Author: Wang Huidong, a member of Yibo Technology Expressway Media
If the circuit board is a person, then the drilling is its soul, especially the via, which is the soul of the board.
In the design of high-speed PCB, via design is an important factor, and via design has become one of the key factors restricting high-speed PCB design. Improper handling may lead to the failure of the entire design. Vias are conductors that connect different layers of routing in multi-layer PCBs. At low frequencies, vias will not affect signal transmission, but as the frequency increases and the signal rise time decreases, vias cannot be simply regarded as electrical connections, but their impact on signal integrity must be considered. Due to the influence of parasitic capacitance and inductance of vias, the equivalent impedance of vias is generally lower than that of transmission lines. At this time, vias appear as impedance discontinuities on the transmission path, which will cause signal reflection, delay, attenuation and other signal integrity problems.
I heard from SI masters that increasing the pad will increase the capacitance of the hole, thereby reducing the impedance, and increasing the pad will cause the signal return loss to deteriorate. Therefore, reducing the pad of the via can increase the impedance of the via. And the smaller the pad, the higher the impedance.
When Niu Niu read this sentence, a light bulb suddenly came to his mind. He thought of a way to increase the impedance of the vias. The more he thought about it, the happier he was. The happier he was, the more he wanted to find someone to share this with. So he thought of his old classmate, good friend, and old roommate Zhao Ligong who worked at Company E.
So he sent a WeChat picture to Zhao Ligong.
Then he attached a sentence, Ligong, I found a way to change the impedance of the vias, and I feel that the effect is particularly good. Today is destined to be an extraordinary day.
When Zhao Ligong saw the picture, he felt that it was a bit abnormal, but he couldn't find where the abnormality was. Anyway, something was wrong.
Zhao Ligong, who was deep in thought, suddenly felt someone behind him. He turned his head and saw that it was his senior brother. The senior brother looked at the graphics on Zhao Ligong's computer screen and said to Zhao Ligong: "Ligong, can you see anything unusual? The size of the drill hole and the pad is 1:1, and it is also connected to the circuit. Ruyan, come and have a look and learn together."
Lin Ruyan stood up from her seat and came to Zhao Ligong and the senior brother.
Lin Ruyan stared at the screen with her beautiful eyes and said in surprise:
"Hey, senior brother, the pad of this PCB design is the same size as the drill hole, how can the circuit be connected?"
"Yes, Ruyan immediately saw the problem. The circuit and the drill hole are connected by the pad. Please see the figure below."
Ideal state of pad design
The drilled hole is not in the middle of the pad due to factors such as hole position deviation and alignment, but it must at least meet the IPC secondary standard, otherwise it will cause a connection failure and an open circuit. As shown in the figure below, although there is a connection, the narrowest part of the line is only 2mil, and there is also a risk of an open circuit during vibration.
Now the pad of this design is the same size as the drill hole, which means that the best state after it is made is that the drill hole and the pad are completely raw, that is, a ring-free electroplated hole. The traces next to it have nowhere to connect, and production cannot be processed, so an open circuit is necessary.
IPC-A-6012D contains the IPC standard for IPC acceptance of annular rings.
"Then what is our normal, normal... drilling and welding ring design standard..." Zhao Ligong asked stutteringly.
The senior brother said: "We usually drill the hole position tolerance of +/-3mil.
If we want to ensure that the finished hole is not broken, we must ensure that the residual ring is at least 2mil
. That is to say, the minimum size of the pad is 5mil on one side.
If it is a via, it is allowed to be broken according to the IPC Level 2 standard, and the minimum welding ring width is 4mil.
So if the customer's designed drilling size is 12mil, then what is the size of the pad?
The IPC Level 2 standard is 12mil+4mil+4mil=20mil.
The IPC Level 3 standard is: 12mil+5mil+5mil=22mil."
After listening to the analysis of the senior brother, Zhao Ligong suddenly realized it. He quickly sent a message to Maomao on WeChat, saying that Maomao's design can only be viewed and cannot be actually produced. Then he repeated the results of the senior brother's analysis to Maomao.
If you want to meet the IPC Level 2 standard, the width of the pad must be at least 4mil larger than the drilling size on one side.
To meet the IPC Level 3 standard, the width of the pad must be at least 5 mil larger than the drilling size on one side.
If the drilling and pad sizes are the same, it is equivalent to an open circuit.
After Zhao Ligong finished talking to Mao Mao, he suddenly seemed to remember something and turned on his computer without saying a word. A picture appeared on his screen. He clicked on the pad setting interface as shown below.
"Hey, Mr. Zhao, you are still thinking of being my teacher. Look, your pad design is 16mil, and the drilling setting is 12mil. But yours is also broken, and it is completely broken.
When Ligong Zhao heard Lin Ruyan's words, he jumped up from his seat, grabbed Lin Ruyan's hand, and ran to the dark shadow behind the office. The sudden change made Lin Ruyan confused. She followed Ligong Zhao obediently, and Lin Ruyan took the initiative to stick to the wall and closed her eyes. Ligong Zhao supported the wall with one hand, staring at Lin Ruyan's face full of baby fat, then he lowered his head and slowly moved his mouth closer to Lin Ruyan's face. Listening to Ligong Zhao's rapid breathing, Lin Ruyan's heart was like a deer beating wildly, her mind was blank, and the air was instantly filled with an ambiguous atmosphere. Suddenly Ligong Zhao moved, he pulled open his jacket and covered Lin Ruyan in it.
"Ligong Zhao, what are you doing? This is an office." Lin Ruyan suddenly woke up and pushed Ligong Zhao hard.
"Ruyan, what are you thinking about? Look at the new watch I bought yesterday. It has a luminous one. Can you see the time clearly?" Ligong Zhao said.
"Yes, yes, I can see clearly..." Lin Ruyin said excitedly under his jacket, then he opened Zhao Ligong's jacket, suddenly pulled up Zhao Ligong's right hand, and bit his wrist.
"Ah...", Zhao Ligong's miserable cry echoed in the office.
"Zhao Ligong, what do you think of this watch I gave you? This one has a built-in red light. When the red light is gone, tell me and I will help you add it." Zhao
Ligong looked at the red teeth marks on his wrist left by Lin Ruyan, and then looked at Lin Ruyan's smiling eyes, and knew something was wrong, and ran away with a bang, faster than a rabbit...
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