When the circuit board is reflow soldering, it is easy for the board to bend and become warped. If it is serious, it will even cause the situation of empty soldering and stele. How should it be overcome?
1. Damage of PCB circuit board deformation
On the automatic surface mount line, if the circuit board is not smooth, it will cause inaccurate positioning, the components cannot be inserted or pasted to the holes of the board and the surface mount pad, and even the automatic insert machine will be damaged. The circuit board fitted with the components is bent after welding, and the component feet are difficult to be cut in order. The boards can’t fit into the case or the socket, so it’s equally annoying for an assembly plant to get stuck on the board. At present, the surface mount technology is developing towards high precision, high speed and intelligent direction, which puts forward a higher flatness requirement for PCB board which is home to various components.
In the IPC standard, the maximum allowable deformation of PCB board with surface mount device is 0.75%, and the maximum allowable deformation of PCB board without surface mount device is 1.5%. In fact, in order to meet the requirements of high precision and high speed mounting, some electronic mounting manufacturers have more stringent requirements on deformation amount. For example, the maximum deformation amount allowed by several customers of our company is 0.5%, and even some customers require 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. When pressed together, thermal stress residue will inevitably occur, leading to deformation. At the same time in the process of PCB processing, through high temperature, mechanical cutting, wet process and other process, will produce a significant influence on the plate deformation, in short can cause of PCB deformation is complicated, how to reduce or eliminate caused by different material properties and processing, the deformation of the PCB manufacturers one of the most complex problems.
2. Analysis of causes of deformation
The deformation of PCB board needs to be studied from the aspects of material, structure, graphics distribution and processing process. In this paper, various reasons for possible deformation and improvement methods are analyzed and expounded.
The area of the copper surface on the circuit board is not uniform, which will worsen the bending and warping of the board.
General circuit boards will be designed with a large area of copper foil for grounding, sometimes Vcc layer has designed a large area of copper foil, when these large areas of copper foil can not evenly distributed in the same circuit boards, will cause uneven heat and cooling speed, circuit boards, of course, also can heat bilges cold shrink, if increases and cannot at the same time can cause different stress and deformation, the temperature of the board at this time if the Tg has reached the upper limit value, the board will start to soften, causing permanent deformation.
The connection points (vias, via holes) on each layer of the circuit board will limit the expansion and shrinkage of the board
Nowadays, the board is mostly multi-layered, and there will be rivet-like connection points (vias) between the layers. The connection points are divided into through-hole, blind hole and buried hole. Where there are connection points, the effect of expansion and contraction of the board will be limited, and also the bending and warping of the board will be indirectly caused.
The weight of the board itself can deform the indentation of the board
Generally, the backweld furnace will use the chain to drive the circuit board forward in the backweld furnace, that is, the whole board will be supported by the fulcrum on both sides of the board.
The depth and connection of the v-cut will affect the deformation of the panel
Basically, v-cut is the culprit of damaging the structure of the board, because v-cut is to Cut the groove on the original large sheet of board, so the place where v-cut is prone to deformation.
2.1 sound analysis of compression materials, structures and graphics on the deformation of plates
PCB board is formed by the compression of core board, semi-solidified sheet and outer copper foil, in which the core board and copper foil are subject to thermal deformation when pressed together, and the deformation amount depends on the coefficient of thermal expansion (CTE) of the two materials.
The thermal expansion coefficient (CTE) of copper foil is about 17×10-6.
However, the Z CTE of common fr-4 substrate at Tg point is (50~70) x10-6.
The TG point above is (250~350) x10-6, and x-direction CTE is generally similar to copper foil due to the presence of glass cloth.
Notes on TG points:
When the temperature of high Tg printing plate increases to a certain area, the base plate will change from “glass state” to “rubber state”. The temperature at this time is called the vitrification temperature (Tg) of the plate. That is to say, the Tg is to maintain the rigidity of base material the highest temperature (℃). That is to say, common PCB substrate material not only produces softening, deformation, melting and other phenomena, but also exhibits a sharp decline in mechanical and electrical characteristics.
Generally, the plate of Tg is above 130 degrees, high Tg is above 170 degrees and medium Tg is above 150 degrees.
Tg of 170 ℃ or more usually PCB printed circuit board, called high Tg, PCB.
The Tg of substrate is improved, and the thermal resistance, humidity resistance, chemical resistance and stability of printed board can be improved. The higher the TG value, the better the temperature resistance of the plate, especially in lead-free process, the higher TG is applied.
High Tg refers to high heat resistance. With the rapid development of the electronics industry, especially the electronic products represented by computers, the PCB substrate material needs higher heat resistance as an important guarantee. The appearance and development of high-density installation technology represented by SMT and CMT have made the PCB more and more inseparable from the support of the high-heat resistance of the substrate in the aspect of small aperture, fine wiring and thin molding.
