How to Design Your HDI PCB Stackup

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HDI PCBs are commonly used in many devices nowadays. Its ability to compact advanced functionality and more components on a smaller space has made it desirable for smaller devices that pack a punch. The trend of miniaturization fits well with HDI PCBs design guidelines. So, how to design an HDI PCB stackup for your project?


There are a lot of things that go into consideration when designing an HDI PCB, such as routing density, layer count, aperture, and much more. Thus, manufacturers need to know how to arrange component space, how to distribute space lines, and more. So let’s take a detailed look into the manufacturing process of HDI PCBs.


Steps in HDI PCB Manufacturing Process


Aperture


For manufacturing, mechanical drilling is utilized for traditional PCBs, but this was an expensive method, so now laser drilling is used. Not only is it cost-effective, but it also helps further reduce the size of the circuit board by decreasing the aperture ratio.


For traditional PCBs, the hole aperture should be 0.15mm, and the thickness-to-aperture ratio should be more than 8:1. For laser drilling, the aperture ratio is in the range of 3 to 6mil, and the depth-to-aperture ratio is 1:1.


A great deal of technicality is involved in this process. The thicker the board, the smaller is the aperture. Sometimes, it becomes difficult for the chemical solution to reach the depth of the drilling holes. Suppose you use pressing and oscillating to press the solutions. In that case, a slight opening is created, which makes the board vulnerable to breaking in case of increased voltage or severe temperature.


Layer Count


Next is layer count; there is no definite answer because the layer count depends on your application and the number of traces you need to fit in the board. There are 3 things you need to look out for when considering the layer count for your HDI PCB circuit board.


1. Thickness and Traces


To ensure controlled and desired trace impedance, you need to determine the layer thickness. There is no one way of doing this, so you need to use your expertise. Just get an initial estimate of the layer thickness based on the BGA pitch and then size your traces on the circuit board accordingly.


2. Nets per Layer


After the trace width and layer thickness are determined, you can finally estimate the required layers. Depending on your layout, you can determine the space taken by a signal layer. You can do this by using a specific board size template then multiplying the number of BGA channels per area with the board area. And this net per layer calculation will be used for determining the stackup type.


3. Layer Count


Now that you know the number of nets per layer, you can divide the net count with the answer and use it to determine the layer count. Remember that this calculation will give you an estimate for signal layers, thus, you need to add the power and ground plane layers to get the final answer.


HDI PCB Stackup Type


There are many types of HDI PCB stackups. HDI stackups are based on the number and order of layers of blind holes. For example, 1-HDI, non-stacked 2-HDI, stacked HDI (resin-filled and non-resin filled), etc. Proper board design depends on the stack design and buried or blind holes distribution. If you stack up the layers asymmetrically, where stress distribution on both sides is non-uniform, the board performance and yield will decrease drastically.


HDI PCB Process


The designing and manufacturing process of an HDI PCB stackup is lengthy and complex. For example, if we discuss the process flow of a 6-layer 2 stacking HDI PCB manufactured using laser drilling, the steps are as follows:


• 3-4 layer mechanical drilling of buried holes
• Drilling buried holes on 2-5 layer
• Blind holes on 2-3 layer and 5-4 layer
• Through holes on 1-6
• Lastly, the drilling of the blind hole on 1-2 and 6-5


Laser drilling is an intricate process and requires pre-operation before electroplating. Moreover, you need professional manufacturers to get the job done as high temperature is used to cauterize the hole wall and oxidize the second layer.


Component Layout


After the layer count and stackup type are decided, we move on to the component layout. This is a crucial step in the manufacturing process as you need to keep things like installability, solderability, and maintainability in check. Even with miniaturization, you need to have ample space for easy installation of each component and rework them if required.


You also need to ensure that high-power signal layers are kept far from other layers. Moreover, the digital and analog parts must be kept separate, even if they are on the same side or on different sides, by enlarging the space.


Line Width


Aside from safe spacing, you also need to care for uniformity and line width. For example, if the line width is too small, an open circuit will be caused due to weak film. And if the spacing is too short, the film residue will dry and cause a short circuit. Thus, the circuit needs to have uniform resin flowing speed and copper thickness distribution.


There are many other things you need to take into consideration when designing an HDI PCB stackup, such as top and bottom component spacing, blind or buried holes, signal quality, routing density, etc. And all of this depends on the manufacturing process. Thus, it’s best to consult a professional manufacturer for your project and ask them about the specifications, circuit board size, etc., to increase control impedance and good signal routing.


Hemeixin offers free PCB quotes and consultation. They provide multiple services like PCB assembly, rigid-flex PCB, PCB technology, and much more. They are the leading PCB manufacturers and can create HDI PCBs with up to 58 layers, laser-drilled micro vias, heavy copper, and much more. Call them now and get a free quote and consultation today!

 

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