Tom
10-15-2002, 03:39 PM
IPC Advanced Study Guide Page Reference: Pages 230 & 231, Section 4.8
The formation of blind and buried vias may be accomplished through different processes. They may be mechanically drilled and use part of the standard equipment for making multilayer boards. The main difference is that the diameter is usually smaller, and greater care is required in the drilling process by the fabricator to make certain that the wall of the blind or buried via is clean and all debris is removed in order to ensure proper plating on the wall surfaces.
If blind and buried vias are prepared prior to lamination, they can be considered as any other through-hole plated feature; if they are prepared after the lamination process, the drilling step requires a "controlled depth" to make certain that the drill goes down only to the layers that are to be connected and does not pierce conductors that are directly below the center point of a hole prepared by controlled depth drilling.
With the advent of high-density interconnecting boards, the industry has seen other techniques for making vias. The term "microvia" has been coined to represent those holes that are part of the HDI process. A microvia can be either a buried via or blind via, since HDI products are made in sequential lamination steps. A sequential processing step (adding additional material through coating or lamination), is that which starts as a blind via may become a buried via due to the addition of extra layers.
Nevertheless, techniques such as laser drilling, wet/dry etching, photoimaging, conductive ink, and mechanical drilling are used to prepare microvias that are part of the HDI process. The main difference in the term microvia is that a microvia is no larger than 0.15mm [0.006 in.], whereas standard blind and buried vias can meet that criteria or be larger.
Some of the main differences between mechanical drilling and the other techniques are that the material constructions may vary; laser ablation differs from mechanical drilling in the focus beam used to create the via can produce a smaller hole. These lasers are generally categorized by their wavelength of light and the technique for the removal of copper and resin can vary dramatically, thus impacting the manner in which the vias are produced.
Vias formed through etching techniques can be done by dry etching. Dry etching is a plasma gas similar to what has been used to remove resin smear in polyimide flexible films. There is also chemical removal of resin material, and although many holes are produced in one process load, the etchant moves in many directions, etching both through and to other surfaces.
If photoimaging is used to remove dielectric coatings and material, the microvias are formed by a photoimaging process, thus making it imperative that the core material consists of photopolymers that can be hardened so that the unhardened material can be removed in the etching process.
The illustration shows relatively different wall constructions for the vias. It should also be noted that the most difficult part of the fabrication process is to take a via that's on the outer surface and plate down into the via to make connection to what is termed as the target land in the outer two surfaces. Illustration 6 in the figure shows a wet-etched or laser drilled construction. There is no copper on the surface and the copper is added as a part of the HDI process. Nevertheless, the shape of that hole is most conducive to getting plating down into the via and on the hole wall.
Cross-sectional views of methods for making vias
The formation of blind and buried vias may be accomplished through different processes. They may be mechanically drilled and use part of the standard equipment for making multilayer boards. The main difference is that the diameter is usually smaller, and greater care is required in the drilling process by the fabricator to make certain that the wall of the blind or buried via is clean and all debris is removed in order to ensure proper plating on the wall surfaces.
If blind and buried vias are prepared prior to lamination, they can be considered as any other through-hole plated feature; if they are prepared after the lamination process, the drilling step requires a "controlled depth" to make certain that the drill goes down only to the layers that are to be connected and does not pierce conductors that are directly below the center point of a hole prepared by controlled depth drilling.
With the advent of high-density interconnecting boards, the industry has seen other techniques for making vias. The term "microvia" has been coined to represent those holes that are part of the HDI process. A microvia can be either a buried via or blind via, since HDI products are made in sequential lamination steps. A sequential processing step (adding additional material through coating or lamination), is that which starts as a blind via may become a buried via due to the addition of extra layers.
Nevertheless, techniques such as laser drilling, wet/dry etching, photoimaging, conductive ink, and mechanical drilling are used to prepare microvias that are part of the HDI process. The main difference in the term microvia is that a microvia is no larger than 0.15mm [0.006 in.], whereas standard blind and buried vias can meet that criteria or be larger.
Some of the main differences between mechanical drilling and the other techniques are that the material constructions may vary; laser ablation differs from mechanical drilling in the focus beam used to create the via can produce a smaller hole. These lasers are generally categorized by their wavelength of light and the technique for the removal of copper and resin can vary dramatically, thus impacting the manner in which the vias are produced.
Vias formed through etching techniques can be done by dry etching. Dry etching is a plasma gas similar to what has been used to remove resin smear in polyimide flexible films. There is also chemical removal of resin material, and although many holes are produced in one process load, the etchant moves in many directions, etching both through and to other surfaces.
If photoimaging is used to remove dielectric coatings and material, the microvias are formed by a photoimaging process, thus making it imperative that the core material consists of photopolymers that can be hardened so that the unhardened material can be removed in the etching process.
The illustration shows relatively different wall constructions for the vias. It should also be noted that the most difficult part of the fabrication process is to take a via that's on the outer surface and plate down into the via to make connection to what is termed as the target land in the outer two surfaces. Illustration 6 in the figure shows a wet-etched or laser drilled construction. There is no copper on the surface and the copper is added as a part of the HDI process. Nevertheless, the shape of that hole is most conducive to getting plating down into the via and on the hole wall.
Cross-sectional views of methods for making vias