Common Problems of PCB Production Line, Solder Mask and Text

1. Why is the BGA located in the solder mask hole? What are the acceptance criteria?

First of all, the purpose of soldering the plug hole is to protect the service life of the via hole. Because the hole diameter of the plug hole required for the BGA position is generally small, between 0.2 and 0.35mm, some potions in the hole are not easy to be baked during the post-processing process. Dried or evaporated, it is easy to leave residues. If the solder mask is not plugged or the plug is not full, there will be residual foreign bodies or tin beads in the post-process processing such as tin spraying and immersion gold. As soon as it is heated during soldering, foreign objects or tin beads in the holes will flow out and adhere to the components, causing defects in the performance of the components, such as open and short circuits. BGA is located in the solder mask plug hole a. It must be fully plugged. b. There should be no redness or false copper exposure. c. 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?

The table glass of the exposure machine does not refract light when light shines through it. If the reflector of the exposure lamp is flat and smooth, then when the light hits it, according to the principle of light, it forms only one reflected light on the board to be exposed. In principle, the light that hits the concave and the light that hits the convex will form countless scattered rays, forming irregular but uniform light on the board to be exposed, improving the effect of exposure.

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

The bottom width area of the part where the green oil on one side of the solder mask is opened is called side development. When the side development is too large, it means that the area of the green oil that is developed to be in contact with the substrate or the copper skin is larger, and the greater the degree of suspension it forms, in the post-process processing such as: spray tin, immersion tin , Immersion gold and other side developing parts will form oil loss when they are attacked by high temperature, pressure and some potions that are more aggressive to green oil. will cause a bridge short circuit.

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

After being processed by the solder mask process, the pads for mounting components in the post-process or the places to be soldered are exposed. During the solder mask alignment/exposure process, this part is caused by the problem of the light shield or the exposure energy and operation. The outside or all of the covered green oil is exposed to light, and a cross-linking reaction occurs. During development, this part of the green oil will not be dissolved by the solution, and the outside or all of the pad to be soldered cannot be exposed. This is called poor solder exposure. . Poor exposure in the post-processing process will lead to inability to mount components, poor soldering, and severe open circuits.

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

a. The circuit board surface includes a foil-clad substrate and a substrate pre-coated with copper after hole metallization. In order to ensure the firm adhesion of the dry film to the surface of the substrate, the surface of the substrate is required to be free of oxide layers, oil stains, fingerprints and other contaminants, no drilling burrs, and no rough coating. In order to increase the contact area between the dry film and the substrate surface, the substrate is also required to have a microscopically rough surface. In order to meet the above two requirements, the substrate should be carefully treated before the film is applied. The treatment methods can be summarized into two categories: mechanical cleaning and chemical cleaning.

b. The same is true for the same solder mask. The grinding plate before solder mask is to remove some oxide layers, oil stains, fingerprints and other dirt on the surface of the board. In order to increase the contact area between the solder mask ink and the board surface and make it firmer It is also required to have a micro-rough surface on the board surface (just like a tire on a car, the tire must be roughened to a better bond with the glue). If there is no oxide layer, oil stain, etc. on the surface of the board to be filmed or to be printed on the solder mask, it will directly separate the solder mask and the circuit film from the board surface. When isolation is formed, the film at this place will be peeled off and peeled off in the post-processing process.

6. What is viscosity? How does the viscosity of solder mask ink affect PCB production?

Viscosity - A measure of resistance or resistance to flow. The viscosity of the solder mask ink has a considerable impact 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 surface will increase, which is easy to cause oil to enter the hole. and local sub-oil book. Relatively speaking, when the copper thickness of the etched outer layer is thick (≥1.5Z0), it is necessary to control the viscosity of the ink to be lower. If the viscosity is too high, the fluidity of the ink will be reduced. Will form but oil, dew lines.

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

The same point: a is that there is solder mask oil remaining on the surface where copper/gold needs to be soldered after solder mask exposure, b, the reasons are basically the same, the baking time, temperature, exposure time and energy.

Differences: The area formed by poor exposure is large, the residual solder mask oil is from the outside to the 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 rays. irradiation of light. The solder mask oil remaining after the development is not clean is only a thin layer of oil at the bottom. Its area is not large, and it only forms a thin film state. This part of the ink is mainly due to different curing factors, which is different from the surface layer ink. A layered shape that generally occurs on pads with holes.

8. Why are there bubbles in the solder mask? How to prevent?

a. Solder mask oil is generally prepared by mixing 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 is squeezed by the scraper and the screen After pressing, it flows to the board surface. When encountering strong light or equivalent temperature in a short period of time, the gas in the ink will flow rapidly with the ink and quickly volatilize outward.

b. The line spacing is too narrow and the lines are too high. The solder mask ink cannot be printed on the substrate during screen printing, resulting in the existence of air or moisture between the solder mask ink and the substrate. During curing and exposure, the gas is heated to expand and cause bubbles.

c. A single line is mainly caused by the line being too high. When the scraper is in contact with the line, the angle between the scraper 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 small bubble will then form.

Prevention: a. The prepared ink is still for a certain period of time before it is used for printing; b. The printed board is also still for a certain period of time, so that the gas in the ink on the surface of the board gradually evaporates with the flow of the ink, and then take it out. Go to a certain temperature to bake.