Factors affecting PCB welding quality
From PCB design to the completion of all components welding for a high quality circuit board, PCB design engineers and even the level of welding process, welding workers and many other links have strict control. It mainly includes the following factors: PCB diagram, quality of circuit board, quality of device, oxidation degree of device pin, quality of solder paste, printing quality of solder paste, programming accuracy of SMT machine, mounting quality of SMT machine, setting temperature curve of reflow furnace, etc.
The insurmountable link of welding factory itself is the link of PCB drawing. Because people who do circuit design often do not weld circuit boards so that they can not obtain direct welding experience, do not know the various factors affecting the welding; But the welding factory worker does not understand the drawing board, they only completes the production task, does not have the thought, does not have the ability analysis to cause the bad welding reason. Due to the two aspects of talent in their respective roles, it is difficult to organic combination.
Suggestions for PCB drawing
Next, I will give some Suggestions to the design and wiring engineers who draw PCB drawings in terms of the link of PCB drawing, hoping to avoid various bad drawing methods that affect the welding quality in the drawing process. It will be mainly introduced in the form of pictures and texts.
About locating hole
Four holes (minimum diameter 2.5mm) shall be left in the corners of the PCB board for locating the board when printing solder paste. The center of the circle in the direction of X axis or Y axis should be on the same axis, as shown in the figure below:
The two devices with higher thickness should not be closely aligned
As shown in the figure below, such a cloth plate will cause the SMT machine to encounter the previously pasted device when attaching the second device, and the machine will detect the danger, resulting in automatic power failure of the machine. ,
Screen characters should not be so small that they cannot be read clearly. Characters should be placed in staggered positions to avoid misinterpretation. Also do not mark only model number, do not mark mark. As shown in the figure below, the SMT programming cannot be carried out. About screen printing and logo
Please hide the device model. Especially the circuit board with high device density. Otherwise, the dazzling effect of finding the welding position.
About the mark point
Used for positioning of the SMT machine. Mark points shall be marked on the PCB board. The specific position: on the diagonal of the board, the pads can be round or square. Do not mix with the pads of other devices. If you have devices on both sides, label both sides.
When designing PCB, please pay attention to the following points:
A. the shape of Mark is as follows. (upper and lower symmetry or left and right symmetry)
The size of b and A is 2.0mm.
C. From the outer edge of the Mark point to the range of 2.0mm, there shall be no change in shape or color that may cause false identification. (pad, paste)
D. The color of Mark should be different from the color of surrounding PCB.
E. To ensure the identification accuracy, the Mark point shall be plated with copper or tin to prevent surface reflection. Dots of light do not recognize shapes that have only line markings.
Leave a 5mm edge around
When drawing a PCB, leave no less than 3mm edge in the long side direction for the SMT machine to transport the circuit board, within which the SMT machine cannot mount the device. Do not place patch devices within this range.
Circuit boards with double-sided devices should be considered that the second reflux will rub off the soldered side of the device, or in serious cases, rub off the solder pad, damage the circuit board.
Therefore, it is recommended that no SMT devices should be placed within 5mm of the long side of the Bottom side of the chip. If it is true that the area of the circuit board is limited, the long side can be added to the process, please refer to article 17 “Suggestions on assembly and adding process”.
Do not drill directly into the pad
The defect of directly going through the hole on the solder pad is that the solder paste melts and flows into the hole during the reflux, causing the solder pad of the device to be short of tin, thus forming virtual welding.
Polarity labeling of diodes and tantalum capacitors
The polarity labeling of diodes and tantalum capacitors shall comply with the industry regulations, so as to prevent the workers from welding in the wrong direction by experience.
The IC welding pad should be extended
About the width of IC pad
As for IC encapsulated by SOP, PLCC, QFP, etc., please pay attention to the width of solder pad when drawing PCB. The width of solder pad a on PCB is equal to the width of IC foot (that is, the Nom in datasheet), please do not widen it, and ensure that b(that is, between the two solder pads) has enough width, so as not to cause continuous welding.
Do not rotate the device at any Angle
Due to SMT machine can’t rotate at any Angle, can rotate 90 ℃, 180 ℃, 270 ℃ and 360 ℃. 1 ℃ below B rotation, SMT machine after SMT device pins and circuit boards welding plate will stagger the Angle of 1 ℃, which affect the welding quality.
The problem that should pay attention to when adjacent pin is shorted
The short connection method in figure a below is not conducive for workers to identify whether the pin should be connected, and it is not beautiful after welding. If drawing according to the method of figure b, figure c short and add resistance welding, the effect of the welding is not the same: as long as each pin is not connected, the chip will not short circuit phenomenon, and the appearance is beautiful.
The problem of the middle solder pad under the chip
Under the chip in the middle there is a solder disc chip drawing if according to the chip packaging picture in the middle of the solder disc, it is easy to cause short-circuit phenomenon. It is recommended to reduce the middle solder pad so as to increase the distance between it and the surrounding pin solder pad, thus reducing the chance of short circuit.
About the BGA
Due to the special BGA package, the welding pads are under the chip, and the welding effect cannot be seen outside. In order to facilitate repair, it is recommended to drill two Hole Size:30mil positioning holes on the PCB board, so as to locate the steel mesh (used for scraping the solder paste) during repair.
Warm hint: the size of the location hole should not be too large or too small, to make the needle inserted after not to drop, do not slosh, insert a little bit tight is appropriate, or positioning is not allowed.
In addition, it is recommended to leave a space around the BGA for devices, so that the solder paste can be scraped under the screen during repair.
About the color of PCB board
Red is not recommended. Because the red circuit board is white under the red light source of the camera of the mounter, it cannot be programmed and is not convenient for the mounter to conduct welding.
About the small devices underneath the big ones
Some people like to put small devices underneath big ones in the same layer, such as a nixie tube with resistance underneath it
Such typesetting will be difficult to repair, repair must be removed digital tube, there may be damage to the digital tube. It is suggested to arrange the resistance under the nixie tube to the Bottom surface
On the copper-clad and solder pad connected to affect molten tin
As the copper coating will absorb a lot of heat, resulting in the solder is difficult to fully melt, thus forming virtual welding.
The device welding pad is directly connected with the copper coating; Although the 50Pins connector in figure b is not directly connected to the copper coating, the middle two layers of the four-layer plate are covered with large areas of copper, so both figure a and figure b can’t fully melt the solder paste due to the large amount of heat absorbed by the copper coating. The body of the 50Pins connector in figure b is a plastic that is not resistant to high temperature. If the temperature is set high, the connector will melt or deform. If the temperature is set low, the copper coating will absorb a lot of heat and cause the solder paste to not fully melt. Therefore, it is recommended that the welding pad be isolated from the large area of copper coating.
Nowadays, there are more and more engineers who can draw, wire and design PCB with software. However, once the design is completed and the welding efficiency is improved, the author believes that the above elements should be paid attention to. And develop good drawing habits, can be a good processing plant for good communication, is every engineer should consider.