PCB News - Analysis on the reliability of printed boards

One of the basic functions of the printed board is to carry the transmission of electrical signals.

Research on the reliability of the PCB printed board is to study that its basic functions are not lost or some of its electrical performance indicators are not decayed, that is, the durability of its functions. This article intends to study the reliability of printed boards from three aspects of printed board downstream users’ post-installation quality, direct user debugging quality and product use quality, so as to characterize the pros and cons of printed board processing quality and provide high-reliability printed boards. The basic way.

1 Reliability analysis of printed boards

1.1 Quality characterization of printed board after installation

After the printed circuit board is installed, the direct reflection of its quality is:

Visually inspect whether there are blistering, white spots, warping, etc. on the printed board.

One of the most concerned is bubbling, which is called "explosion or delamination" in the industry, and the high-reliability printed boards should not have "bubbling" defects after installation. In order to obtain high-reliability printed boards, you must start with the following aspects.

1.1.1 Selection of printed board materials

The performance of the same type of PCB printed board substrate differs greatly from different manufacturers, and the performance difference of different types of printed board substrates is even greater. When selecting a substrate for printed circuit board processing, both the heat resistance of the material and the electrical performance of the material must be considered. As far as installation is concerned, we should consider the heat resistance of the material more. The heat resistance of materials is generally based on the glass transition temperature (Tg) and thermal decomposition temperature (Td) as a reference. At present, the printed board installation is divided into leaded, lead-free, and mixed installation according to the solder joint composition of the components (leaded and lead-free), and the corresponding peak temperature of reflow soldering is 215 degree Celsius, 250 degree Celsius, and 225 degree Celsius. Therefore, for different installation methods, printed board materials should be selected separately. For lead-free soldering, use plates with Tg higher than 170 degree Celsius; for mixed-assembly soldering, use plates with Tg higher than 150 degree Celsius.

For leaded soldering, all materials are suitable, but usually plates with Tg higher than 130 degree Celsius are used. In addition to considering Tg, it is generally necessary to pay attention to the manufacturer's brand and model. At present, the commonly used boards with stable performance include Tuc, IsoIa, Hitachi, Neleo, etc.

1.1.2 Control of the production process

Printed boards must be sampled for delivery and thermal stress experiments before they leave the factory, the purpose of which is to ensure that the installation is not layered. Although products that are fully qualified in the delivery state and thermal stress test cannot be guaranteed to be installed without defects, there must be hidden dangers in the installation of defective products in the delivery state. Therefore, the delivery state and thermal stress test are the early predictions of the installation quality. In this way, the delivery state and thermal stress are necessary conditions for printed board delivery. For this reason, the following aspects should be paid attention to in the processing of printed boards to ensure that the delivery state and thermal stress test are qualified, and to improve the quality after installation.

1.1.2.1 Clarify the processing requirements of printed boards

The number of layers and thickness of the printed board, the pitch of the BGA (or the minimum center distance between the hole walls), and the thickness of the conductor copper affect the results of the printed board thermal stress test. For boards with more than 12 layers and a thickness greater than 3.0 mm, due to the large Z-axis expansion and contraction value, it is easy to produce micro-cracks after thermal stress, resulting in hole wall defects.

BGA pitch is less than 0.8 mm or the hole wall center distance is less than 0.5 mm. Due to the large heat capacity, the heat is concentrated during installation, which is easy to cause delamination of the dielectric layer. Therefore, substrates with Tg greater than 170 degree Celsius should be selected for this type of printed board processing.

The thickness of the conductor is greater than 35 μm, the heat capacity is large, and the resin flow resistance is large. When laminating, try to use multiple prepregs with high fluidity. For printed boards with a hole diameter of less than 0.3 mm, the quality of the drilling directly affects the quality of the hole wall. The drilling parameters should be strictly controlled to ensure that the hole wall is clean, flat, and has little tearing.

1.1.2.2 Refined process control

Delamination in the delivery state and thermal stress experiment is mainly due to the poor bonding strength between the copper and the prepreg due to the quality defects of the oxidation treatment of the inner conductor or the contamination or moisture absorption of the prepreg. The oxidation process is different due to different materials. High Tg materials are hard and brittle, and use velvety brown oxidation, while conventional materials may be crystalline black oxidation. [2] Of course, the roughness of the conductor surface directly affects The bonding strength of copper and prepreg. Therefore, no matter what kind of oxidation treatment, the surface roughness of oxidation must be clearly specified. At the same time, during the lamination process, try to avoid material contamination and moisture absorption. For this reason, the baking conditions of the single chip must be quantitatively controlled, the prepreg must be dehumidified, and the cleanliness of the environment and the operation standard of the laminated board must be controlled. In the lamination process control, effective lamination parameters must be established according to the board type and board volume to ensure sufficient resin wetting and rheological speed to avoid the generation of voids.

1.2 Characterization of printed board debugging quality

Printed board debugging quality is mainly based on whether the debugging results meet the design requirements smoothly, and whether the printed board debugging after installation is smooth, involves the processing quality of the printed board, and is also an important information for the reliability of the printed board. Generally, a board that is debugged smoothly has high reliability; on the contrary, a board that is not debugged smoothly has hidden dangers in its reliability. The processing quality of the printed board mainly involves the line, disk, and media layer of the printed board. .

1.2.1 The influence of printed board wires on the quality of printed boards

With the refined development of electronic products and the continuous improvement of printed board processing technology, the wires of printed boards are no longer simple signal transmission, but are supplemented by many functional requirements such as impedance lines, equal length lines, and reactance lines. Wait. Therefore, wire defects such as gaps, burrs, shape corners, etc., have more and more obvious effects on the performance of the printed board (3). The 10% deviation of the line width may bring about 20% of impedance changes. The wire gaps and burrs make the signal The delay can be up to 0.1 ns, and the difference in the shape of the wire will cause interference such as reflection and noise to affect the integrity of signal transmission. It can be seen that the quality of the line cannot be ignored in the production process of the printed board. On the one hand, strict process control is required. On the other hand, high-precision production equipment and appropriate process technology (such as semi-additive method and additive method) are required to ensure that the accuracy of the line meets the design requirements.

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