Circuit board in the surface treatment process on the application of lead - free
PCB Final Pad Finish The purpose of the Finish process is to solder copper, through-hole pads, and parts of the PCB onto the PCB by metalizing or coating the organic protective film so that the solder has a tin- Solderability. If it is untreated copper surface, due to oxidation of copper and easy to produce oxide film, then this layer of oxidized surface as non-stick pot, melt-like solder is not on the stick, and can not weld assembly of electronic components.
The simplest, most common and cheapest process for the surface treatment of solder pads is the Hot Air Solder Leveling (HASL), which directly immerses the PCB into a melting furnace at about 200 ° C. Scraping the excess surface of the solder with hot air and blow out residual tin in the through hole, mainly used in low-level circuit board, it can make the PCB to maintain good solderability before assembly, the industry has been used for many years, the current PCB process surface welding HASL was still used in 60% of pad treatments. However, HASL uses tin-lead (Sn / Pb composition ratio of 63/37) solder, which is bound to be discontinued under the RoHS-restricted lead-containing products.
Second, the type of lead-free surface welding technology
As the green trend of lead-free, in recent years, replaced by a variety of surface treatment methods developed rapidly. The current lead-free solderable surface treatment technology is divided into metal coating (Metallic coatings) and organic coating (Organic coatings) two categories. The metal coating mainly contains ENIG, ImAg and ImSn, while the organic coating is mainly organic solder flux (OSP). The four lead-free surface welding technology is more mature at this stage, is hereby outlined as follows:
About organic solderability (OSP) surface treatment technology, as early as the 1960s Enthone company that developed the Entek treatment of copper film technology, the use of Banzotriazo (BTA) material on the bare copper surface Solder pad) to protect the copper of a transparent film processing, copper protection and to achieve the dual purpose of welding can be used instead of tinned thin wire can be welded layer, also known as preflux (preflux). In 1997, the fourth-generation OSP material Substituted Benzimidazoles (SBA) began mass production, the Japanese in 2002 for the lead-free solder development of the fifth generation of OSP series, Aryl Phonylimidazole class materials, to achieve better heat resistance , The cracking temperature of up to 355 ° C, a substantial increase in the copper gasket antioxidant capacity; In addition, the Japanese industry has developed water-soluble OSP, for example: Shikoku Glicoat chemical products, Dafeng of Solderite WPF series, The Entek 106A of Enthone, which has been transferred to Enthone by American Electrochemical Corporation, is further improved in terms of pad flatness, compatibility with flux solder paste, cost, and environmental load.
2. Nickel-gold (ENIG)
Electroless Ni & Immersion Gold (ENIG) includes electroless nickel and immersion gold plating processes. Electroless nickel is a self-catalyzed reaction, the use of redox deposition of nickel metal on the copper, the reducing agent can be divided into sodium hypophosphite (NaH2PO2H2O, Sodium Hypophosphate) and sodium borohydride (NaBH4, Sodium ) Two series, the latter because of expensive, so the commercial process to sodium hypophosphite bath mainly; dip gold plating for a displacement reaction (Exchange reaction), the use of metal standard electrode potential difference, the standard electrode potential A lower metal will reduce and deposit the metal with a higher standard electrode potential, meaning that any metal ion with a higher electromotiv potential in the solution will replace and deposit on a lower electromotive force metal And the metal on the substrate is dissolved in the bath, and the displacement reaction is stopped as the gold is completely covered with the nickel substrate.
3. Chemical silver (ImAg)
Immersion Silver (Immersion Silver) is a lead-free surface treatment process that replaces copper with Ag (Ag) and uses copper (Cu) deposited on the copper surface, when the copper surface is completely covered by silver, the reaction is self-stop. The preparation process of silver is relatively simple, including pre-cleaning, micro-etching, surface pretreatment and dip-plating of copper surface. The silver-plating solution is mainly composed of silver nitrate, good weldability and good solderability Strength is also very reliable; but the silver surface in the air is easy to produce sulfide and oxidation phenomenon, it is necessary to dip in the plating tank to add organic inhibitors to prevent surface discoloration of the situation; In addition, in order to avoid organic silver migration phenomenon (Silver migration ), Dipping bath need to add surface wetting agent and buffer agent.
4. Chemical tin (ImSn)
Immersion Tin (Immersion Tin) technology is also a replacement reaction, the same principle and immersion gold, the use of Sn +2 replacement Cu, Sn0 deposited on the copper surface; but the copper substrate Replacement of tin is required to join the wrong agent (Complex agent) to react, commonly used as the wrong mixture of thiourea (Thiourea), then, when copper ions are no longer dissolved in the bath, the replacement reaction to stop. However, the process of dip tin can easily cause the problem of green paint discoloration or side erosion, after the welding machine plate pads and solder joints between the rapid growth of the brittle metal layer, will make the weldability affected; In addition, the surface of pure tin electroplating Tin whiskers (Tin whisker) problems, but also dip tin plating technology, a major concern.
Third, the application of lead-free surface welding technology trends
PCB manufacturers in different regions of the use of lead-free surface treatment process technology is somewhat different, in general, Japanese manufacturers use the majority of the OSP, such as CMK, Ibiden, Sony Chemical, etc .; As Europe and the United States (such as: Multek, Ruwel) Preferred metal coating approach, most of the use of nickel gold, silver or tin (as shown in Table 1).
Table 1 international manufacturers of lead-free solder surface treatment technology used in the status quo
Circatex in 2003 predicted that OSP technology in the North American PCB industry market share is expected to be 19% in 2003 to 22% in 2005; the European region from 2003 to 5% of the rapid increase in 2005 10%. As for chemical silver in recent years, Europe and the United States to promote a large number of technology, estimated in 2005 in North America using chemical silver market share will grow from 2003 to 7%; and chemical tin market share is about to grow to 3%.
According to the survey data show that, in fact, what kind of lead-free process in addition to the regional customers in addition to the requirements, the other consideration is that product characteristics. For example: As the OSP can meet the trend of electronic products, light and thin, so in the current lead-free products, NB board with the use of OSP surface treatment technology; and almost all mobile phone board selective gold mining in the key part of gold processing, The other part of the OSP; As for the photovoltaic panels are used high reliability of the gold surface treatment process.
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