what should be paid attention to when drawing PCB diagram from the welding angle（1）

Factors affecting PCB welding quality

From PCB design, all components welding to a high-quality circuit board, there is a need for strict control of the PCB design engineer and even the welding craft, the level of the welding workers. The main factors: PCB diagram, quality of the board, quality of the device, oxidation degree of the device pins, quality of the solder paste, printing quality of the solder paste, accuracy of the SMT machine, and mounting quality of SMT machine , the setting of the temperature of the reflow oven, and so on.

Advice for PCB layout

1.About the positioning hole

Four holes (minimum aperture 2.5mm) are required at the four corners of the PCB to position the board during solder paste printing. Require the center of the X-axis or Y-axis to be on the same axis

2.About the mark point

Used for SMT machine positioning. Mark points should be marked on the PCB. The specific position: diagonally on the board, can be round, or square pads, not mixed with the pads of other devices. If there are devices on both sides, both sides should be marked.

When designing the PCB, please note the following:

A.The shape of the Mark point (up and down symmetry or bilateral symmetry)

b. From the outer edge of the Mark point within 2.0 mm, there should be no shape or color change that may cause erroneous recognition. (pad, solder paste)

c.There should have a difference between color of the Mark point and the color of the surrounding PCB.

d. In order to ensure the recognition accuracy, the surface of the Mark point is plated with copper or tin to prevent surface reflection. For a mark with only a line shape, the light spot cannot

3.About leaving 5mm side

When drawing a PCB, leave a side of not less than 3 mm in the long-side direction for the SMT machine to transport the board. In this range, the SMT machine cannot mount the device. Do not place the chip device within this range.

4. Do not directly via the hole on the welding pad

The defect of the via hole directly on the pad is that the solder paste melts and flows into the via hole during over-flow, causing the device pad to be deficient in tin, thereby forming virtual welding.

5.On the polarity mark of the diode, tantalum capacitor

The polarity of the diode and tantalum capacitor should be marked in accordance with the rules, so as to prevent the worker from welding the wrong direction.

6.On silk screen and logo

Please hide the device model. Especially for boards with high density device. Otherwise, dazzling influences finding the welding location .

Also don't just mark the model, no label. Causing the SMT machine programming can not be carried out.

The size of the silkscreen characters should not be too small to be seen. Character placement should be staggered via to avoid misreading.

7. About the IC welding pad should be extended

SOP, PLCC, QFP and other packaged ICs should extend the welding pad when drawing the PCB. The pad length on the PCB = IC leg length × 1.5 is suitable, so it’s convenient for soldering iron by hand, the chip pins and PCB welding pads, tin the three melts into one.

8.About the width of the IC welding pad

SOP, PLCC, QFP and other packaged ICs, pay attention to the width of the welding pad when drawing the PCB, the width of the pad a on the PCB = the width of the IC pin (ie: the Nom. value in the datasheet), please do not widen, guarantee enough width between the two pads to avoid joint welding.