PCB Tech - Why is the PCB board deformed and how to prevent it 1

Why is the PCB board deformed and how to prevent it 1

When PCB boards undergo reflow soldering, most of them are prone to board bending and warping. In severe cases, it may even cause components such as empty soldering and tombstones. How to overcome it?

1. The hazards of PCB board deformation

In the automated surface mount line, if the circuit board is not flat, it will cause inaccurate positioning, components can not be inserted or mounted on the hole and surface mount pad of the board, and even the automatic insertion machine will be damaged. The circuit board with the components is bent after soldering, and the component feet are difficult to cut neatly. The board cannot be installed on the chassis or the socket inside the machine, so it is also very annoying for the assembly plant to encounter the board warping. The current surface mount technology is developing in the direction of high precision, high speed, and intelligence, which puts forward higher flatness requirements for PCB boards that are home to various components.

In the IPC standard, it is specifically pointed out that the maximum allowable deformation of PCB boards with surface mount devices is 0.75%, and the maximum allowable deformation of PCB boards without surface mounting is 1.5%. In fact, in order to meet the needs of high-precision and high-speed placement, some electronic assembly manufacturers have stricter requirements for deformation. For example, our company has multiple customers who require a maximum deformation of 0.5%, and even some customers require it. 0.3%.

PCB board is composed of copper foil, resin, glass cloth and other materials. The physical and chemical properties of each material are different. After being pressed together, thermal stress will inevitably be left and cause deformation. At the same time, in the PCB processing process, it will go through various processes such as high temperature, mechanical cutting, and wet treatment, which will also have an important impact on the deformation of the board. In short, the reasons that can cause the deformation of the PCB board are complex and diverse. Distortion or deformation caused by processing has become one of the most complicated problems faced by PCB manufacturers.

2. Analysis of the causes of PCB board deformation

The deformation of PCB board needs to be studied from several aspects such as material, structure, pattern distribution, processing process, etc. This article will analyze and explain various reasons and improvement methods that may occur.

The copper surface area on the circuit board is uneven, which will worsen the bending and warping of the board.

Generally, a large area of copper foil is designed on the circuit board for grounding purposes. Sometimes there is also a large area of copper foil designed on the Vcc layer. When these large area copper foils cannot be evenly distributed on the same circuit board When it is installed, it will cause uneven heat absorption and heat dissipation. Of course, the circuit board will also expand and contract. If the expansion and contraction cannot be done at the same time, it will cause different stress and deformation. At this time, if the temperature of the board has reached At the upper limit of the Tg value, the board will begin to soften, causing permanent deformation.

The connection points (vias, vias) of each layer on the circuit board will limit the expansion and contraction of the board.

Today's circuit boards are mostly multi-layer boards, and there will be rivet-like connection points (vias) between the layers. The connection points are divided into through holes, blind holes and buried holes. Where there are connection points, the board will be restricted. The effect of expansion and contraction will also indirectly cause plate bending and plate warping.

Reasons for PCB board deformation:

(1) The weight of the circuit board itself will cause the board to dent and deform

Generally, the reflow furnace uses a chain to drive the circuit board forward in the reflow furnace, that is, the two sides of the board are used as fulcrums to support the entire board. If there are heavy parts on the board, or the size of the board is too large, It will show a depression in the middle due to the amount of seed, causing the plate to bend.

(2) The depth of the V-Cut and the connecting strip will affect the deformation of the jigsaw

Basically, V-Cut is the culprit that destroys the structure of the board. Because V-Cut cuts grooves in the original large sheet, the V-Cut is prone to deformation.

2.1 Analysis of the influence of pressing materials, structures and graphics on plate deformation

PCB board is formed by pressing the core board and the prepreg and the outer copper foil. The core board and the copper foil are heated and deformed when they are pressed together. The amount of deformation depends on the coefficient of thermal expansion (CTE) of the two materials;

The coefficient of thermal expansion (CTE) of copper foil is about 17X10-6;

The Z-direction CTE of ordinary FR-4 substrate at Tg point is (50~70)X10-6;

TG point above is (250~350)X10-6, X-direction CTE is generally similar to copper foil due to the presence of glass cloth.

Notes on TG point:

High Tg printed board When the temperature rises to a certain area, the substrate will change from "glass state" to "rubber state", and the temperature at this time is called the glass transition temperature (Tg) of the board. That is, Tg is the highest temperature (°C) at which the base material maintains rigidity. That is to say, ordinary PCB substrate materials not only produce softening, deformation, melting and other phenomena at high temperatures, but also exhibit a sharp decline in mechanical and electrical properties.

The general Tg of the sheet material is above 130 degrees, the high Tg is generally greater than 170 degrees, and the medium Tg is approximately greater than 150 degrees.

Usually the PCB printed circuit board with Tg≥170 degree Celsius is called high Tg printed circuit board.

The Tg of the substrate is increased, and the heat resistance, moisture resistance, chemical resistance, stability and other characteristics of the printed board will be improved and improved. The higher the TG value, the better the temperature resistance of the plate, especially in the lead-free process, where high Tg applications are more common.

High Tg circuit board sheet refers to high heat resistance. With the rapid development of the electronics industry, especially the electronic products represented by computers, the development of high functionality and high multilayers requires higher heat resistance of PCB substrate materials as an important guarantee. The emergence and development of high-density mounting technologies represented by SMT and CMT have made PCBs more and more inseparable from the support of high heat resistance of substrates in terms of small aperture, fine wiring, and thinning.

Therefore, the difference between general FR-4 and high Tg FR-4 is the mechanical strength, dimensional stability, adhesion, water absorption, and thermal decomposition of the material in the hot state, especially when heated after moisture absorption. There are differences in various conditions such as thermal expansion, and high Tg products are obviously better than ordinary PCB substrate materials.

Among them, the expansion of the core board with the inner layer pattern is different due to the difference between the pattern distribution and the thickness of the core board or the material characteristics. When the pattern distribution is different from the thickness of the core board or the material characteristics, it will be different. Will be deformed. When the PCB laminate structure has asymmetry or uneven pattern distribution, the CTE of different core boards will vary greatly, which will cause deformation during the lamination process. The deformation mechanism can be explained by the following principles.

Suppose there are two kinds of core boards with a large difference in CTE that are pressed together by prepregs. Among them, the CTE of A core board is 1.5*10-5/ degree Celsius, and the length of the core board is 1000mm. In the pressing process, the prepreg, which is used as the bonding sheet, will bond the two core boards together through three stages of softening, flowing and filling with graphics, and curing.

According to the above analysis, it can be seen that whether the laminated structure and material type of the PCB board have been distributed uniformly, which directly affects the CTE difference between different core boards and copper foils. The difference in expansion and contraction during the lamination process will pass through the solid sheet of the prepreg. The process is retained and finally forms the deformation of the PCB board.