Introduction Of Surface Treatment in PCB Copy Engineering
At present, the PCB Copy Engineering industry uses a wide range of industrial application of hot air solder coating (HASL, hot air solder leveling) alternative technology. Over the past decade, numerous technical papers have been published that predict that HASL will be impregnated with organic solderability preservatives (OSP), electroless nickel / immersion gold (ENIG) or new metal Technologies such as silver and tin in PCB Copy Engineering OAA. However, so far, no one predicted to become a reality.
HASL is the ultimate surface treatment technology for major applications around the world. A predictable, well-known coating, HASL is used today on billions of solder joints. Nonetheless, three key drivers: cost, technology and the need for lead-free materials, have driven the electronics industry to consider HASL alternatives.
From the standpoint of cost, many electronic components such as mobile communications and personal computers of PCB Copy Engineering are becoming free-to-use goods, sold at a cost or lower price, to secure an Internet or telephony service contract. This strategy makes these commodities mass production and commodityization. Therefore, cost and long-term environmental impact must be taken into account. Environmental concerns typically focus on potential lead leakage into the environment. Even though legislation in North America prohibits the use of lead for a few years, original equipment manufacturers (OEMs) must meet European and Japanese environmental laws to make their products sell globally. This consideration has given rise to a number of topics that evaluate alternative methods of eliminating lead at each major OEM.
Alternative HASL methods allow for lead-free printed circuit boards (PCBs) that also provide a flat coplanar surface to meet increased technical requirements. The denser pitch and area array elements have been allowed to increase electronic functionality. Often, the higher the technology, the lower the cost. However, most alternatives improve high-tech assembly and long-term reliability while still lowering costs.
Cost savings are a function of the overall process cost, including process chemistry, labor, and general overhead. Alternative technologies such as OSP, immersion silver and immersion tin, Immersion Gold provide a 20-30% reduction in the final surface treatment cost. While the savings per board in high-level multilayer board products may be low, the cost savings of everyday electronics, along with greater functionality and elimination of lead, will drive a dramatic increase in the use of alternative methods.
The use of alternative methods will not only increase, but will replace HASL as the final surface treatment option. The problem of today's alternatives is the number of choices and the pure convolution of the published data. Alternative methods such as ENIG, OSP, immersion tin, and immersion silver provide a lead-free, solderable, flat, coplanar surface that provides significant improvements in production through the first pass assembly pass rate. In order to uncover the mystery of the final surface treatment, these alternative methods of HASL can be distinguished by comparing the advantages of each coating to the assembly requirements and the PCB design.
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First, the assembly requirements
The effect of the HASL alternative on the assembly process reflects the solderability of the surface and how it interacts with the welding material used. Each type of alternative surface coating - OSP, organometallic organometallic (immersion tin and silver) or metal (ENIG) - has a different welding mechanism. This difference in the welding mechanism affects the assembly process and the reliability of the solder joint.
OSP is a protective coating that must be removed during soldering. The flux must be brought into direct contact with the OSP surface to penetrate and solder to the copper foil on the PCB surface.
A dip coater, such as silver or tin, co-precipitates to remove the final surface oxide. Unlike OSP, tin and silver dissolve in the solder inside and will become part of the solder joint that will help melt the wet speed. Both tin and silver form solder joints directly on the copper surface of the PCB.
If properly precipitated, the gold on the ENIG surface is pure, and since it can be fused to the solder, it will provide the fastest rate of melting of the weld. However, when ENIG is used, solder joints are formed above the nickel barrier layer, not directly on the copper surface of the PCB.
All three alternative coatings offer the best printing surface, the same for all types of solder paste. Solder paste is printed directly on the surface coating, providing direct contact with flux, penetration of OSP and wetting the PCB surface. Printed stencils are ideal for depositing perfect solder pastes, creating an effective seal that eliminates HASL print paste and solder bridges. The result is that the three alternative coatings have a high first-pass assembly pass rate and a small difference in soldering wetness. The difference lies in the strength and reliability of the solder joint. Several studies have confirmed that the use of OSP, directly welded to the copper surface, provides the best strength of the welding point. 2,3 The strength of the solder joint becomes important when the smaller pad of the area array chip package is used.
