4. Quality Problems in Actual Production
In the process of laser drilling, there are many quality problems, and we do not intend to give a comprehensive description, but only put forward the most likely quality problems for reference by peers.
(1) Misalignment between the CO2 laser drilling position and the bottom target position of the copper window method.
In laser drilling, the beam positioning system is critical to the accuracy of aperture shaping. Although the precise positioning of the beam positioning system is used, due to the influence of other factors, the lack of flame with hole shape deformation often occurs. The reasons for the quality problems in the production process are analyzed as follows:
A. The film for making the pads and wire patterns of the inner core board, and the film for opening the window after the resin-coated copper foil (RCC) layer is added, both of which will be potential factors for size increase and shrinkage due to the influence of humidity and temperature.
B. When the core board makes the wire pad pattern, the size of the substrate expands and shrinks, and after the high-temperature press-coated resin-coated copper foil (RCC) layer is added, the inner and outer substrate materials appear again.
C. The size and position of the copper window opened by etching will also cause errors.
D. The error caused between the spot of the laser machine and the displacement of the table.
E. The second-order blind hole alignment is more difficult and more likely to cause position errors.
According to the analysis of the above reasons, according to the relevant technical data and the experience of the actual operation process, the main technical countermeasures are as follows:
a. To reduce the size of the typesetting, most manufacturers use 450 × 600 or 525 × 600 (mm) for the typesetting of multi-layer boards. However, for the mobile phone board with a wire width of 0.10mm and a blind hole diameter of 0.15mm, it is best to use the upper limit of the typesetting size of 350×450 (mm).
b. Increase the diameter of the laser: The purpose is to increase the coverage of the copper window. The specific method is “beam diameter = hole diameter + 90~100 μm. When the energy density is insufficient, one or two shots can be used to solve it.
c. The process method of opening a large copper window is adopted: At this time, the size of the copper window is enlarged but the aperture is not changed, so the diameter of the laser hole is no longer completely determined by the position of the window, so that the hole position can be directly determined according to the bottom of the core board. Pad target location to burn holes.
d. Change from photochemical imaging and etching to YAG laser window opening method: Use YAG laser light spot to open the window according to the reference hole of the core board, and then use CO2 laser to burn out the hole at its window level to solve the problem caused by imaging.
e. Laminate twice and then make second-order micro-blind via method: After a layer of resin-coated copper foil (RCC) is laminated on both sides of the core board, if it is necessary to laminate once more RCC and make second-order blind vias (ie, double-layered ), the alignment of the blind holes of “product two” must be aimed at “product one” to form holes. The original target of the core board cannot be reused. That is to say, when “piling up” a hole and a pad, the edge of the board will also make a target. Therefore, after the RCC of “Jiji 2” is pressed and pasted, four mechanical reference holes of “Jiji 2” can be drilled through the X-ray machine to the target on “Jiji 1”, and then the holes and lines are formed to take This method can make “product two” align with “product one” as much as possible.
(2) Incorrect Hole Pattern
According to the accumulation of many production experiences, it is mainly due to the quality problems of the used substrate molding. The main quality problem is that the thickness of the dielectric layer after the resin-coated copper foil is pressed and pasted is inevitably different. Under the same drilling energy, The bottom pad of the thinner part of the dielectric layer not only has to withstand more energy, but also reflects more energy, so the hole wall is made into a pot shape that expands outward. This will have a greater impact on the quality of electrical interconnections between layers of a build-up multilayer board.
The reliability of the high-density interconnects structure of the build-up multilayer printed circuit board will bring a series of technical problems due to the incorrect hole pattern.
Therefore, technological measures must be adopted to control and solve them. The following processes are mainly used:
(1) Strictly control the thickness difference of the dielectric layer when the resin-coated copper foil is pressed to be between 510μm.
(2) Change the energy density and pulse number (number of guns) of the laser, and find out the process conditions for mass production through the test method.
(3) Poor removal of the slag at the bottom of the hole and the broken slag on the hole wall.
This type of quality problem is the most likely to occur, because a little improper control will produce this type of problem. Especially when dealing with multi-hole type laminates on large impositions, it is impossible to guarantee 100% that there will be no quality problems. This is because the number of micro-blind holes on the processed large-scale board is too large (about 60,000 to 90,000 holes on average), and the thickness of the dielectric layer is different. When the laser drilling with the same energy is used, the residue left on the bottom pad. The thickness is also different. It is impossible to ensure that all the residues are completely clean after the decontamination treatment. In addition, the inspection methods are relatively poor. Once defects are found, the bonding force between the subsequent copper plating layer and the bottom pad and the hole wall is often caused.
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