Solder paste printing solution for miniaturized components

In recent years, with the increase in performance requirements of smart terminal devices such as smart phones and tablet computers, the SMT manufacturing industry has a stronger demand for miniaturized and thin electronic components. With the rise of wearable devices, this demand is even greater. Increasingly. The picture below is a comparison of I-phone 3G and I-phone 7 motherboards. The new I-phone mobile phone is more powerful, but the assembled motherboard is smaller, which requires smaller components and more dense components. Assembly can be done. With smaller and smaller components, it will become more and more difficult for our 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.

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 open area ratio. 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.

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.

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 stencil. 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.