Therefore, the difference between the general fr-4 and the high Tg fr-4 is that under the thermal state, especially under the heat after moisture absorption, the mechanical strength, dimensional stability, adhesion, water absorption, thermal decomposition and thermal expansion of the materials are different, and the high Tg product is significantly better than the common PCB substrate material.
The expansion of the core board with good interior graphics is different due to the difference of the graph distribution with the thickness of the core board or material characteristics. When the graph distribution is different with the thickness of the core board or material characteristics, when the graph distribution is more uniform and the material type is consistent, no deformation will occur. When the laminated structure of PCB plate is asymmetric or the graph distribution is not uniform, the CTE of different core plates will be greatly different, which will lead to deformation during the compaction process. The deformation mechanism can be explained by the following principles.
FIG. 1 dynamic viscosity curve of ordinary semi-cured tablets
Suppose there are two kinds of CTE large difference of core board through A curing press together, in which A core board CTE is 1.5 x10-5 / ℃, core board length is 1000 mm. In the pressing process, the semi-solidified sheet, which is used as the bond sheet, is bonded together with two core boards through three stages of softening, flowing and filling.
Figure 1 for ordinary FR – 4 resin under different heating rate in the bottom of the kinematic viscosity curve, in general, materials from 90 ℃ to flow, and achieve above TG for the start of crosslinking curing, and a half before curing curing state for freedom, the free expansion core board and copper foil in a heated state, its deformation can through their worth to the CTE and temperature change.
To simulate pressing conditions, temperature from 30 ℃ to 180 ℃,
At this time, the deformation of the two core plates are respectively
Delta LA = (180 ℃ ~ 30 ℃) x1.5 x10-5 m / ℃ X1000mm = 2.25 mm
Delta LB = (180 ℃ ~ 30 ℃) X2.5 X10-5 m / ℃ X1000mm = 3.75 mm
At this time, because the semi-solidification is still in the free state, the two core plates are one long and one short, non-interference, and have not yet deformed.
, as shown in figure 2, and press fit will remain at high temperatures for A period of time, until A cure completely cure, resin to curing condition, at this time can’t flow, two kinds of core board together. When the temperature drops, without interlayer resin bound, core board will be back to the initial length, will not produce deformation, but it’s actually two core board at high temperature has been cured resin bond, not voluntary contraction in the cooling process, in which A core board contraction should be 3.75 mm, actually when contraction is greater than 2.25 mm is hampered by A core board, to achieve the force balance between the two core board, B core board can’t shrink to 3.75 mm, and A core The contraction of the plate will be greater than 2.25mm, so that the whole plate will curve towards the B core plate, as shown in figure 2.
Fig.2 deformation diagram of different CTE core plates during pressing
Based on the above analysis, it can be seen that whether the lamination structure and material type of PCB board have been distributed uniformly or not has a direct impact on the CTE difference between different core boards and copper foil. In the process of compaction, the difference of expansion and contraction will be retained through the fixation process of the half solidified sheet and finally form the deformation of PCB board.
2. 2 deformation during PCB processing
The reason of deformation in PCB sheet machining process is very complex, which can be divided into thermal stress and mechanical stress. Among them, the thermal stress is mainly produced in the pressing process, while the mechanical stress is mainly produced in the process of piling, transporting and baking. The following is a brief discussion in process order.
Copper clad sheet material: the copper clad sheet is double-faced, with symmetrical structure and no graphics. The copper foil is almost the same as CTE of glass cloth, so the deformation caused by different CTE will hardly occur in the process of compaction. However, due to the large size of the copper clad plate press and the temperature difference in different areas of the hot plate, the curing speed and degree of the resin in different areas in the pressing process will vary slightly, and the dynamic viscosity at different heating rates will also vary greatly, so local stress caused by the differences in the curing process will also occur. In general, the stress will be balanced after compression, but will gradually release in the later processing and produce deformation.
Pasting: PCB pressing is the main process of generating thermal stress, among which the deformation caused by different materials or structures is analyzed in the previous section. Similar to the compaction of copper clad sheet, local stress caused by different curing processes will also occur. Due to thicker thickness, diversified graphics distribution and more semi-cured sheets, thermal stress of PCB sheet will also be more difficult to be eliminated than that of copper clad sheet. The stress in the PCB is released in the process of subsequent drilling, shape or grilling, resulting in the deformation of the PCB.
Resistance welding, characters such as baking process: as the solder resist ink curing can’t stack with each other, so the PCB will be set on the shelf in the baking plate curing, resistance welding temperature about 150 ℃, just over middle and lower Tg material point of Tg, Tg above resin as high elastic state, plate easy under the action of gravity or oven blast out of shape.
