The most important questions and answers
They are used in smartphones, airplanes and medical devices and enable high technical performance in the smallest of spaces: HDI printed circuit boards have become an integral part of the electronics industry. However, with advancing miniaturization and increasingly complex circuits, the demands on the design and manufacture of multilayer PCBs are increasing.
What characterizes an HDI printed circuit board?
HDI PCBs are compact multilayer PCBs with four or more layers, which are usually built up sequentially in several press cycles using SBU (Sequential Build Up) technology. HDI stands for High Density Interconnect and means high circuit density. This can be achieved through a multi-layer structure of the PCB, line structures and distances of < 150 µm and vias with a size of < 150 µm (often MicroVias).
Technology & design to cost at a glance
What requirements are placed on PCB manufacturers?
PCB manufacturers must firstly ensure that the base materials are suitable. This means keeping laminate and copper thicknesses as low as possible and ensuring a low coefficient of thermal expansion. On the other hand, ever smaller line-space structures are accompanied by higher demands on the system technology. This is where DI (direct imaging) exposure is gaining in importance, offering not only high layout resolution but also better positioning accuracy. DI imaging is also gaining ground in the solder-stop sector. Direct exposure of the resist makes it possible to reduce the stop varnish bar and reduce the stop varnish clearance by pads. High demands are also placed on the etching process in the HDI area, where a uniform etching rate of top and bottom must be guaranteed.
What needs to be considered for the PCB layout of HDI circuit boards?
It is generally advisable to contact the PCB manufacturer as early as possible to find out about their options. At the end of the layout process, a layout should be ready for the manufacturer. On the developer and layouter side, the first step is to describe what the future PCB should look like. The following questions, among others, must be answered:
- Which base material should be used?
- How many layers are necessary?
- Where are the holes in the stack?
- What base copper thicknesses are required?
- Are components with a high connection density (e.g. BGA) used?
These often result in the routing strategy for the entire layout and the layer structure as well as the constraints for all parameters, for example conductor widths, distances and sizes for blind or buried vias, can be defined.
Example: BGA unbundling
What role do closure technologies play in HDI printed circuit boards?
Even if they are associated with higher process costs, closure technologies such as microvia-copper filling or plugging in accordance with IPC-4761 Type VII a number of advantages. These include, in particular, extended design and routing options as well as space savings thanks to an almost planar surface topography. In addition, air and media inclusions can be prevented and assemblies can be protected with potting varnish. If microvias are also used for heat dissipation, the thermal performance of HDI PCBs can also be increased.
Where is HDI headed - what else is possible in the future?
In the medium and long term, it will be possible to use even thinner materials with better properties and increase the number of pressings. There will also be further progress in miniaturization, so that even smaller structures can be realized both in the conductor pattern and in the stop varnish and even smaller holes can be processed safely.
Do you have any further questions?
You can find even more information on the topic of HDI PCBs in our XPERTS online seminar "HDI - Is it still possible: possibilities for unbundling highly integrated components" with Thomas Doberitzsch and Dirk Deiters. You can register for the on-demand version, which is available at any time register here.
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