PCB Technical - Solder Mask? What is the use for PCB? Is it only green?

The term Solder Mask is often heard in articles and discussions on circuit boards. What is Solder Mask? What role does it play in the circuit board?

What is Solder Mask?

The correct Chinese name of Solder Mask should be called "solder mask" or "solder mask", because more than 99% of circuit boards use green solder mask, so it is commonly called "green paint" or "green oil" )”, there are red paint, black paint, blue paint... etc. In order to cater to the color appearance of different occasions, for example, in the NPI stage, some RDs will specifically use “red paint” to distinguish them from mass-produced boards. The exposed board chooses to use black to match the color of the case. With the evolution of the times and the introduction of laser barcodes, in order to be able to use optical scanning to interpret laser barcodes on circuit boards, it is generally not recommended to use solder masks other than green. In fact, Solder Mask has a more vivid English name called Solder Resist, but it is used by fewer people.

The purpose and function of Solder Mask

Solder Mask is located on the top layer and bottom layer of the circuit board copper foil circuit, used to protect the copper foil from being oxidized and accidentally being touched by solder and affecting the function of the circuit board, so Solder Mask Generally, resin is used as the main material, and PCB printing technology is used to cover the Solder Mask on the circuit board and the lower surface that does not need to be soldered to meet the requirements of moisture resistance, insulation, solder resistance, high temperature resistance and aesthetics. A few may use inkjet printing.

The Mask in Solder Mask actually represents the meaning of the mask

Generally, when making Solder Mask, PCB manufacturers first print the entire PCB board with solder mask, then put it into baking for pre-baking, and then use film for contact exposure, and the film on the The transfer of the image to the surface of the solder mask is actually similar to the principle of a photocopier.

Please note: The image on the negative is a negative, that is, the part with black shadows on the negative is not intended to be preserved, while the transparent green paint will be preserved. It acts as a mask, so in English Will use the term mask (mask).

Then use UV (ultraviolet) light to dry the solder resist that is not covered by the mask. Only then does the solder resist really solidify and absolutely adhere to the circuit board. It is also said that the previous drying is only "pre-drying", And finally enter the chemical tank to clean the area with the mask. Generally, the copper surface that can be soldered is exposed after the solder resist is cleaned.

Please note: only the copper surface appears after the printing process of the solder resist. In the subsequent process of the circuit board, the surface treatment (Finished), such as gold, silver, tin, etc., is required to prevent the copper surface from being soldered. Oxidized before.

Solder Mask printing will affect the quality of SMT solder

Although the solder resist process seems simple, and its greatest effect on the circuit board is only to insulate and resist solder, but if the solder resist is poorly operated, it may also cause serious quality problems, except for the solder resist of the PCB board factory itself. In addition to the possible defects such as poor cleaning, holes, missing print, exposed copper, uneven printing, poor plugging, etc., sometimes the printing offset and thickness of the solder mask are also one of the quality factors that affect the quality of SMT solder, especially The parts and solder joints of mobile phone boards are getting smaller and smaller.

▪ Will the printed offset of the solder mask on the circuit board cause BGA short circuit?

▪ The green paint and silk screen layer thickness of the original PCB will affect the amount of solder paste and cause BGA short circuit?

Because Solder Mask uses film transfer to determine where to retain or not to retain the solder mask, the alignment between the Solder Mask solder mask and the PCB becomes very important, because once the alignment shifts too much, the solder mask (Ink) may cover and affect the size of the copper foil pad that should be exposed.

As for the printing height of the solder resist, it will affect the amount of solder paste, because the thicker the solder resist, the greater the height difference between it and the solder pad, and the more solder paste printed on the steel plate, there is no problem with large solder pads., But if the amount of solder paste on the small solder pad is too much, the problem of solder short circuit is prone to occur.

Process capability of Solder Mask printing

Because most PCB board factories use scrapers and screens to print the solder resist green paint on the circuit board, but if you look at the circuit board carefully, you will find that the surface of the circuit board is not as flat as you think. There will be traces of copper foil, and there will also be a large area of copper surface. These copper foils that float on the surface of the circuit board will actually affect the thickness of the green paint printing more or less, and because of the influence of the squeegee, the corners of the circuit The position of (Trace corner, B) is sometimes extremely thin.

PCB_Soldermask_thickness Because most PCB manufacturers use scrapers and screens to print the solder mask green paint on the circuit board, but if you look at the circuit board carefully, you will find that the surface of the circuit board is not as flat as you think. There will be traces of copper foil on the surface, and there will also be a large area of copper surface. These copper foils floating on the surface of the circuit board will actually affect the thickness of the green paint printing more or less, and because of the influence of the squeegee, the circuit board The position of the corner (Trace corner, B) is sometimes extremely thin.

Generally speaking, the PCB factory will do some simple size and tolerance control for the soldermask thickness of the four positions in the above figure, but the tolerance range is actually different for each PCB factory. There seems to be no industrial standard. PCB board manufacturers generally do not print the solder mask green paint too thinly, for fear of exposed copper and insufficient filling of the via holes, and the thickness of the screen, the type and pressure of the squeegee, the number of squeegees (usually once for each round trip)... etc. Conditions will affect its thickness.

In addition, in addition to the thickness of the solder resist green paint printing, the accuracy of the printing position is more important. Limited to current technology, the general PCB board factory only uses low-cost image copy technology for the solder resist green paint printing. Therefore, the alignment is not accurate, which has caused some minor troubles for some system operators with high-precision requirements. However, at the current price orientation, the ability is like this, unless the price is increased. Higher-end imaging technology.

The following lists the ability of most PCB board manufacturers to offset the printing of solder mask green paint and the minimum printable width of solder mask green paint:

List the ability of most PCB board manufacturers to offset the printing of solder mask green paint and the minimum printable width of solder mask green paint

▪ E: Minimum soldermask dam/bridge (Minimum solder mask dam/bridge): 4 mil (some PCB manufacturers can achieve 3mil). Solder dam is a solder resist green paint printed between two adjacent solder pads. The main purpose is to prevent solder short circuit. The reason why it is called dam is not that the height of the solder resist green paint is sufficient to isolate the solder. Overflow causes a short circuit, but the surface tension of the solder resist green paint is relatively small relative to the molten solder paste. Even if the original printing of the solder paste will be printed on the green paint, the cohesion of the solder is large, and the solder paste is generally not very far away after melting. The far solder will retract into the solder pad by itself, just as water always flows down, and the molten solder paste will flow to the place where the tin can be eaten. Therefore, printing solder resist green paint between adjacent pads can prevent the problem of solder short circuit.

▪ F: The tolerance of soldermask exposure registration (precision of solder mask green paint printing): +/-2mil (some manufacturers claim to be able to achieve +/-1mil).

If it is "Non-Solder Mask Defined" wiring, the accuracy of this dimension is related to the unilateral distance between the solder mask opening and the solder pad.

▪ G: This size should be the same as E.