Summary of common problems in PCB dry area (circuit, solder mask, text) (1)

1. Why should the BGA solder mask of PCB board contain holes? What is the reception standard?

Answer: First of all, the solder mask plug hole is to protect the service life of the via, because the hole required for the BGA position is generally smaller, between 0.2-0.35mm, some of the liquid in the hole is not easy to be Drying or evaporating away, it is easy to leave residues. If the solder mask does not plug the hole or the plug is not full, there will be residual foreign matter or tin beads in the subsequent processing such as spraying tin or immersion gold. Install components on the customer When heated during high-temperature soldering, foreign matter or tin beads in the hole will flow out and adhere to the component, causing defects in component performance, such as open and short circuits. BGA is located in the solder mask plug hole A, must be full B, no redness or false copper exposure is allowed, C, no too full bumps are higher than the pads that need to be soldered next to it (which will affect component mounting Effect).

2. What is the difference between the countertop glass of the exposure machine and ordinary glass? Why is the reflector of the exposure lamp uneven? .........

 Answer: The table glass of the exposure machine will not produce light refraction when the light passes through it. If the reflector of the exposure lamp is flat and smooth, when the light shines on it, according to the principle of light, it forms only one reflected light shining on the board to be exposed. If it is pits and bumps, it is based on the light. The principle is that the light shining on the recesses and the light shining on the protrusions will form countless scattered rays of light, forming irregular but uniform light on the board to be exposed, improving the exposure effect.

3. What is side development? What are the quality consequences caused by side development?

Answer: The bottom width area of the part where the green oil on one side of the solder mask window is developed is called side development. When the side development is too large, it means that the green oil area of the part that is developed and which is in contact with the substrate or the copper skin is larger, and the degree of dangling formed by it is larger. The subsequent processing such as tin spraying, tin sinking , Immersion gold and other side developing parts are attacked by high temperature, pressure and some potions that are more aggressive to green oil. Oil will drop. If there is a green oil bridge on the IC position, it will be caused when the customer installs the welding components. Will cause a bridge short circuit.

4. What is poor solder mask exposure? What quality consequences will it cause?

Answer: After being processed by the solder mask process, it is exposed to the solder pads or the places where the components need to be soldered in the later process. During the solder mask alignment/exposure, it is caused by the light barrier or the exposure energy and operation problems. The outside or all of the green oil covered by this part is exposed to light to cause a cross-linking reaction. During development, the green oil in this part will not be dissolved by the solution, and the outside or all of the pad to be soldered cannot be exposed. This is called welding. Poor exposure. Poor exposure will result in failure to mount components in the subsequent process, poor soldering, and, in severe cases, an open circuit.

5. Why do we need to pre-process the grinding plate for circuit and solder mask?

Answer: 1. The circuit board surface includes the foil-clad board substrate and the substrate with pre-plated copper after hole metallization. In order to ensure that the dry film adheres firmly to the surface of the substrate, the substrate surface is required to be free of oxide layers, oil stains, fingerprints and other dirt, no drilling burrs, and no rough plating. In order to increase the contact area between the dry film and the surface of the substrate, the substrate is also required to have a micro-rough surface. In order to meet the above two requirements, the substrate must be carefully processed before filming. The treatment method can be summarized as mechanical cleaning and chemical cleaning.

2. The same principle is true for the same solder mask. Grinding the board before solder mask is to remove some oxide layers, oil stains, fingerprints and other dirt on the board surface, in order to increase the contact area of the solder mask ink and the board surface and make it stronger. It is also required that the board surface has a microscopic rough surface (just like the tire of a car repair, the tire must be ground into a rough surface to be better combined with the glue). If you do not use grinding before the circuit or solder mask, the surface of the board to be pasted or printed solder mask has some oxide layers, oil stains, etc., it will directly separate the solder mask and the circuit film from the board surface The isolation is formed, and the film in this area will fall off and peel off in the later process.

6. What is viscosity? What effect does the viscosity of solder mask ink have on the production of PCB boards?

Answer: Viscosity-is a measure of preventing or resisting flow. The viscosity of the solder mask ink has a considerable influence on the production of PCB. When the viscosity is too high, it is easy to cause no oil or stick to the net. When the viscosity is too low, the fluidity of the ink on the board will increase, which is easy to cause oil to enter holes. And local oil book. Relatively speaking, when the outer copper layer is thicker (≥1.50Z), the viscosity of the ink should be controlled to be lower. If the viscosity is too high, the fluidity of the ink will decrease. At this time, the bottom of the circuit and the corners will be It will not be oily or exposed.

7. What are the similarities and differences between poor development and poor exposure?

Answer: The same points: a. There is solder mask oil on the surface where the copper/gold needs to be soldered after the solder mask. The cause of b is basically the same, the time, temperature, exposure time and energy of the baking sheet.

Differences: The area formed by poor exposure is larger, and the remaining solder mask is from outside to inside, and the width and Baidu are relatively uniform. Most of them appear on the non-porous pads, mainly because the ink in this part is exposed to ultraviolet light. The light shines. The remaining solder mask oil from poor development is only thinner at the bottom of the layer. Its area is not large, but forms a thin film state. This part of the ink is mainly due to different curing factors, which is formed by the surface layer ink. A hierarchical shape, which generally appears on a holed pad.

8. Why does the solder mask produce bubbles? How to prevent it?

Answer: (1) Solder mask oil is generally mixed and formulated by the main agent of the ink + curing agent + diluent. During the mixing and stirring of the ink, some air will remain in the liquid. When the ink passes through the scraper, silk The nets are squeezed into each other and flow to the plate surface. When they encounter strong light or equivalent temperature in a short time, the gas in the ink will flow rapidly with the mutual acceleration of the ink, and evaporate sharply. (2 ), the line spacing is too narrow, the lines are too high, the solder mask ink cannot be printed on the substrate during screen printing, resulting in the presence of air or moisture between the solder mask ink and the substrate, and the gas is heated to expand during curing and exposure, causing bubbles. (3) A single line is mainly caused by the high line. When the squeegee is in contact with the line, the angle of the squeegee and the line increases, so that the solder mask ink cannot be printed to the bottom of the line, and there is gas between the side of the line and the solder mask ink , A kind of small bubbles will be formed when heated.

Prevention: a. The formulated ink is static for a certain period of time before printing, b. The printed board is also static for a certain period of time so that the gas in the ink on the surface of the board will gradually volatilize with the flow of the ink, and then take it for a certain amount Bake at the temperature.

9. What is resolution?

Answer: Within a distance of 1mm, the resolution of the lines or spacing lines that can be formed by the dry film resist can also be expressed by the absolute size of the lines or spacing. The difference between the dry film and the resist film thickness The thickness of the polyester film is related. The thicker the resist film layer, the lower the resolution. When the light passes through the photographic plate and the polyester film and the dry film is exposed, due to the light scattering effect of the polyester film, the lighter side Seriously, the lower the resolution.

10. What is dry film etching resistance and plating resistance?

Answer: Etching resistance: The dry film resist layer after photopolymerization should be able to withstand the etching of ferric chloride etching solution, persulfuric acid etching solution, acid chlorine, copper etching solution, sulfuric acid-hydrogen peroxide etching solution. In the above etching solution, when the temperature is 50-55°C, the surface of the dry film should be free of hair, leakage, warping and shedding. Electroplating resistance: in acidic bright copper plating, fluoroborate common lead alloy, fluoroborate bright tin-lead alloy plating and various pre-plating solutions of the above electroplating, the dry film resist layer after polymerization should have no surface hair , Infiltration, warping and shedding.

To be continued..............

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