PCB Tech - PCB manufacturer, OSP for lead-free soldering surface treatment

PCB manufacturer, OSP for lead-free soldering surface treatment

In addition to the lead-free solder (Solder) must be completely lead-free, the solder pads on the PCB surface (including various packages), through-L solder rings and parts feet and other surface treatments must also be lead-free. First, I will write about the soldering pads on the board. It is expected that there will be seven or eight kinds of solderable treatments for mass production options. For those who can go online for mass production, it seems that only OSP and I-Sn or There are only two or three types of I-Ag.

As for the solderable surface treatment of parts feet or pads, in addition to the early various IC metal tripods (LeadFrame, also known as lead frames), which are continuously electroplated with reel-to-reel (ReeltoReel), the rest of the bulk components (Discrets are separated) The formula is a poor translation) Regardless of whether it is passive or active, most of them adopt a large number of "barrelplating" (Barrelplating) processing methods. The current and future mainstream bath solutions will include:
1. Electroplating pure tin (with acidic tin sulfate TinSulfate or tin methane sulfonate as the bath, it can be used as rack plating or barrel plating. The surface tin plating of this kind of copper material can be divided into bright tin and matte tin. The former is prone to tin whiskers, while the latter is more expensive.)
2. Silverplating (mainly alkaline cyanide bath, and can also be used as rack plating and barrel plating.)
3. NickelandTinplating (Plating the surface of copper with a layer of nickel and then plating with pure tin will reduce the problem of tin whiskers.)
4. There are also electroplating of nickel and lithium (Ni/Pd), and electroplating of nickel and gold (Ni/Pd/Au), and even electroplating of various other tin alloys. However, due to the lack of two aspects of too expensive or insufficient technological maturity, the current It is not enough to form a climate and will not be introduced.

1. Organic flux OSP:
Simply put, OSP is to grow a layer of organic film on the clean bare copper surface to protect the copper surface in a normal environment from rusting (oxidation or sulfide, etc.); but in the subsequent welding high temperature, This kind of protective film must be easily removed and removed by the flux, so that the exposed clean copper surface can be immediately combined with the molten solder to form a firm solder joint in a very short time. This can be protected Copper anti-rust organic film is called "OrganicSolderabilityPreservatives" (OrganicSolderabilityPreservatives).
In the early days, some types of coating containing rosins or active resins (Preflux) were formulated as the early products of this type of OSP. However, in the field of single-sided panels in the past, it was often called "full-surface treatment" (finishing the copper surface), which was a physical organic protective film coated on the copper surface; ) Chemicals, which directly react with the copper surface to form a chemical protective film of "organic copper complex". The principle is not exactly the same, but the effect of copper protection and welding is very similar, so it is also called at present. OSP.
Enthone uses Azole chemicals (such as Benzo-Triazole) to protect the copper surface, which is the well-known product Entek in the industry. This is a temporary water-based copper protection agent from CU-56 that IBM first used that year. After years of experience and improvement, this type of OSP has made great progress. It may become the main method of processing the board surface pad and the cheap and flat surface in the lead-free soldering era in the future.

The following will be introduced according to the five generations of the main chemicals used:
(1) Brenzotriazole (BTA), the first generation
This BTA protects the copper surface from corrosion and oxidation. It can be traced back to the temporary coating CU-56 (1% BTA aqueous solution) used by IBM to protect the copper surface in its PCB manufacturing process in the 1960s. Enthone continued to study and improve, and became the well-known Entek processing method (Enthone Technology). Even so far, there are still many industry players who only know the business name of Entek, but not the scientific name of the originator of BTA.

The reason why BTA can protect copper against corrosion is that it reacts immediately and directly with the cuprous oxide Cu20 on the surface of the copper material, and then generates a polymer-state organic copper complex salt (Complexor Tailu translated as complexing agent, It seems to be more clever than the one that quotes Japanese complexing agent), the following formula is the schematic structure of the reaction process, that is, the imaginary diagram of the formation of multiple thin films on the copper surface. The film formed by these BTA and cuprous oxide (Cu20) is a bovine transparent and colorless film (turns brown after aging), which will grow thicker in the bath, depending on temperature, time, pH, etc. In 1989, scholars Tornkvist and others published a special article in the "Journal of the Electrochemical Society", pointing out that when the BTA molecule first reacts with cuprous oxide, BTA will interact with its special orientation (orientation) of the "triple seat" in the molecule., And make it face outwards to form a long chain of [Cu(I)BTA)n. With other adsorption mechanisms, a planar molecular film can be formed and attached to the copper surface.
The following is the P. of "Circuit Board Information Magazine" in June 1996. 80 for the textual explanation of the copper protection agent BTA:
The so-called BTA is the abbreviation of Benzotriazole, its official scientific name is 1,2,3-Benzotriazole, which means that there are three interconnected azos at the 1, 2, and 3 positions and form a 5-carbon heterocyclic compound. It is called "nitrogen-containing heteropentane" or Azole benzene compound.
BTA is a white, yellowish, odorless crystalline powder. It is very stable in acid and alkaline solutions, and it is not prone to oxidation and reduction reactions, so it is quite stable. It can form stable chemicals with metals. This BTA is not easily soluble in water, but it can be dissolved in alcohols or benzene, and is usually used as a photo protective film or ultraviolet absorber.
More than ten years ago, Enthone, a famous American supplier of circuit board chemicals, sold it in a solution of methanol and water as a tarnishandoxideresist on the copper surface, and the trade name was ENTEKCU-55 and CU- 56. It has been recognized by IBM. The latter are well-known, and most of the domestic circuit board factories use its 0.25% dilute aqueous solution as a copper protection agent. After the thickening of the copper is completed, the board is only immersed in the bath 30-60 It can be processed in seconds, and then dried with hot air to get a good protection effect. It can directly complete the image transfer work without brushing (either dry film or printing). Dilute sulfuric acid activation and cleaning before entering the secondary copper It can be easily removed, which helps the bonding and adhesion between copper and brocade.
Its high-concentration (l%) bath immersion treatment has stronger protection for bare copper, and it can replace SMOBC for long-term copper protection, and is better than the soldering performance of the board during assembly. The processes of melting, spraying, and rolling tin are not too much. This article is an IBM research article. Today, when the concept of "waste reduction" is growing and the cost of circuit boards is being forced down, the full text is hereby translated to read apricots as a reference for process improvement.