Overview of PCB design from the perspective of welding

From PCB design to the completion of all components welding to a circuit board with high quality, PCB design engineers and even PCB welding process, the level of welding workers and many other links need to be strictly controlled.

There are mainly the following factors: PCB drawing, quality of circuit board, quality of devices, oxidation degree of device pins, quality of solder paste, printing quality of solder paste, accuracy of programming of the placement machine, placement quality of the placement machine, setting of temperature curve of reflow soldering furnace, etc.

The insurmountable link of the welding plant itself is the link of PCB design. Because people who do circuit design often do not weld the circuit board, they cannot obtain direct welding experience and do not know various factors that affect welding; However, the workers in the welding plant do not understand the drawing board. They just finish the production task without thinking and ability to analyze the causes of poor welding. It is difficult to organically combine these two talents because they perform their respective duties.

Suggestions for PCB design

Now I will give some suggestions to the design and wiring engineers who draw PCB drawings on the PCB design link, hoping to avoid various bad drawing methods that affect the welding quality in the drawing process.

About Locating Holes

Four holes (the minimum hole diameter is 2.5mm) shall be reserved at the four corners of the PCB board for positioning the circuit board when printing solder paste. The center of the circle in the X or Y direction is required to be on the same axis,

About mark point

Used for placement machine positioning. Mark points shall be marked on the PCB board. The specific position is: on the diagonal corner of the board, it can be round or square pads, and it shall not be mixed with pads of other devices. If there are devices on both sides, both sides should be marked.

When designing PCB, please pay attention to the following points:

a. The shape of the Mark point is symmetrical from top to bottom or from left to right

Talking about the Problems in PCB Design from the Angle of Welding

b. The dimension of A is 2.0mm.

c. Within 2.0mm from the outer edge of the Mark point, there should be no shape and color changes that may cause incorrect identification. (pad, solder paste)

d. The color of the Mark point should be different from that of the surrounding PCB.

e. To ensure the identification accuracy, the surface of the Mark point is plated with copper or tin to prevent surface reflection. The shape is marked with only lines, and the light spot cannot be recognized.

Talking about the Problems in PCB Design from the Angle of Welding

About leaving 5mm side

When drawing PCB, at least 3mm edge shall be reserved in the long side direction for the placement machine to transport the circuit board. The placement machine cannot mount components within this range. Do not place SMD devices within this range.

Talking about the Problems in PCB Design from the Angle of Welding

For the circuit board with components on both sides, it should be considered that the components on the edge of the soldered side will be wiped off during the second over-current, and in serious cases, the bonding pad will be wiped off and the circuit board will be damaged.

Talking about the Problems in PCB Design from the Angle of Welding

Therefore, it is recommended not to place SMD devices within 5mm from the long side of the side with few chips (usually the Bottom side). If the area of the circuit board is really limited, the edge can be processed on the long side. See Article 17 "Suggestions on the panel and the edge processing" in this paper.

Do not directly through holes on the pad

The defect of via directly on the pad is that the solder paste melts and flows into the via during over reflow, resulting in a lack of tin in the device pad, thus forming a false solder.

Talking about the Problems in PCB Design from the Angle of Welding

Polarity marking of diode and tantalum capacitor

The polarity of diodes and tantalum capacitors shall be marked in accordance with the line rules to avoid the wrong direction of welding by workers based on experience.

Talking about the Problems in PCB Design from the Angle of Welding

About silk screen and logo

Please hide the device model. Especially the circuit board with high device density. Otherwise, dazzle will affect finding the welding position.

Talking about the Problems in PCB Design from the Angle of Welding

Don't just mark the model, not the number. As a result, the chip mounter cannot be programmed.

Talking about the Problems in PCB Design from the Angle of Welding

The font size of silk screen characters should not be too small to see clearly. The characters shall be placed in staggered vias to avoid misreading.

Talking about the Problems in PCB Design from the Angle of Welding

About IC pad should be extended

When drawing PCB for IC packaged by SOP, PLCC, QFP, etc., the pad shall be extended, and the pad length on PCB=IC foot length × 1.5 To be suitable, it is convenient to fuse the chip pin with PCB pad and tin when soldering with soldering iron manually.

Talking about the Problems in PCB Design from the Angle of Welding

About the width of IC pad

For IC packaged by SOP, PLCC, QFP, etc., pay attention to the width of the pad when drawing a PCB. The width of pad a on the PCB=the width of the IC foot (that is, the Nom value in the datasheet). Please do not increase the width, and ensure that b (that is, between two pads) has enough width to avoid continuous welding.

Talking about the Problems in PCB Design from the Angle of Welding

Do not rotate the device at any angle

Since the mounter cannot rotate at any angle, it can only rotate at 90 ℃, 180 ℃, 270 ℃ and 360 ℃. As shown in Figure B below, after the placement machine is rotated by 1 ℃, the device pin and the pad on the circuit board will stagger by 1 ℃, thus affecting the PCB welding quality.