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In recent years, with the increase in the performance requirements of smart terminal devices such as smart phones and tablet computers, the PCB manufacturing industry has a stronger demand for miniaturization and thinning of electronic components. With the rise of wearable devices, this demand is even greater. Increasingly.
With smaller and smaller components, it will become more and more difficult for the PCB production process. The improvement of the one-time through rate has become the main goal of the SMT process engineer. Generally speaking, more than 60% of the defects in the SMT industry are related to solder paste printing, which is a key process in SMT production. Solving the problem of solder paste printing is equivalent to solving most of the process problems in the entire SMT process. At present, British 01005 SMD devices and 0.4 pitch BGA/CSP are commonly used in SMT production. A small number of metric 03015 SMD devices are also used in production, while metric 0201 SMD devices are currently only in the trial production stage and are expected to be gradually used in production in the next few years.
To understand the challenges brought by miniaturized components to solder paste printing, we must first understand the area ratio of stencil printing (Area Ratio).
The area ratio of stencil printing (Area Ratio)
If the stencil opening area ratio does not meet the requirements (the stencil is too thick), the following picture will appear. When the solder paste is printed and demolded, the solder paste of small components sticks to the wall of the steel mesh and falls off to the soldering The amount of solder paste on the disk is small.
If the stencil opening area ratio does not meet the requirements (the stencil is too thick), the following picture will appear
For the solder paste printing of miniaturized pads, the smaller the pad and the stencil opening, the more difficult it is for the solder paste to separate from the stencil hole wall. To solve the solder paste printing of miniaturized pads, there are the following solutions for reference:
1. The most direct solution is to reduce the thickness of the steel mesh and increase the area ratio of openings.
As shown in the figure below, after using a thin steel mesh, the soldering of the pads of small components is good. If the substrate produced does not have large-size components, then this is the simplest and most effective solution, but if there are large components on the substrate, the large components will be poorly soldered because of the small amount of tin. So if it is a high-mix substrate with large components, we need other solutions listed below.
The most direct solution is to reduce the thickness of the steel mesh and increase the area ratio of openings.
2. Use the new steel mesh technology to reduce the requirement for the opening ratio of the steel mesh.
1) FG (Fine Grain) steel mesh
FG steel sheet contains a kind of niobium element, which can refine the grain and reduce the overheating sensitivity and temper brittleness of steel, and improve the strength. The hole wall of laser-cut FG steel sheet is cleaner and smoother than that of ordinary 304 steel sheet, which is more conducive to demolding. The opening area ratio of the steel mesh made of FG steel sheet can be lower than 0.65. Compared with the 304 steel mesh with the same opening ratio, the FG steel mesh can be made slightly thicker than the 304 steel mesh, thereby reducing the risk of less tin in large components .
2) Electroformed steel mesh
The manufacturing principle of electroforming steel mesh: by printing photoresist material on the conductive metal base plate, and then making the electroforming template by shielding the mold and ultraviolet exposure, and then placing the thin template in the electroforming liquid for electroforming. In fact, electroforming is similar to electroplating, except that the nickel sheet after electroforming can be stripped from the bottom plate to form a steel mesh.
Electroformed Steel Mesh
The electroforming steel mesh has the following characteristics: there is no stress inside the steel sheet, the hole wall is very smooth, the steel mesh can be any thickness (within 0.2mm, controlled by the electroforming time), the disadvantage is that the cost is high. The picture below is the comparison of the laser steel mesh and electroformed steel mesh wall. The smooth hole wall of the electroformed steel mesh has a better demoulding effect after printing, so that the opening ratio can be as low as 0.5.
Comparison of laser steel mesh and electroformed steel mesh wall map
3) Ladder steel mesh
The stepped steel mesh can be locally thickened or thinned. The partially thickened part is used to print the solder pads that require a large amount of solder paste, and the thickened part is realized by electroforming, and the cost is higher. The thinning is achieved by chemical etching. The thinned part is used to print the pads of miniaturized components, which makes the demolding effect better. Users who are more cost-sensitive are recommended to use chemical etching, which is lower in cost.
4) Nano coating. (Nano Ultra Coating)
Coating or plating a layer of nano-coating on the surface of the steel mesh, the nano-coating makes the hole wall repel the solder paste, so the demolding effect is better, and the volume stability of the solder paste printing is more consistent. In this way, the quality of printing is more assured, and the number of cleaning and wiping of the steel mesh can also be reduced. At present, most domestic processes only apply a layer of nano-coating, and the effect is weakened after a certain number of printing. In foreign countries, there are nano-coatings directly plated on the steel mesh, which have better effect and durability, and of course the cost is also higher.
3. Double solder paste molding process.
1) Printing/Printing
Two printing machines are used to print and form solder paste. The first one uses ordinary stencil to print small component pads with fine pitch, and the second uses 3D stencil or step stencil to print large component pads. This method requires two printers, and the cost of the stencil is also high. If a 3D stencil is used, a comb-shaped scraper is also used, which increases the PCB production cost, and the production efficiency is also low.
2) Printing/spray tin
The first solder paste printer prints close-pitch small component pads, and the second inkjet printer prints large component pads. In this way, the solder paste molding effect is good, but the cost is high and the efficiency is low (depending on the number of large component pads).
Double solder paste molding process
Users can choose to use the above several solutions according to their own situation. In terms of cost and production efficiency, reducing the thickness of the stencil, using low-required aperture area ratio stencils, and step stencils are more suitable choices; users with low output, high quality requirements, and cost-insensitive users can choose printing/jet printing plan.
