PCB Tech - Green circuit board manufacturing process (3) circuit board sheet and surface treatment

Green circuit board manufacturing process (3) circuit board sheet and surface treatment

Under the trend of "green electronics", the production of circuit boards will be subject to multiple impacts. New lead-free solders will lead to a rapid increase in reflow temperature and operating time, heat-resistant requirements for substrate composition and downstream solderability The expectation of surface treatment such as wire bonding, etc. will bring unprecedented new challenges to the process from inner layer treatment to finished board surface treatment.

The need for halogen-free is a considerable challenge for the brimness of circuit board dielectric materials. Sheet glass transition. The increase in temperature (Tg) and the decrease in water absorption have become important goals for the next generation of panels. This article will provide a brand-new solution to the artistic realm of RCF back adhesive copper foil. Even for the halogen-free difficult HDI sheet, it has been proved in practice that it can show the most sophisticated properties.

The final surface treatment of the circuit board surface is closely related to the downstream lead-free assembly, and the various effects produced should be discussed in depth. In addition, the industry may abandon the heat torture of lead-free spray tin (HASL) and look for other alternatives, such as chemical immersion tin and chemical immersion silver. This article will also organize and present the various improvements made to the new immersion tin, immersion silver and electroless nickel gold in the past few years.
1. Improvement of the board
(1) Adhesive copper foil
RCF or RCC has been widely used in the production of micro-blind vias on mobile phone circuit boards. The newly developed ruler RCC has greatly reduced the environmental hazard. The content of the material part can be divided into two parts: the first is the change of the dielectric layer formula, and the second is the revolution of the back-adhesive copper foil itself. Atuo company decided to improve both parts at the same time.
(2) The manufacturing process of friendly adhesive-backed copper foil
At present, the standard procedure for the existing circuit board manufacturers to make back adhesive copper foil is to first mix and stir all the components (resin, hardener, additives, flame retardant, etc.) in an organic solution, and then mix the fluid with a precision coater The material is coated on the copper foil, and then the solvent in the wet film is driven away by an oven for drying

A new solvent-free (So1Vent-1eSS) type back adhesive copper foil has been developed: the solid raw materials are uniformly mixed first, and then the overall uniform powder is obtained through melt extrusion, so that no solvent is needed. In fact, this kind of manufacturing technology has already been used in the decorative field, and it has also been very mature and widely used. The new method just applies the original powder technology to the surface of the copper foil with a special dense coating method, and then uses the improved system of the coating equipment to obtain a uniform powder layer. Then gradually use the oven to melt it under specific conditions, and after cooling, a uniform film can be obtained on the copper foil. The film obtained in this way, due to its limited fluidity, will have a slightly rough appearance, but it also allows the produced RCF to quickly exhaust gas in the subsequent press industry.
(3) Composition of halogen-free board
In the past, epoxy resins containing odorous flame retardants have been the main components of substrate boards, films, and back adhesive copper foils. However, since EU legislation explicitly banned the application of certain toxic substances in electronic products, the industry has also accelerated the replacement of such toxic substances. In the future, these toxic substances will no longer exist in the formulation of medium materials.
The latest dielectric material developed by ATTO is a halogen-free epoxy resin, which has passed the relevant halogen-free specifications. This kind of phosphide is used as a flame retardant and changed to a powder formula. In terms of the performance of the material properties, it has not been sacrificed due to non-halogenation. Especially in the key water absorption, it can still be kept at a very low water level. The right table is the odorless board pressure. An overview of the various characteristics after integration.

(Four), summary
Atuo Technology has developed a new coating technology for adhesive copper foil that meets environmental protection requirements, combined with a halogen-free formula and applied to dielectric materials. The success of this technology has taken a big step toward IS] green materials and manufacturing processes. . In addition, relative to the requirements of the various thicknesses of the dielectric layer, the thickness tolerances have also been accurately grasped. For designers and producers who are committed to environmental protection, it will provide the best weapon in the production of micro-blind holes.
2. Surface treatment and welding
This part is mainly to compare the various final surface treatments of circuit boards, and the solder wettability in general air or nitrogen. The surface treatments tested by ATTO include: chemical tin, chemical silver, organic solder mask, lead-free tin spray, medium phosphorus and high phosphorus chemical nickel gold and nickel tin gold, etc.
In order to clearly understand the best combination of various surface treatments and lead-free soldering, it is necessary to measure the diameter of the loose tin after the solder paste has been melted after reflow. The board used for the multi-function test board is FR4 of Tg170, the board thickness is 1.6mm, and the thickness of the copper plating on the surface is 30μm.
(1) Layer Thickness
The various surface treatments tested are all used in the most suitable thickness range for lead-free soldering, and the thickness of various coatings is carefully measured and verified in different ways.
(Two), reflow soldering (Reflow Soldering)
The reflow soldering trial done by Berlin Ato’s head office uses the “REHM NITRO2100” five-stage heating reflow oven. The temperature-time profile (Profile) used is based on the curve set by J-STD-020-C. The highest temperature of the board surface is 260 degree Celsius, and the entire reflow soldering process does not exceed 10 minutes.
Each test welding is carried out separately in a nitrogen environment (residual oxygen rate <100PPm) and a general air environment (180KppmO2). The selected solder paste is SAC305 lead-free solder paste of "KOKI S3X58AM406". The solder paste printing uses a stainless steel plate of "DEK248" with a thickness of 125μm.

(3) Solder spread diameter
This method includes solder paste printing (the printed diameter is 1000 m), after reflow soldering, observe the diffusion of liquid and solid solder paste. When the surface tension of the solder paste is small and the diffusion ability is good, it will show a large coverage on the solder pad. Therefore, the size of the diffusion area can be used to evaluate the wettability of the surface treatment film.

(4) Comparison of various coatings after repeated welding
For the various surface treatment layers that participated in the test, firstly, they were directly reflowed without aging, and then each film was subjected to high temperature aging of one, two and three simulated reflow soldering, and then went to solder paste printing and reflow. weld. The surface of each test sample is pre-printed with 30 solder paste test points, and then reflow soldering is performed.

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