HDI blind hole design, have you noticed this detail?

author ： Mr. Yibo technology Expressway member Wang Huidong

Raoxiaoxiao said ： My love has three , The sun 、 The moon 、 Circuit board .
Mao said ： I'll just see .
Ah Mao, who just came to the workshop for a walk , From a distance PMC Rao Xiaoxiao, a beautiful woman, has a beautiful face , Beautiful eyebrows , A faint frown , The delicate face shows the shallow anxiety , I feel pity for you , More is the kind of calm and focused beauty .
Let people sigh at this moment ： The world of mortals is noisy and flashy , A lifetime passes in an instant . Beauty is like a dream , A lifetime of youth .
Mao walked quietly to Xiao Xiao's back , I saw her staring at the piece in front of the monitor PCB see , Enlarged PCB plate .
Mao feels this PCB A little familiar , Look again , Oh dear ！ This is not the piece you re cast and processed outside HDI Board ？ Yes , It is the board to be launched soon .
A picture is displayed on the screen , The surface of the pad is pitted .
Mao asked Xiao Xiao softly , How did these pads look like this .

Xiao Xiao said ： Ah Mao , You have to ask your PCB Processing factory , The sunken degree on the blind hole processed in this batch （dimple） It obviously exceeds the standard , Compared with the last time, one is in the sky and the other is underground .

Archive the pictures made in the last batch

What is? HDI

HDI（High Density Interconnection） High density interconnection PCB English abbreviations , It is based on the traditional multilayer circuit board manufacturing technology , Through high-density micro wiring and micro through hole technology , Assisted by laser drilling , level / Upright PTH Multi layer burying produced by wet process and other processes / Blind hole PCB For short .

according to IPC-2226 The definition of it ：
Diameter of blind hole or buried hole ≤0.15mm[0.00591 in], Disc diameter ≤0.35mm[0.0138 in], By laser or mechanical drilling , dry / Wet etching , Graphic transfer , A conductive coating is formed by electroplating .
remarks ： Aperture ＞0.15mm[0.00591 in] Refer to through holes in this standard .

HDI Common drill hole size range ：
3-5mil, Generally, the intermediate value is taken 4mil There are a lot of design and production .

HDI frequently-used IPC standard
1）IPC/JPCA-2315- Design Guide for high density interconnection structures and micropores
2）IPC-2226- High density interconnection （HDI） Part of printed circuit board design criteria
3）IPC/JPCA-4104- High density interconnection （HDI） Specification for verification and performance of dielectric materials for structures
4）IPC-6016, High density interconnection （HDI） Specification for verification and performance of structures

HDI Processing principle of blind hole ：
Non mechanical drilling , Aperture at 0.15mm（6mil） The following blind holes , Bottom of blind hole Pad stay 0.25mm（10mil） The following , It's called Microvia Micro pilot hole or micropore .
Micropores are usually processed by laser , Light types mainly include infrared light and ultraviolet light .

common LASER There are usually two ways of excitation ：
One is UV light , One is sealing CO2 Gas

Using infrared heat , When the temperature rises or the energy increases to a certain extent , Such as the melting point of organic matter 、 At ignition or boiling point , Then the interaction or binding force of organic molecules will be greatly reduced to make organic molecules separate from each other into free state or free state , Due to the continuous supply of energy by the laser , And make organic molecules escape or burn with oxygen in the air to become carbon dioxide or water gas and disperse away , Because the laser is processed with a certain diameter of infrared beam , Thus, tiny holes are formed .

solid state Nd：YAG The ultraviolet laser emits a high-energy ultraviolet beam , Using its optical energy ( High energy photons ), Destroy the molecular bonds of organic matter ( Such as covalent bond ), Metal crystal ( Such as metal key ) etc. , Form suspended particles or atomic groups 、 A molecular group or atom 、 Molecules and escape , Finally, a blind hole is formed .

Filling method of laser hole ：

Advantages of electroplated hole filling ：
1. It is beneficial to the design of stacked holes (Stacked) And the hole on the disc (via on Pad)
2. Improve electrical performance , Contribute to high frequency design
3. Helps to dissipate heat and increase current carrying
4. Plug hole and electrical interconnection are completed in one step
5. The blind hole is filled with electroplated copper , Higher reliability

Because the aperture of radium perforation is small, it is 0.075-0.2mm, By electroplating , Increase the thickness of copper in the hole , So as to achieve the purpose of plug hole .

The ratio of plate thickness to hole diameter for hole filling electroplating ( Thickness diameter ratio ) The limit is 1:1, Usually in 0.8：1, Also is to use 4mil The laser hole in the hole , The maximum depth of hole filling is 4mil, The normal depth is 3.2mil about . The figure below is a typical HDI Laminated drawing of plates , You can see it , The dielectric thickness of all blind hole layers shall not exceed 1：1 Thickness diameter ratio of .

Influence of blind hole depression on welding :
Because the diameter of blind hole or through hole is larger than that of electroplated hole filling ( Pore structure ) reason , And the basic principle of electroplating additives , Inevitable Dimple value . Since the blind hole involves the subsequent welding process , Usually yes Dimple Values have different specifications , General requirements Dimple≤15um.

The first batch of dimple, It meets the production requirements .

The second batch dimple Obviously exceeding the standard .

If... On the blind hole disc dimlpe excessive , As shown in the figure below ,BGA When welding, the welding ball of , Will create a void .

The figure below shows that the concave degree on the disc hole and blind hole pads is relatively large , It is easy to produce voids and bubbles during welding .

If the pad is not in the middle of the solder joint , This will cause cracking on one side of the solder joint , This is where the defect arises , Especially after aging and vibration test , This kind of defect will be more prominent .

After listening to Xiao Xiao's analysis , Mao's mouth widened in surprise , This is this. …… Beauty is not terrible , The terrible thing is that beautiful women have culture , It's still very professional .
Mao stayed for a long time , I just asked ：“ Xiao Xiao , You said I would do it later HDI plate , This sag , What to do with .”
Xiao Xiao said angrily ：
1. When the design , The thickness diameter ratio of blind hole shall not exceed the standard , Control in 0.8：1 about .
2. Special for laser blind hole VCP Hole filling electroplating treatment , The sag shall be controlled within 15um following .
3. We will try to solve this batch , Because you are our God .

In the end, it's still PCBA The factory carried all .

The problem is coming.
When designing laser blind holes , Whether the thickness diameter ratio of the blind hole is considered , Have you paid attention to the abnormity of over standard concavity in hole filling electroplating ,PCBA Whether the factory has put forward improvement suggestions , Come on , Wusu opens , The more you talk, the more hi ……