This article clears up confusion about different via structures in PCB fabrication and lists the pros and cons of each.
There are three high frequency asked questions about blind & buried via sand filled via: what are they?why do I need them? Why do they cost a lot more?
To understand the reason behind their extra cost — and to appreciate their importance in modern, complex PCB designs — we need a basic review. Below is a list of industry terms and their definitions, followed by an explanation of how blind, buried or filled via are created in a fabrication shop.
There are many industry terms and acronyms in this article, if you do not understand them clear, please check this article:Industry Terms And Acronyms In PCB manufacturing
Via explanation and process
Thru-hole, Buried, Blind, Stacked
Thru-hole via
The via hole is the means of connecting an electrical trace to the opposite side of a PCB. It is in essence a cylindrical trace created by copper plating a drilled hole to a typical wall thickness of 1 mil (.001). To create the via hole, a landing pad is required on each side of the PCB. When viewed from an edge-on perspective, the cross section of a plated via hole looks like a rivet with a button head on the top and the bottom. Because a landing pad is required to create the plated via, each via consumes a significant amount of area on the PCB. The smallest drilled via is going to have a landing pad of about 20 mils (.020 or .5 mm)in diameter. A typical MLB will have hundreds of via holes. These create a great challenge for the designer as they attempt to rout traces n’ spaces around the inconvenient via landing pads.
Buried via
A buried via is employed to greatly increase the density of the traces n’ spaces within a PCB. It allows a significant increase in the area available to the PCB designer for traces n’ spaces on the outer two layers since the buried via holes do not penetrate the outer layers. The same is true of any internal signal layers that do not contain the buried via.
The designer can make connections between traces without going “up” or “down” to an outer layer. This is a huge benefit when the traces must emulate coaxial cables and have a characteristic AC impedance. Thru-hole via provide a significant barrier to the routing of these controlled impedance traces.
A buried via is created by first processing one or more double side PCB. A very thin, double-sided core (2 layers within a multi-layer PCB) is processed as a complete plated-through DSB. This requires different process equipment from a standard double sideboard due to the thickness of the core — which can be as thin as a sheet of paper. Yet, all the thru-hole processes are required with the exception of solder mask and final surface finish.
These thin, PTH cores are sandwiched between resin-rich prep reg (above and below) and then pressed together to create a multi-layer PCB.
At this stage, the “brick”, as it is called in the industry, can be processed as a double-sided PCB.
The prep reg must have sufficient resin (epoxy) content to not only fill the 3-D geometry created by the traces n’ spaces, but ALSO the additional geometry of the center of the plated via holes.
Sequential lamination for buried vias
For additional density, each layer pair in a MLB can have its own buried via. During the layup process, these layer pairs are then laminated into the final brick for outer layer processing. But even this structure can be further expanded by combining pairs of internal layers containing buried via in a process called sequential lamination. Sequential lamination creates an intermediate brick of 4 or more layers, with each layer pair containing its own buried via. Then the intermediate brick can have thru-hole via that join the layer pairs. An intermediate brick can be drilled and processed with its own buried via.
Two or more intermediate bricks can then be combined to create an additional intermediate brick with even higher density. This process can be performed a maximum of 5–6 times. Three is the recommendation due to copper thickness accumulating on the inner layers and registration issues resulting from this many photos/chemical/mechanical operations.
Buried via are very robust and are often employed in controlled impedance designs. However, they add cost due to the need to process a complete DSB and then start the conventional MLB process. When you purchase buried via, you are purchasing a double side PCB as well as a multi-layer PCB. With sequential lamination, you are purchasing multiple DSBs as well as a multi-layer PCB.
Blind via
The blind via has become popular as more fabrication shops have developed the ability to perform a controlled-depth drill operation with a laser drill. The most common application of the blind via is to remove the congestion created by the fan-out of “dog bone” traces to a thru-hole via under high density BGAs. These fan-out traces and their terminating thru-hole via use huge amounts of real estate, require the use of very small traces n’ spaces, and require the use of very small, fragile solder mask dams between the geometry. With its accompanying landing pad, the terminating via consumes routing space for traces that need to go to other BGA pads, requiring these to happen on an inner layer. The small traces n’ spaces add cost due to the yield at the fabricator to produce these small traces n’ spaces on outer layers.
The landing via pads also create a physical limit to the size of the BGA balls/pads that can used. There are many new high-density BGAs that cannot use a copper “dog bone” PCB pattern with landing pads for thru-hole via.
