1. The hazards of PCB board deformation
On an automated surface mount line, if the circuit board is not flat, it will cause inaccurate positioning, components cannot be inserted or mounted on the board's hole and surface mount pad, and even the automatic insertion machine will be damaged. The circuit board on which the components are mounted bends after soldering, and the component feet are difficult to cut flat. The board can not be installed in the case or the socket in the machine, so the assembly factory is also very troubled when it encounters the board warp. The current surface mount technology is developing in the direction of high precision, high speed, and intelligence, which puts forward higher flatness requirements for the PCB board as a home of various components.In the IPC standard, the maximum allowable deformation of PCB boards with surface-mounted devices is 0.75%, and the maximum allowable deformation of PCB boards without surface-mount devices 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 on the amount of deformation.
The 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 residue will inevitably occur, resulting in deformation. At the same time, in the PCB processing process, it will go through various processes such as high temperature, mechanical cutting, wet treatment, etc., and it will also have an important impact on the deformation of the board. In short, the reasons for the PCB board deformation are complex and diverse. How to reduce or eliminate the material characteristics Distortion caused by different or processing has become one of the most complicated problems faced by PCB manufacturers.
2. Analysis of the causes of deformation
The deformation of the PCB board needs to be studied from several aspects such as material, structure, graphic distribution, processing process, etc. This article will analyze and elaborate on various causes and improvement methods that may cause deformation.The uneven copper surface area on the circuit board will worsen the board bending and board warpage.
Generally, a large area of copper foil is designed on the circuit board for grounding. Sometimes, a large area of copper foil is designed on the Vcc layer. When these large area copper foils cannot be evenly distributed on the same circuit board When it is on, it will cause the problem of uneven heat absorption and heat dissipation speed. Of course, the circuit board will also expand and contract. If the expansion and contraction can not simultaneously cause different stress and deformation, then 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) of the various layers on the circuit board will limit the board's expansion and contraction
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. The connection points will limit the board. The effect of expansion and contraction will also indirectly cause bending and warping.
The weight of the circuit board itself will cause the board to sag and deform
Generally, the reflow furnace will use a chain to drive the circuit board in the reflow furnace, that is, use the two sides of the board as a fulcrum to support the whole board. If there are too many parts on the board, or the size of the board is too large, It will show the phenomenon of depression in the middle because of its own species, causing the plate to bend.
The depth of V-Cut and the connecting bar will affect the amount of panel deformation
Basically, V-Cut is the culprit of destroying the structure of the board, because V-Cut cuts grooves on the original large plate, so the V-Cut is prone to deformation.
2.1. Analysis of the influence of PCB Lamination materials, structure and graphics on the deformation of the plate
The 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 deformed by heat when they are pressed. The amount of deformation depends on the coefficient of thermal expansion (CTE) of the two materials.The coefficient of thermal expansion (CTE) of the copper foil is about
The Z-direction CTE of the ordinary FR-4 substrate at the Tg point is;
Above TG point is (250 ~ 350) X10-6, X-directional CTE is generally similar to copper foil due to the existence of glass cloth.
Notes on TG points:
When the temperature of a high Tg printed board rises to a certain area, the substrate will change from "glass state" to "rubber state". The temperature at this time is called the glass transition temperature (Tg) of the board. That is, Tg is the maximum temperature (° C) at which the base material remains rigid. In other words, ordinary PCB substrate materials not only produce softening, deformation, melting and other phenomena at high temperature, but also show a sharp decline in mechanical and electrical characteristics.
The general Tg sheet is above 130 degrees, the high Tg is generally greater than 170 degrees, and the medium Tg is approximately greater than 150 degrees.
Usually PCB printed board with Tg≥170 ℃ is called high Tg printed 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 sheet, especially in the lead-free process, high Tg applications are more.
High Tg 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 apertures, fine wiring and thinning.
Therefore, the difference between the general FR-4 and the high Tg FR-4: the mechanical strength, dimensional stability, adhesion, water absorption, thermal decomposition of the material in the hot state, especially under the heat after moisture absorption , Thermal expansion, and other conditions are different, high Tg products are significantly better than ordinary PCB substrate materials.
The expansion of the core board in which the inner layer graphics are done is different due to the difference in the graphic distribution and the thickness or material characteristics of the core board. When the graphic distribution is different from the thickness or material characteristics of the core board, when the graphic distribution is more uniform, the material type is consistent, not Will deform. When the laminate structure of the PCB board is asymmetric or the pattern distribution is uneven, it will cause a large difference in CTE of different core boards, thereby causing deformation during the lamination process. The deformation mechanism can be explained by the following principles. Suppose that there are two kinds of core plates with a large difference in CTE which are pressed together through the prepreg. The CTE of the A core plate is 1.5 × 10-5 / ℃, and the core plate length is 1000mm. In the compressing process, the prepreg, which is used as the adhesive sheet, is bonded to the two core boards through the three stages of softening, flowing, filling graphics, and curing.
Figure 1 is the dynamic sticky bottom curve of ordinary FR-4 resin at different heating rates. In general, the material starts to flow from about 90 ° C, and cross-linking and curing begins when it reaches above the TG point. The semi-cured sheet is in a free state before curing. At this time, the core board and the copper foil are in a state of free expansion after being heated, and the amount of deformation can be obtained from the respective CTE and temperature change values.
Simulate pressing conditions, the temperature rises from 30 ℃ to 180 ℃,
At this time, the deformation of the two core plates is
△ LA = (180 ℃ ~ 30 ℃) x1.5 × 10-5m / ℃ X1000mm = 2.25mm
△ LB = (180 ℃ ~ 30 ℃) X2.5X10-5M / ℃ X1000mm = 3.75mm
At this time, since the semi-curing is still in a free state, the two types of core plates are long and short, do not interfere with each other, and have not yet deformed.
When pressed, it will be kept at high temperature for a period of time until the semi-cured is completely cured. At this time, the resin becomes cured and cannot flow freely. The two core plates are combined together. When the temperature drops, such as without interlayer resin binding, the core The board will return to its original length without deformation, but in fact the two core boards have been bonded by the cured resin at high temperature and cannot shrink at will during the cooling process. The A core board should shrink by 3.75mm. When the shrinkage is greater than 2.25mm, it will be hindered by the A core board. In order to achieve the balance of the force between the two core boards, the B core board cannot shrink to 3.75mm, and the A core board shrinkage will be greater than 2.25mm, so that the whole board toward the B core Board direction curvature According to the above analysis, it can be seen that the laminated structure and material type of the PCB board have a uniform graphic distribution, 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 The curing process of the prepreg is retained and eventually deforms the PCB.