Although reduced in use, wave soldering is still a part of today's assembly process as a whole. The welding mechanism of each final surface coating will affect the choice of flux composition and wave soldering process settings. Metallic and organometallic coatings contribute to the wetting of the through-hole solders, often requiring less flux, less active flux, and less turbulence in the peaks. Disposable materials are good with OSP under production conditions, but require some optimization to increase penetration of flux and / or solder into the via. Often, this optimization increases the flux usage, substitutes for a particular type of flux chemistry, or increases solder penetration by higher turbulence or temperature.
A global replacement for the traditional wave soldering process is underway. Intrusive reflow, selective solder fountains and compliant pins are practically used on all final surface coatings. The work done so far has shown that the turbulence of the selective solder fountain improves the wettability of the through-hole. The paste-in-hole or intrusive reflow will contact the flux and flux carrier directly to the surface of the PCB so that the wettability of the via is similar for all final surface coatings. Finally, the HASL alternative is stronger than the HASL using a compliant pin due to predictable hole errors. In the alternative, a thicker dip provides the smoothest surface for the insert, providing the widest operating window for the compliant pin. 4
The assembly industry is now evaluating lead-free soldering alternatives. While some alloys seem to be the choice of particular OEMs, they also have to choose alloys that are accepted throughout the industry. Nonetheless, all alloys under test require higher reflow temperatures and slower melt rates. Solder paste suppliers have engineered a special flux chemistry to improve the wetting of these new alloys. Initial studies have shown that higher reflow temperatures do not affect the solderability or bond strength of OSP, immersion silver, or immersion tin. The higher melting temperature significantly facilitates penetration of the OSP and wetting of the tin and silver surfaces, or even double-sided reflow. Additional testing is underway to evaluate the effect of melt rate and to optimize the specific reflow parameters for the final surface coating.
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Second, PCB design& PCB Copy Engineering
As discussed, the assembly process can be optimized to suit all of the final surface coatings. The PCB design will ultimately determine the best HASL alternatives for each application, but more specifically the types of packaging and interconnections:
Applications such as key contact, component shielding, and edge connector require low contact resistance throughout the life of the device.
1, flexible circuit boards usually require aluminum or stainless steel to strengthen the components or radiators.
2, component packaging and some PCB requires wire bonding (wire bonding) or with the direct chip attached to the conductive adhesive compatibility. 3, HDI (high-density interconnect) on the PCB geometry dramatically affect the use of traditional electroless plating coating the pass rate.
4. You have seen field failures due to the insufficient bonding strength of the area array packages assembled on ENIG.
To meet all of these requirements, the electronics industry is focusing on three major alternatives: OSP, immersion silver and immersion tin. Each of these three coatings provides the benefits of being tailored to specific PCB design requirements.
OSP is the lowest cost alternative to multi-metal surface compatibility, providing the highest bond strength. There is now a new formulation that provides a thinner settling layer that is as strong as the original, which eliminates the rusting of the multi-metal surface. Due to wear resistance or solderability of the electrolytic gold precipitate, a multi-metal coating such as electrolytic nickel / gold for plug-in connectors or gold wire bonding is required. High cost and embrittlement of the gold in the solder joints require the OSP to have a second coating of the soldered connections.
ENIG can also be used in conjunction with the new OSP process, if the strength of the solder joint is critical to the PCB design. This high bond strength makes the OSP a choice for mobile electronics and area array packages. OSP also shows greater compatibility with conductive adhesives used in flip-chip applications. Finally, the radiator or rigid member may be readily applied to the plate form prior to final surface treatment. Unlike OSP, dip or electroless plating processes will be plated on stainless steel or aluminum, causing discoloration.
Compared to OSP and ENIG, immersion silver is a relatively new technology. However, in the past six years, a wide range of tests and a large number of production has confirmed the reliability of this process 5,6. The wetting properties of the solder make this coating more suitable for an existing lead-free wave soldering process. This surface treatment method is a potential alternative to most applications, including shielding, aluminum wire bonding, keying and soldering.
The contact resistance of this coating remains low after aging or reflow processes. Initial studies have shown that the contact resistance remains low after 300,000 contact with the conductive polymer, although more work needs to be done. As a metallic coating, immersion silver can also be checked with low magnification or no magnification, making it easy for both the applicator and the assembler to determine their presence.