Issues that should be paid attention to when drawing PCB diagrams from the perspective of welding

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Issues that should be paid attention to when drawing PCB diagrams from the perspective of welding [Copy link]

Factors Affecting PCB Welding Quality

From PCB design to the completion of all component welding to a high-quality circuit board, it is necessary for PCB design engineers and even welding processes, welding workers and many other links to have strict control. The main factors are: PCB diagram, circuit board quality, device quality, device pin oxidation degree, solder paste quality, solder paste printing quality, placement machine programming accuracy, placement machine placement quality, reflow oven temperature curve setting and other factors.

The link that the welding factory itself cannot overcome is the PCB drawing link. Since circuit designers often do not solder circuit boards, they cannot gain direct soldering experience and do not know the various factors that affect soldering; and workers in welding factories do not know how to draw boards. They only complete production tasks and have no thoughts or ability to analyze the causes of poor soldering. Since the talents in these two areas perform their respective duties, it is difficult to combine them organically.

Suggestions for drawing PCB diagrams

Here I would like to give some suggestions to the design and wiring engineers who draw PCB diagrams on the PCB drawing process, hoping that various bad drawing methods that affect the welding quality can be avoided in the process of drawing. It will be mainly introduced in the form of pictures and texts.

About positioning holes

Four holes (minimum aperture 2.5mm) should be left at the four corners of the PCB board to position the circuit board when printing solder paste. It is required that the center of the circle in the X-axis or Y-axis direction is on the same axis, as shown in the following figure:

About mark point

Used for placement machine positioning. Mark points should be marked on the PCB board. The specific location is at the diagonal corner of the board. It can be a round or square pad. Do not mix it with the 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 as shown below. (Up and down symmetry or left and right symmetry)

b. The size of A is 2.0mm.

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

d. The color of the Mark point should have a light and dark difference from the color of the surrounding PCB.

[font=-apple-system-font, BlinkMacSystemFont,e. To ensure recognition accuracy, the surface of the Mark is plated with copper or tin to prevent surface reflection. The light spot cannot be recognized for marks with only lines.

As shown below:

About leaving 5mm margin

When drawing PCB, leave a margin of no less than 3mm in the long direction for the placement machine to transport the circuit board. The placement machine cannot place components within this range. Do not place patch components within this range.

As shown in the figure:

For circuit boards with devices on both sides, it should be considered that the second reflow will rub off the devices on the side that have been soldered. In serious cases, the pads will be rubbed off and the circuit board will be damaged. As shown in the following figure:

Therefore, it is recommended not to place SMD components within 5mm of the long side of the side with fewer chips (usually the Bottom side). If the area of the circuit board is really limited, you can add process edges to the long sides. Refer to Article 17 of this article "Suggestions on panel assembly and process edges".

Do not directly make vias on the pads

[color=rgb(51, 51, The defect of directly drilling vias on pads is that during reflow, the solder paste melts and flows into the vias, causing the device pads to lack tin, thus forming cold solder joints. , 51, 51)]

About the polarity marking of diodes and tantalum capacitors

The polarity markings of diodes and tantalum capacitors should comply with industry regulations to prevent workers from soldering in the wrong direction based on experience. As shown in the figure:

About silk screen and logo

Please hide the device model. Especially for circuit boards with high device density. Otherwise, the dazzling view will affect finding the soldering position. As shown below:

[color=rgb(51, 51,Also, do not mark the model without marking the number. As shown in the figure below, it will make the placement machine programming impossible.

The font size of the silk-screen characters should not be too small to be clearly seen. The characters should be placed staggered with the vias to avoid misreading.

About the extension of IC pads

When drawing PCB, the pads of ICs packaged such as SOP, PLCC, and QFP should be extended. The pad length on the PCB should be equal to the length of the IC foot × 1.5. This will facilitate manual soldering with a soldering iron, and the chip pins, PCB pads, and tin will be melted into one. As shown in the figure:

About the width of IC pads

For ICs in SOP, PLCC, QFP and other packages, pay attention to the width of the pads when drawing the PCB. The width of pad a on the PCB = the width of the IC foot (i.e., the Nom. value in the datasheet). Please do not widen it to ensure that pad b (i.e., between the two pads) has enough width to avoid continuous welding. As shown in the figure:

Do not rotate the device at any angle when placing it.

