ແຜງວົງຈອນພິມ after reflow welding is prone to plate bending plate warping, serious words will even cause components empty welding, monument and so on, how to overcome it?

1. Harm of PCB circuit board deformation

In the automatic surface mounting line, if the circuit board is not smooth, it will cause inaccurate positioning, components can not be inserted or mounted to the hole and surface mounting pad of the board, and even damage the automatic inserting machine. The circuit board loaded with components is bent after welding, and the component feet are difficult to cut neatly. Boards can not be installed into the chassis or machine socket, so the assembly plant encountered a board tilt is also very troublesome. At present, surface mounting technology is developing towards high precision, high speed and intelligent direction, which puts forward higher flatness requirements for PCB board as the home of various components.

The IPC standard specifically states that the maximum allowable deformation is 0.75% for PCB board with surface mount device and 1.5% for PCB board without surface mount device. In fact, in order to meet the needs of high precision and high speed mounting, some electronic mounting manufacturers have more stringent requirements for deformation.

PCB board is composed of copper foil, resin, glass cloth and other materials, all of which have different physical and chemical properties. After pressed together, thermal stress residue will inevitably occur, resulting in 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. Cause analysis of deformation

The deformation of PCB board needs to be studied from the aspects of material, structure, graphic distribution, processing process and so on. This paper will analyze and elaborate various reasons for possible deformation and improvement methods.

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

On the general circuit board design has 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 the expansion and contraction can not be simultaneously caused by different stresses and deformation, at this time if the temperature of the board has reached the upper limit of Tg value, the board will begin to soften, resulting in permanent deformation.

The connecting points (ViAs) of the layers on the board limit the expansion and contraction of the board

Nowadays, the circuit board is mostly multilayer board, and there will be rivets like connection point (VIAS) between the layer and the layer, the connection point is divided into through hole, blind hole and buried hole, where there is a connection point will limit the effect of plate expansion and contraction, will also indirectly cause plate bending and plate warping.

The weight of the circuit board itself can cause the board to sag and deform

General welding furnace will use the chain to drive the circuit board in the welding furnace forward, that is, when the two sides of the board when the fulcrum to support the whole board, if the board above the overweight parts, or the size of the board is too large, because of the amount of its own and appear in the middle of the depression phenomenon, resulting in plate bending.

The depth of V-cut and the connecting strip will affect the deformation of the panel

Basically, V-cut is the culprit of destroying the sub-structure of the board, because V-cut is to Cut grooves on the original large sheet, so it is easy to deform the place of V-cut.

2.1 Impact analysis of pressed materials, structures and graphics on plate deformation

PCB board is made by pressing core board, semi-solidified sheet and outer copper foil. The core board and copper foil are heated and deformed during pressing. 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

The Z-cTe of ordinary FR-4 substrate at Tg point is.

Above TG point, it is (250-350) x10-6, and x-cTE is generally similar to copper foil due to the presence of glass cloth.