HDI's blind hole design, have you noticed this detail?

What is HDI

HDI (High Density Interconnection) is the abbreviation of High Density Interconnection PCB. It is based on the traditional multi-layer circuit board manufacturing technology, realized by high-density micro-wiring and micro-via technology, supplemented by laser drilling, horizontal/vertical Abbreviation for multilayer buried/blind via PCB produced by PTH wet process and other processes.

According to the definition in IPC-2226:
Blind or buried hole diameter ≤ 0.15mm [0.00591 in], disk diameter ≤ 0.35mm [0.0138 in], by laser or mechanical drilling, dry/wet etching, pattern transfer, formed by electroplating Conductive wrap.
Remarks: Aperture > 0.15mm[0.00591 in] refer to the via hole in this standard.

HDI commonly used drilling size range:
3-5mil, generally take the middle value of 4mil for design and production.

IPC standards commonly used by HDI
1) IPC/JPCA-2315-High Density Interconnect Structure and Microvia Design Guidelines
2) IPC-2226-High Density Interconnect (HDI) Printed Circuit Board Design Guidelines Part
3) IPC/JPCA-4104 - Specifications for the Verification and Performance of Dielectric Materials for High Density Interconnect (
HDI) Structures , Blind holes with aperture below 0.15mm (6mil) and blind holes with bottom Pad below 0.25mm (10mil) are called Microvia vias or microvias. Microholes are usually processed by laser, and the light types mainly include infrared light and ultraviolet light. There are two common LASER excitation methods: one is UV light, the other is sealed CO2 gas

Using the thermal energy of infrared rays, when the temperature increases or the energy increases to a certain extent, such as the melting point, ignition point or boiling point of the organic matter, the interaction force or binding force of the organic matter molecules will be greatly reduced to make the organic matter molecules detach from each other and become free. State or free state, due to the continuous supply of energy by the laser, the organic molecules escape or burn with oxygen in the air to become carbon dioxide or water gas and disperse. Since the laser is processed by an infrared beam of a certain diameter, it forms tiny holes.

The solid-state Nd: YAG ultraviolet laser emits a high-energy ultraviolet light beam, and uses its optical energy (high-energy photons) to destroy the molecular bonds (such as covalent bonds) of organic matter, metal crystals (such as metal bonds), etc., forming suspension Particles or atomic groups, molecular groups or atoms, molecules escape out, and finally form blind holes.

Filling method of laser hole:

The advantages of electroplating and filling :
1. It is beneficial to design stacked holes (Stacked) and via on Pad (via on Pad)
2. Improve electrical performance, help high frequency design
3. Help heat dissipation and increase current carrying
4. Plug Holes and electrical interconnections are completed in one step
5. Blind holes are filled with electroplated copper for higher reliability

Since the diameter of the laser hole is as small as 0.075-0.2mm, electroplating is used to increase the copper thickness in the hole, so as to achieve the purpose of plugging the hole.

The thickness-to-aperture ratio (thickness-diameter ratio) limit of hole-filling electroplating is 1:1, usually 0.8:1, that is, using a 4mil laser hole, the maximum depth of the hole filling is 4mil, and the normal depth is about 3.2mil. The figure below is a stack-up diagram of a typical HDI board. It can be seen that the dielectric thickness of all blind via layers does not exceed a 1:1 aspect ratio.

The effect of blind hole concavity on soldering:
Due to the blind hole diameter or through hole aspect ratio electroplating filling (hole structure), and the basic principle of electroplating additives, the Dimple value is inevitable. Since the blind hole involves the subsequent welding process, there are usually different specifications for the Dimple value, generally requiring Dimple≤15um.

The first batch of dimples meets production requirements.

The second batch of dimples obviously exceeded the standard.

If the dimlpe on the blind hole plate exceeds the standard, as shown in the figure below, the solder ball of the BGA will produce a void when it is soldered.

The picture below shows that the depression on the hole in the plate and the blind hole pad is relatively large, and it is easy to generate voids and air bubbles during soldering.

If the pad is not in the middle of the solder joint, one side of the solder joint will be cracked, and the defect will be generated, especially after the aging and vibration test, the defect will be more prominent.

1. When designing, the thickness-diameter ratio of blind holes should not exceed the standard , controlled at about 0.8:1.
2. The laser blind hole is treated with special VCP hole filling and electroplating, and the concave degree is controlled below 15um.