Since the placement machine cannot rotate at any angle, it can only rotate 90°C, 180°C, 270°C, and 360°C. As shown in Figure B below, if it rotates 1°, the device pins and the pads on the circuit board will be offset by 1° after placement by the placement machine, thus affecting the welding quality.

Problems that should be paid attention to when adjacent pins are short-circuited

The short-circuit method in Figure a below is not conducive to workers identifying whether the pin should be connected, and it is not beautiful after welding. If you short-circuit and add solder mask according to the methods of Figure b and Figure c when drawing, the welding effect will be different: as long as each pin is not connected, the chip will not have short circuit phenomenon and the appearance is also beautiful.

About the problem of the middle pad under the chip

When drawing a chip with a pad in the middle under the chip, if you draw the middle pad according to the chip package drawing, it is easy to cause a short circuit. It is recommended to reduce the size of the middle pad and increase the distance between it and the surrounding pin pads, thereby reducing the chance of a short circuit. As shown in the following figure:

:

Two devices with higher thickness should not be arranged closely together

As shown in the figure below, this layout will cause the placement machine to touch the previously placed device when placing the second device. The machine will detect the danger and cause the machine to automatically power off.

About BGA

Due to the special nature of BGA packaging, its pads are all under the chip, and the soldering effect cannot be seen from the outside. For the convenience of rework, it is recommended to make two positioning holes of Hole Size: 30mil on the PCB board to facilitate the positioning of the steel mesh (used to scrape solder paste) during rework.

Tips: The size of the positioning hole should not be too large or too small. The needle should not fall off or shake after insertion, and it should be slightly tight when inserted, otherwise the positioning will be inaccurate. As shown below:

It is also recommended to leave some space around the BGA to avoid placing components, so that the stencil can be placed to scrape the solder paste during rework.

About PCB color

It is not recommended to make it red. Because the red circuit board appears white under the red light source of the camera of the placement machine, it cannot be programmed and is not convenient for the placement machine to solder.

About small devices under large devices

Some people like to arrange small devices under large devices on the same layer, for example: there are resistors under the digital tube, as shown in the following figure:

Such layout will cause difficulties for repair. When repairing, the digital tube must be removed first, and the digital tube may be damaged. It is recommended to place the resistor under the digital tube to the Bottom side, as shown below:

About the influence of copper cladding and pad connection on tin melting

Because the copper cladding absorbs a lot of heat, it makes it difficult for the solder to fully melt, thus forming a cold solder joint. As shown in the picture:

In Figure a, the device pad is directly connected to the copper clad; in Figure b, although the 50Pins connector is not directly connected to the copper clad, the middle two layers of the four-layer board are large-area copper clad, so both Figure a and Figure b will cause the copper clad to absorb a lot of heat and cause the solder paste to not fully melt. The body of the 50Pins connector in Figure b is a plastic that is not resistant to high temperatures. If the temperature is set too high, the body of the connector will melt or deform. If the temperature is set too low, the copper clad will absorb a lot of heat and cause the solder paste to not fully melt. Therefore, it is recommended to isolate the pad from the large area of copper clad. As shown in the figure:

Suggestions on panelization and processing edges

Summary

Nowadays, there are more and more engineers who can use software to draw, wire and design PCB. However, once the design is completed, it can greatly improve the welding efficiency. The author believes that it is necessary to pay attention to the above factors. And cultivate good drawing habits and be able to communicate well with the processing factory, which is something every engineer should consider.

Source: Internet compilation. If copyright is involved, please contact us to delete.

yjguohua

Thank you for the summary!

懒猫爱飞

Every engineer who draws PCBs must practice on the production line for half a year to achieve better results.