Design aspect:
(1) Expansion and contraction coefficient matching
Generally, a large-area copper foil is designed on the circuit board for grounding, and sometimes the Vcc layer is also designed with a large-area copper foil. When these large-area copper foils cannot be evenly distributed on the same circuit board When it is on the board, it will cause the problem of uneven heat absorption and heat dissipation speed. Of course, the circuit board will also expand with heat and contract with cold, but if the expansion and contraction cannot match, it will cause different stress changes and eventually cause deformation. At this time, the board If the temperature has reached the Tg value, the board will start to soften, and after curing, it will become a permanent deformation.
(2) The connection points (vias, vias) of each layer on the circuit board limit the expansion and contraction of the board
Today’s circuit boards are mostly multi-layer boards, and between layers, there will be connection points like rivets (through holes, blind holes, buried holes), where there are connection points, it will limit the thermal expansion and contraction of the board As a result, it will also indirectly cause board bending and board warping.
(3) The design of V-cut will affect the amount of panel deformation
V-cut can easily become the culprit of PCB deformation caused by external force, because V-cut cuts grooves on the original large sheet, so the V-cut is prone to deformation.
Materials:
(1) Difference in coefficient of thermal expansion (CTE)
PCB boards usually use FR-4 copper-clad laminates as the board material, and FR-4 copper-clad laminates are composed of copper foil and a dielectric layer, while resin and glass fiber cloth form the dielectric layer. However, under normal circumstances, the resin and glass fiber cloth are not distinguished on the CTE, but are considered as a dielectric layer.
Because the materials of the copper foil and the dielectric layer are different, their CTEs are naturally different, so when they are affected by temperature, the changes caused are also different. When this change is reflected in the deformation, the difference between the two is reflected in the deformation of the PCB board.
(2) Local differences in the material itself
Questions about the material itself are extremely complicated, such as uneven thickness, and will not be discussed in depth here.
In terms of production process:
The causes of deformation during PCB board processing are also very complex, which can be divided into thermal stress and mechanical stress. Among them, the thermal stress is mainly generated during the pressing process, and the mechanical stress is mainly generated during the plate stacking, handling, and baking processes. The following is a brief discussion in the order of the process:
(1) incoming copper clad laminate
Most copper-clad laminates are double-sided, with symmetrical structures and no graphics. The CTE of copper foil and glass fiber cloth is almost the same, so there is almost no deformation caused by the difference in CTE during the lamination process. However, the size of the copper clad laminate press is large, and there are temperature differences in different areas of the hot plate, which will cause slight differences in the curing speed and degree of resin in different areas during the pressing process. Localized stresses due to differences in the curing process. Generally, this stress will maintain balance after pressing, but it will be gradually released and deformed in the future processing.
(2) Pressing
The lamination process is the main process for generating thermal stress on multi-layer PCB boards. Similar to the CCL lamination, due to the local stress caused by the difference in the curing process, the thermal stress of the PCB board will be more difficult to eliminate than the CCL due to reasons such as thicker thickness, various pattern distribution, and more prepregs. However, the stress existing in the PCB board is released in the subsequent drilling, shape, baking and other processes, resulting in deformation of the board.
(3) Baking process of solder mask, characters, etc.
Since the solder resist inks cannot be stacked on top of each other when they are cured, the PCB boards will be placed vertically on the shelf and baked to cure. The solder resist temperature is about 150°C, and the boards are easily deformed under their own weight or strong wind in the oven.
(4) Hot air solder leveling
Usually, when the hot air solder is leveled, the temperature of the tin furnace is 225°C~265°C, the time is 3S-6S, and the temperature of the hot air is 280°C~300°C. When the solder is leveled, the board is put into the tin furnace from room temperature, and within two minutes after being out of the furnace, it is washed with post-treatment water at room temperature. The whole hot air solder leveling process is a sudden heating and quenching process. Due to the different materials and uneven structure of the circuit board, thermal stress will inevitably appear during the heating and cooling process, resulting in microscopic strain and overall deformation, forming warpage.
other aspects:
(1) storage
The storage of PCB boards in the semi-finished stage is generally inserted vertically into the shelf. If the shelf is not adjusted properly, or the boards are stacked during storage, the board will be mechanically deformed. Especially for thin plates below 2.0mm, the impact is more serious.
(2) The weight of the circuit board itself will cause the board to sag and deform
Generally, the reflow furnace will use the chain to drive the circuit board forward in the reflow furnace, that is, use the two sides of the board as the fulcrum to support the whole board. If there are heavy parts on the board, or the size of the board is too large, It will show a phenomenon of depression in the middle due to the amount of its own species, causing the plate to bend.
