HDI circuit board

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Structure of a 6-layer HDI / SBU multilayer (orange: conductor made of copper; conical: microvia)

The HDI circuit board ( High Density Interconnect circuit board ) is a compactly designed circuit board .

Advantages over conventional printed circuit boards

The ever advancing miniaturization and the ever more complex required circuits as well as components with high pin numbers are pushing the classic multilayer boards more and more to the physical limits of their possibilities. HDI circuit boards offer finer line structures and smaller vias. The microvias create space and also have better electrical properties than classic "thick" vias or blind holes .

By pressing additional layers using SBU (Sequential Build Up) technology, signals on the inner layers can be connected and disentangled without blocking the space for components with a high pin density. With a little experience and with a good layout, these components can even be placed on the circuit board so that they overlap . Thin circuit boards with 100 µm and 125 µm structures enable impedance-controlled cables for high and highest frequencies .

The triumphant advance of the HDI circuit board began at the end of the 1990s with the products in the DECT Gigaset Pocket 2000 and in the Siemens C + S25 GSM cellular network. The circuit boards came from the former ppe plant in Schopfheim (today the plant belongs to the Würth Group) and later from Pulversheim also from AT&S Leoben. In 2008 around 98 percent of all mobile GSM and UMTS devices worldwide will be manufactured as HDI circuit boards.

Disadvantages compared to ordinary printed circuit boards

Due to the more complex manufacturing process, you are more closely tied to a manufacturer (mastered manufacturing processes, tolerances, layer thickness, dielectric constant of the carrier material, etc.). High-frequency applications with striplines cannot be implemented with the high tolerances in the microvial layers. Another layer is required for this. The reference position of the stripline must not be in the 2nd copper layer (Cu) below the stripline.

Structure and manufacturing steps

To the picture at the top. Figure is a schematic representation. Cores cannot be used on layers with galvanic copper plating (buried vias). The structuring only takes place after the LBA / through-hole plating process.

symbol signature description Layout specifications comment
1 Outer layer structures
A. Outer layer structure > 75 µm Depending on the Cu thickness
B. Track spacing > 75 µm Depending on the Cu thickness
2 Inner layer structure
C. Track width > 75 µm Depending on the Cu thickness
D. Track spacing > 75 µm Depending on the Cu thickness
3 Microvias from top to L2,
standard or con. Micro
drill tool
E. Hole diameter entry > 100 µm If conical, then depending on the drilling depth
(dielectric thickness)
F. Hole diameter target pad > 100 µm Is defined by the tool
G Drilling depth Depending on the
dielectric thickness		 Note aspect ratio> 1: 1!
H Microvia entry pad > E + 200 µm 100 µm around the hole required
I. Microvia landing pad > 350 µm F + 125 µm all around the hole diameter
on the landing pad
4th Buried Via from L2 to L5
J Drilling diameter > 150 µm Note aspect ratio> 1: 8!
K Pad diameter > L + 200 µm
5 Through hole
L. Drilling diameter > 150 µm Note aspect ratio> 1: 8!
M. Paddle diameter outer layers > L + 200 µm 100 µm around the hole required
N Paddle diameter inner layers > L + 250 µm 125 µm around the hole required

Keywords from the HDI area

Blind Via (blind hole)
Contact ending on an inner layer
Buried Via (Buried Via)
Through-hole plating located in the core layers and not visible from the outside
HDI (High Density Interconnect)
Circuit with microvias and the finest structures
Microvia
Connection or through-hole plating with a diameter of less than 200 µm
SBU (Sequential Build Up)
Sequential layer structure: requires at least two pressing processes for multilayer circuits
LBA (conductor pattern structure)
Application of galvanic copper in order to be able to produce vias

Production of a 6-layer HDI / SBU multilayer

  1. Fabrication of the structures of the inner layers 1 and 2 with L2 + L3 and L4 + L5 (see picture above)
  2. Press the inner layers 1 and 2 with the prepregs inside to form a multilayer core
  3. Drilling the buried vias as through holes (4)
  4. Through-hole plating of the multilayer core (L2 to L5)
  5. Hole Filling (optional): Filling the sleeve 4 with filler material and subsequent surface grinding
  6. Structuring the core (layers 2 and 5)
  7. Pressing with the prepreg laminates on the outside
  8. Drilling the microvias 3 and the vias 5
  9. Finishing (structuring, contacting, external surface treatment) like an ordinary multilayer
  10. Surface finish (recommended: chemical tin or chemical nickel / gold)

Depending on the desired properties and the position and type of contacts, several variants of the structure are possible for a certain number of layers.

Testing of HDI circuit boards

Smaller series

Smaller series are tested most economically with finger testers (also known as flying probe testers ), which can optically detect the test points and thus deflect their test fingers precisely onto the pads. One advantage is that the finger tester can easily be relearned for a new product. This means that small series can also be tested cost-effectively. The disadvantage is that checking a circuit board can take several minutes if there are many connections to be checked. Often, due to time constraints, only impedance measurements are made and the DUT is not subjected to a 100% test.

Larger series

For larger series (sometimes from 50 to 100 circuit boards), the use of a rigid needle adapter makes sense. The circuit boards are measured under a PRS (camera system) (for shrinkage, elongation, pillow shape, barrel shape, twist and offset between the top and bottom layers). Using these correction values, the HDI circuit board is then positioned in the contact and contacted and checked with the rigid needle adapter. With the fine rigid needles, 70 µm structures can be contacted which have a test distance> 150 µm. The concept of this adapter makes it possible to resolve up to 280 test points per cm², whereby a very high test density can be achieved.

Advantages are:

  • Testing with rigid needle adapters is very quick, so that larger series can also be tested quickly.
  • The HDI circuit boards are subjected to a 100% test
  • Rigid needle adapters can also be used to contact populated HDI circuit boards, which means that a function test can also be carried out.

The disadvantage is that rigid needle adapters have to be manufactured specifically for the product, which results in adapter costs per circuit board type.

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