Hot air solder leveling: ordinary plate hot air solder the usual tin stove temperature is 225 ℃ ~ 265 ℃, time of 3 s to 6 s. Hot blast temperature is 280 ℃ ~ 300 ℃. The usual board solder from room temperature into the tin stove, two minutes after post-processing wash and room temperature. The whole process of leveling hot air solder is a quench process. Because the circuit board material is different and the structure is not uniform, the thermal stress will appear inevitably in the process of cold and heat, leading to the micro strain and the whole deformation warping area.
Storage: the storage of PCB boards in the semi-finished product stage is generally firmly inserted into the shelves, which is not suitable for the tight adjustment of the shelves, or the stacking and placing plates in the storage process will cause mechanical deformation of the boards. In particular, the impact on thin plates below 2.0mm is more serious.
In addition to the above factors, there are many factors affecting PCB deformation.
3. Improvement measures
Then how to prevent plate bending and plate warping through the back welding furnace?
1. Effect of reducing temperature on stress of plates
Since “temperature” is the main source of plate stress, as long as the temperature of the backweld furnace is reduced or the rate of heating and cooling of the slow plate in the backweld furnace is adjusted, the situation of plate bending and plate warping can be greatly reduced. But there may be other side effects.
2. High Tg plate
Tg is the glass transition temperature, which is the temperature at which the material changes from glass state to rubber state. The lower the Tg value of the material, the faster the board begins to become soft after entering the back welding furnace, and the longer the time to become soft rubber state, the more serious the deformation of the board will of course be. The use of higher Tg plates can increase their ability to withstand stress and deformation, but the cost of materials is relatively high.
Increase the thickness of the circuit board
Many electronic products in order to achieve the purpose of the thinner, the thickness of the board has the 1.0 mm and 0.8 mm, even as by the thickness of 0.6 mm, the thickness of this to keep the board with no deformation after welding furnace back, really stand, suggested that if there is no frivolous request, the board is best can use 1.6 mm thickness, can greatly reduce the risk of plate bending and deformation.
4. Reduce the size of circuit boards and the number of panels
Since most of the back weld furnace use chain to drive circuit board, the larger the size of the circuit board will be because of the weight of its own, concave deformation in the welding furnace back, so try to put the long side of the circuit board as an edge on the chain back to soldering furnace, can reduce weight sag deformation caused by the circuit board itself, to reduce the number of the makeup is also for this reason, that is to say, a furnace, as far as possible with narrow edge perpendicular to the direction of the furnace, can achieve the lowest sag deformation.
5. Used tray jig
If the above methods are difficult to achieve, the last is to use a stove tray (reflow carrier/template), to reduce the deformation of a furnace tray can reduce the plate bending plate become warped because both thermal and cold, all hope that the tray can hold the circuit board to wait until the temperature of the circuit board below the Tg values began to harden, can also maintain the size of the garden to.
If the single-layer tray is not able to reduce the deformation of the circuit board, it is necessary to add a cover, the circuit board with the upper and lower two tray clamping, so that the problem of the circuit board back welding furnace deformation can be greatly reduced. However, this furnace tray is expensive, but also has to add artificial storage and recovery tray.
6. Use solid connections and stamp holes instead of v-cut divider
Since the v-cut will destroy the structural strength of the boards, try not to use the v-cut divider or reduce the depth of the v-cut.
Real connection: adopts the knife type dividing machine
Stamp hole
Optimization in PCB production engineering:
Influence of different materials on the deformation of plate
The excessive deformation and defect rate of different materials were calculated, and the results were shown in table 1.
Can see from the table, low Tg material deformation defects than high Tg, the high Tg materials are shown as packing materials, however, are less than lower Tg materials, after pressing during processing at the same time, baking temperature 150 ℃, the highest impact on lower Tg materials would be greater than high Tg material.
Engineering design study
The engineering design should avoid the design of structural asymmetry, material asymmetry and graph asymmetry as far as possible, in order to reduce the generation of deformation. At the same time, it is also found in the research process that the direct compression structure of core plate is more prone to deformation than that of copper foil.
It can be seen from table 2 that the disqualified defect rate of the two structures is significantly different, which can be understood as that the compression structure of the core plate is composed of three core plates. The expansion and contraction between different core plates and the stress change are more complicated and difficult to be eliminated.
In engineering design, the form of the plywood frame also has great influence on the deformation. Generally, there will be large continuous copper frame and discontinuous copper point or copper block frame in PCB factory, which also have different differences.
Table 3 shows the comparative test results of two kinds of border design plates. The reason why the deformation of the two kinds of frames is different is that the continuous copper leather frame has high strength, and the rigidity is relatively large in the process of pressing and plywood, which makes the residual stress in the plate not easy to release, and focuses on the release after shape processing, resulting in more serious deformation. The non-continuous copper point frame gradually releases the stress in the pressing and subsequent processing