

Immersion gold

利点： Not easy to oxidize, can be stored for a long time, the surface is flat, suitable for welding fine gap pins and components with small solder joints. First choice for button PCB board. Can be repeated many times over reflow soldering will not reduce its solderability. It can be used as a substrate for COB (Chip On Board) wire bonding.

Disadvantages: higher cost and poor welding strength. Because of the electroless nickel plating process, it is easy to have the problem of black disk. The nickel layer will oxidize over time, and long-term reliability is a problem.

Immersion gold process is different from OSP process. It is mainly used on the board with the functional requirements of the surface and a long storage period, such as the key contact area, the edge connection area of the router shell and the electrical contact area where the chip processor is elastically connected. Due to the flatness of the tin-spray process and the removal of OSP fluxes, Shen Jin was widely used in the 1990s. Later, due to the appearance of black discs and brittle nickel-phosphorus alloys, the application of the immersion gold process has decreased.

However, at present, almost every high-tech PCB factory has a gold wire. Considering that the solder joints become brittle when the copper-tin intermetallic compound is removed, many problems will occur at the relatively brittle nickel-tin intermetallic compound. Therefore, almost all portable electronic products (such as mobile phones) use copper-tin intermetallic compound solder joints formed by OSP, immersion silver or immersion tin, and immersion gold is used to form the key area, contact area and EMI shielding area, so-called selective Immersion gold craft.

 Immersion silver

Immersion Silver is cheaper than Immersion Gold. If the PCB has connection functional requirements and needs to reduce costs, Immersion Silver is a good choice. Coupled with the good flatness and contact of Shenyin, it should choose the Shenyin process. Shenyin is used a lot in communications products, automobiles, and computer peripherals, and it is also used in high-speed signal design. Since Shen Yin has good electrical properties unmatched by other surface treatments, it can also be used in high-frequency signals. The EMS recommends the Shenyin process because it is easy to assemble and has good checkability. However, due to defects such as tarnishing and solder joint voids, Shen Yin’s growth was slow (but did not decline).

Immersion tin

It has been a decade since immersion tin was introduced into the surface treatment process. The emergence of this process is the result of the requirements of production automation. immersion tin did not bring any new elements into the solder joints, and is particularly suitable for communication backplanes. Tin will lose solderability beyond the shelf life of the board, so sink tin needs better storage conditions. In addition, the use of carcinogens in the immersion tin process is restricted.

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