Further, from an assembler’s perspective, the dog bone pattern or (even worse) the solder mask defined pad process are a huge problem, as they attempt to use conventional stencil print processes for the application of the solder paste to the BGA pads. A stencil process requires that the extruded solder paste be able to adhere to the PCB pad such that it is pulled away in a pattern from the aperture of the metal stencil when the stencil snaps away from the PCB surface. The stencil process needs a solid gasket to the corresponding pad in order to be pasted. If the stencil is not able to gasket to the pad, then the volume of paste that is deposited is significantly reduced. Any non-uniformity in the height of the surface of the PCB exacerbates this stencil marketing problem. If the dog bone patterns and the copper floods for solder mask defined pads are viewed end-on, they are at the same height as the BGA pads. However, once the solder mask is applied, the non-uniform thickness of the solder mask over the copper floods and the dog bone traces/via landing pads create a 3-D topography for the stencil. The stencil contacts the solder mask first and then must slightly deform (about 1 mil) to make contact with the BGA pads that are lower than the solder mask. This irregular geometry results in a significant variation in the deposition of the solder paste on a given BGA pad. The advent of very good solder paste inspection (SPI) equipment has reduced the defects experienced by the assembler, since the systems reject PCB that have insufficient paste. But the same systems are creating a significant bottleneck as PCB with insufficient paste deposition are rejected and must be cleaned and re-stenciled.
There are numerous processes for creating the blind via. The oldest is perhaps the easiest to comprehend. In this case, the multi-layer brick is photo-masked and etched to reveal windows in the copper foil. These windows are then laser drilled (CO2 laser) to a controlled depth that is typically 4 mil (.004 or 0.1 mm). The reason for the etching process used to create the windows is to allow a pattern that will deflect the beam of the CO2 laser everywhere but the area under the opening, which is ablated to the copper pad on the next layer. Following laser ablation of the fiberglass covering the inner layer pad, the brick is mechanically drilled for the thru-hole via and then processed as a DSB with the conventional thru-hole plating.
The thru-holes are plated in a normal fashion (see the above DSB process), resulting in a copper cup that is formed in the BGA pad and makes an attachment to the corresponding copper pad on the next inner layer. This copper attachment to an inner layer allows a pads-only outer surface for the attachment of the BGA.
However, the process described will leave a dimple in the BGA pad approximately 3 mils in depth and 4 mils in diameter. For larger BGAs, this is not an issue. But as the diameter of the BGA pad approaches .2 mm (10 mils), the dimple consumes a significant volume of the applied solder paste. Different and modern process are employed with additional process steps that include terms like “reverse pulse plating” (RPP), and are used to solve this problem to copper-fill the dimple or copper-cap the dimple if a resin fill of the thru-hole via is employed. Therefore, while blind via replicate the thru-hole plating process, they are created with different and more expensive equipment for each process operation.
Blind via with modern technology require additional process steps with very expensive capital equipment (laser drill, RPP equipment, laser direct image, etc.). For these reasons, blind via can be thought of as doubling the process required to produce the traces n’ spaces and plated holes for the outer layers.
Blind via and buried via are not mutually exclusive, and a design can employ either or both. Today’s high-density PCB frequently require at least the use of blind via to increase the yield at the fabricator and the assembler. While they do increase the cost of the PCB, the reliability and process consistency will offset the additional PCB cost. It is a false sense of economy to accept the process variation and structural degradation that conventional processes will create when a .25 mm BGA is utilized.
Stacked via
Stacked via are used in extremely high density designs where the real estate to fan out to buried vias is not available. In this case, a blind via is stacked over a blind via on an intermediate brick instead of a copper inner layer pad. This is process requires sequential lamination, but it also requires that the blind via process be performed on the intermediate brick before the next lamination process.
Stacked vias are not for the faint of heart. They can be made reliable, but to do so requires close cooperation between the designer and the fabrication shop. This technology cannot be designed in a vacuum and then sent out for bid. The most common failure is a coefficient of thermal expansion (CTE) mismatch of all the resins in the MLB. During subsequent processing by the assembler, the CTE of each resin (prep reg, core, resin fill) will expand at a different rate. In all thermoplastics, the CTE goes from approximately linear to exponential once the glass transition temperature (TG) of the different resins have been exceeded. If the fabricator simply matches data sheets without performing flow tests on each type of resin, he has set the assembler up for disaster. Any CTE mismatches will result in a fracture of the stacked via during the SMT fusion or even the through-hole soldering process. All MLB resins melt below the fusion temperatures of lead-free solder.
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