A seemingly reasonable PCB design, why does the step area on the board bubble after welding?

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A seemingly reasonable PCB design, why does the step area on the board bubble after welding? [Copy link]

Author: Wang Huidong, member of Yibo Technology Expressway Media

High-end ingredients are often cooked in the simplest way.

Complex PCBs often consider solutions from the design perspective.

A seemingly reasonable PCB design, just because the circuit board is made of a depth control area and copper plating, a strange scene appeared during welding, and the board was layered and bubbling. Is it the processing that messed up the design, or is the welding ignoring the process? Please see today's case analysis.


Xiaopiao works in a product company as a PCB designer. She is a beautiful girl with a slight smile on her face, she is confident and calm, and she is elegant and sunny.

Xiaopiao is designing a PCB recently. In the area near the edge of the board, there is a device that is 0.4mm higher than the board. However, the product has strict requirements on the thickness of the PCB. The device cannot exceed the thickness of the PCB, otherwise it will not be placed in the cavity normally during assembly because the device thickness exceeds the standard.

After discussing with the structural engineer, Xiaopiao decided to solve the problem of the device exceeding the board thickness from the PCB design.

First, design a sinking area on the PCB, and control the depth of the PCB molding during molding to sink the device to meet the assembly requirements of the product.

After discussing with the factory process engineer, the plan is feasible and it was done.

After she quickly adjusted the stacking plan, everything was perfect.

Suddenly, she had another idea: copper plating could be applied to the sunken area of the PCB, which would also improve the heat dissipation of the product.

Her mood was beautiful at the moment. Considering the influence of processing and copper plating thickness, she hummed a little song and made the following notes on the original PCB drawing.

The PCB file was quickly submitted to the factory, and the board was also produced according to Xiaopiao's design.

Waiting is a long and painful process.

Waiting with hope is called waiting, and waiting without hope is called suffering.

Fortunately, this waiting process ended quickly.

The finished PCB board was soon handed over to Xiaopiao.

Tomorrow is the day for PCB online welding and assembly.

Tomorrow is also destined to be an extraordinary day.

Watching the board slowly start the assembly journey from the automatic board loading machine.

Solder paste printing, SPI, high-speed patch, furnace inspection, the last step is to pass the twelve temperature zones of reflow soldering.

A miracle is about to be born, and the time to witness the miracle is coming.

Xiaopiao's mood at this moment is like a freshly opened Coca-Cola, bubbling with joy.

There are always surprises and surprises in life. Sometimes you wait for a long time for the stars to appear, so you look up at the starry sky, but it's dawn...

The moment the board came out from the back of the furnace, Xiaopiao couldn't believe his eyes. Have you seen the steamed buns steamed by northerners?

Yes, at this time, the step area on the board is like full of steamed buns just out of the pot.

Golden bubbles coming out.

All the disappointments came from her blind expectations. Xiao Piao couldn't figure out what went wrong.

Suddenly, she saw a familiar figure in the workshop. He was handsome and elegant. He was Wang Daxian, a PCB process engineer who was known as omniscient and omnipotent.

Xiao Piao's originally dull eyes were instantly filled with expression. She ran to Wang Daxian happily and said excitedly:

"Daxian, help me see what the problem is."

Daxian picked up the blistering board, looked at it for a moment, smiled slightly, and then said:

"Xiao Piao, you have been hit by the most powerful copper foil separation curse in the PCB world. The copper foil and the substrate are not adhered well, and they are separated and blistered after being thermally impacted."

"Can Daxian explain it in detail?" Xiao Piao asked hurriedly.

Daxian said: "Girl, this matter is a bit complicated to talk about, so I'll make it short. It starts with the lamination of PCB."
There are three musketeers in PCB lamination-copper foil, core, and PP.

The copper foil commonly used in hard boards is electroplated with copper sulfate solution. One side is smooth, which we call the glossy side, and the other side is a rough crystal side, called the matte side. The roughness of the two sides is different. The rougher side can produce stronger bonding force with the resin after treatment.

Prepreg is the abbreviation of Pre-pregnant. It is a sheet-like bonding material synthesized by resin and glass fiber cloth carrier.
It exists in the form of a roll solid at room temperature. When laminating, it is cut into sheets. During lamination, when the temperature in the press reaches above its TG, after a period of time, it will slowly change from a solid state to a glass state, that is, (resin) becomes the shape of glue. Resin is a thermosetting material that can undergo polymer polymerization. It can be used as an adhesive between copper foil and glass fiber cloth.

Core is a combination of PP and copper foil after PP is fully cured. CORE is fully cured after high temperature, high pressure and vacuum lamination. During PCB processing in the factory, its shape and thickness are basically stable and will not change much. During PCB production, it has many names, such as core board, core, and substrate. In fact, it is a mixture of glass fiber cloth, epoxy resin and double-sided copper foil.

The following figure shows a conventional 6-layer board stack, with copper foil on the top and bottom surfaces and PP and core in the middle. In the press, the PP is melted through high temperature, high pressure and vacuum, and the core and copper foil are completely bonded together. After complete solidification, it becomes what we often call a multi-layer circuit board.

The circuit board designed and produced by Xiaopiao is processed by depth control after PCB lamination. In the area after depth control, the circuit board has been completely solidified, and then copper is plated on it. This copper foil is simply deposited on the substrate. Its bonding strength is not as good as that of copper foil during lamination. When subjected to high temperature impact, the electroplated copper and the substrate are layered and bubbled.

Master, I am still a little dizzy after what you said. Wang Banxian talked for a long time in the workshop, but Xiaopiao still couldn't understand it.

It was just time to get off work. Wang Daxian pulled Xiaopiao to the bicycle repair stall outside the factory park and let Xiaopiao watch Uncle Li repair the tire there. Uncle Li first picked up a file and rubbed it hard on the tire, then applied glue on it, and finally pressed a piece of rubber on it. After a while, the damaged tire was repaired. Then Daxian asked Xiaopiao what did you see. Xiaopiao said excitedly, Daxian, I understand. It is not easy for Uncle Li to repair the tire. You can see that he is sweating profusely. I will design an artificial intelligence tire repair product in the future to make Uncle Li less tired. The immortal suddenly felt like he couldn't breathe, and after holding his breath for a long time, he said slowly, "If you repair the tire without grinding it, how can you increase the bonding strength between the tire and the rubber? There will be air leakage after the repair."

The outer copper layer of the circuit board is bonded together by high temperature, high pressure, vacuum and melted PP. The rough surface of the uranium foil is combined with the gelatinous resin. After it is completely cured, it is tightly combined and not easy to delaminate. The copper plating in the controlled depth area is just to deposit a layer of copper on the surface of the substrate in the controlled depth area by electroplating. Because the deposited copper does not involve glue, the bonding strength between this layer of copper and the PCB is far less than that between the rough surface of the copper foil and the resin during lamination. During reflow soldering, it will be affected by thermal shock, resulting in delamination and blistering, so the case happened.

After listening to this, Xiao Piao suddenly realized the problem. He then gave up the copper plating plan in the depth control area, re-evaluated and optimized the design, and finally delivered the board successfully.

damiaa

Learned.

90houyidai

If copper plating is required, is there any good way?

minihang

Come and study

专业PCB

If the grinding plate is not ground well, the copper skin will be separated from the copper skin. The separation of copper skin and PP indicates that the process and materials are not up to standard.

xiaobingma

Learned