HDI PCB capability
HDI (High Density Interconnected) boards, one of the fastest growing technologies in PCBs, are now available at Weller Technology. HDI Boards contain blind and/or buried vias and often contain microvias of .004 o in diameter. They have a higher circuitry density than traditional circuit boards.
Pros of HDI PCB
- Increased interconnection density by miniaturization of holes, pads and conductors.
- Provides the possibility to have a via hole connect direct in the SMD or BGA pad.
- Improved signal integrity due to reduced tracklength.
- Improved possibility to maximization of ground connections.
- Opportunities of better thermal enhancement.
- Improved reliability by stepping up technology rather than compromising existing design rules
Cons of HDI PCB
- Some technologies are at early stage yet.
- There are different ways to achieve similar technology (copper filling, drilling).
- Lower Yield.
- Most suppliers “can do everything” (risks costly experience).
- Limits available suppliers.
- Stepping up to advanced technology without proper need (or knowledge).
- Expensive equipment.
Laser drilling with conformal mask is a well-tested way to handle the microvia projects among PCB manufacturers. Yet, the additional photoimaging process for via pattern etching has a slight registration problem, which comes as a direct result of etching capability constraint. Therefore, drilling of copper layer by direct laser irradiation is a promising alternative. This technology is only properly employed when special surface pretreatment is done to improve the surface topography of the copper layer, as well as to enhance the laser absorptivity on copper.
In fact, the copper direct laser drilling is capable of providing higher accuracy, better hole quality and better efficiency for HDI projects. The smallest hole size of it can go down to 2 mil.
HDI circuit boards layers Build-up
Choice of Material
There are a wide range of materials that can be used for build-ups on HDI boards.
- RCC (Resin coated copper)
- LDP (Laser drillable prepreg)
- PP (prepreg)
- NWA (Non woven aramide) epoxy & polyimide
- Speedboard C (Gore)
- The most common are RCC, LDP and PP
RCC (Resin coated copper)
This is as the name indicates a copper foil which are treated with epoxy resin. It is normally applied in two layers one layer with fully cured epoxy (c-stage) and a layer with semi cured epoxy (b-stage). Normally values are 25 or 35μm of c-stage epoxy and 35μm of b-stage epoxy.
- Easy to drill
- Even surface for fine lines
- High CTE values
- Lower amount of resin
- Lower peel strength than LDP
- Requires an even surface that it’s bonded to
- Mismatched Dk with the core layer
- Short shelf life
LDP (Laser Drillable Prepreg)
This is as the name indicates a prepreg that has been more adopted for laser drilling, the fiber bundles are more uniformly distributed so the laser beam always drills in a uniform material.
This is very important since the epoxy is very easy to drill, but the glass fibers are more difficult. The laser drillable prepregs are normally made only in 106, 1080 & 2116 style.
Laser Drillable Prepreg-Benefits
- Same CTE as core layer
- High amount of resin
- Same Dk as core layer
- High peel strength
- Woven material – increased rigidity
- Long shelf life
Laser Drillable Prepreg-Disadvantages
- More difficult to drill then RCC
What material should be used?
Since HDI boards are a more sensible in construction and we now have switched to lead-free soldering there is a need for better material than for conventional multilayer
boards.They key parameters are:
- Total CTE in Z-axis
- Decomposition temperature
- Glass transition temp (Tg)
- Use LDP instead of RCC foil as material for μvia
- Use high quality FR4 in cores and Prepreg
- Specify the material according to IPC standard
Design rules for HDI PCB
- Always aim for symmetrical build-ups even if μvias not are needed on both sides.
- Aspect ratio on blind hole should be kept well below 1:1, preferred 0.7:1
- When using 2 levels of μvias, keep the copper balance good, fill out empty areas with ground plane,so the amount of resin is enough to make a good encapsulation of the tracks.