Different Stackups for HDI PCBs

Written by Rush PCB Inc on . Posted in PCB, PCB Manufacturing

Printed Circuit Board (PCB) manufacturers typically follow three types of stackups for boards they will assemble with high-density packages:

  • Standard Lamination with Vias or Plated Through Holes
  • Sequential Lamination with Plated through, Blind, and Buried Vias
  • Lamination Buildup with microvias

Among the three above, the last is especially suitable for High Density Interconnect PCB (HDI PCB). Rush PCB Inc., eminent HDI PCB manufacturer,recommends using lamination buildup with microvias for HDI printed circuit boards that have high pin-count Ball Grid Arrays (BGA) and other fine-pitch packages, as each type has its own advantages and disadvantages.

For instance, standard lamination with through vias may be low cost for 28 layers and below, but is very difficult to route when multiple BGAs with over 1500 pins and less than 0.8 mm pitch are involved. Likewise, sequential lamination with blind and buried vias have potentially shorter via stubs and fairly simple via models, with smaller via diameters than those required for through hole vias. Costing more than the standard lamination with through vias, sequentially laminated boards maintain the same minimum trace widths and their practical reliability limits their number of layers to a maximum of two or three.

Above limitations and more are leading to a larger number of HDI PCB manufacturers moving towards building up laminations with microvias and other advanced features for HDI PCBs. Advantages of the microvia HDI PCB design include achieving very high route density with fewer layers, as the traces and vias have much smaller dimensions comparatively. In microvia HDI PCBs, the potential for a lower number of layers comes from the effective use of patterns with microvias, as this opens up more space for routing, providing the only applicable way of designing with several large, fine-pitch BGAs with pitch measuring 0.8 mm or lower.

Offering the lowest cost for high frequency, high-density boards, HDI technology, with suitable stackup definition, also improves power and signal integrity in high frequency PCBs. Although typical materials fabricators use for HDI PCBs do well in processes requiring RoHS, use of newer materials has the potential of higher performance with the lowest costs. Notably, these newer materials are not suitable for fabrication of boards using standard or sequential lamination.

Also Read; How to Choose Professional PCB Prototype Assembler

HDI PCB Stackups

Based on the IPC-2315 standard from the Institute of Printed Circuits (IPC), fabricators may use HDI PCB stackup of types I, II, III, IV, V, or Vi. Of the above, types IV, V, and VI are more expensive to fabricate and usually not suitable for high density PCBs with challenges of routing and BGA breakout.

The standard differentiates HDI stackups with the use of via types—micro, blind, and buried, along with plated through vias, although use of the latter is not strictly necessary for flex PCBs.

HDI Type I

HDI PCBs of Type I stackup have a structure of a laminated core with at least a single layer of microvias on one or both of its sides. Type I HDI may use PTH vias and blind vias, but no buried vias.

Rush PCB HDI

Two factors limit the number of layers in the laminated core of Type I HDI PCBs:

  • The aspect ratio or total length to hole diameter for the plated through hole (PTH) via must be less than 10 to maintain reasonable reliability.
  • Very thin FR-4 dielectrics can delaminate under high temperatures required for lead-free soldering.

Therefore, Type I HDI stackup will not be significantly better than the laminate for large dense boards with multiple pin-count BGAs, as the PTH via pads will need to grow larger for higher layer counts. Moreover, the use of a single microvia layer does not offer appreciable benefits with the introduction of special features such as smaller diameter vias and thinner traces.

HDI Type II

HDI Type II structure construction uses microvias, blind and buried vias on a laminated core, with at least a single layer of microvias on one or both sides. Fabricators may stagger microvias from other microvias, and stack them or stagger them relative to buried vias.

Rush PCB HDI 2

Although the HDI PCB Type II stackup is significantly better than Type I for large dense boards using multiple fine pitch components, it has the same limitations as that of the Type I in context of the limitations on the number of laminated core layers.

As Type II HDI PCBs can have microvias only on their outermost layers, it places restrictions on using the outermost layers for a ground or power plane. Additionally, it is not very effective when there is only a single buildup layer for trace routing.

HDI Type III

HDI Type III structure construction also uses microvias, blind and buried vias on a laminated core, with at least two layers of microvias on one or both sides. Fabricators may stagger microvias from other microvias, and stack them or stagger them relative to buried vias.

Rush PCB HDI 3

Type III HDI PCBs offer the best stackup configuration for large dense multilayer PCBs using multiple large fine pitch BGAs, although they have the same limitations on the number of layers as faced by Type I and II when they use PTH holes and thin FR-4 dielectrics.

Using microvias in the inner layers of Type III HDI PCBs allows the outer layers to be used for ground and/or power planes, leaving adequate number of layers for signal routing. Additionally, designers can attain greater routing density with stacked vias, but at higher cost.

Summary

Depending on the application and available budget, designers may use any one of the three types of High Density Interconnect PCB stackup. Type III microvia HDI PCBs offer the widest type of via models and spans. Fabrication cost will depend on the best via models that accommodate the highest route density and signal integrity.

What is Vapor Phase Reflow Soldering?

Written by Rush PCB Inc on . Posted in PCB, PCB Assembly and component, PCB Manufacturing

Although potential electronics manufacturing services (EMS) partners usually look into capabilities and equipment lists, carrying out a comparison can be a daunting task as most EMS partners opt for different brands of equipment, specifically for surface mount assembly (SMT) equipment. Therefore, it is more useful to compare technologies such as flying probes and ICT, and convection and vapor phase reflow.

Convection Reflow Oven

With multiple heating zones, sometimes as many as 12, followed by a cooling element, convection reflow ovens usually have individual temperature controls for each zone. After the SMT assembly process is completed, a conveyor belt carries the populated printed circuit board (PCB) into the oven, which exposes the PCB to a controlled time-temperature profile.

The Printed Circuit Board production lines usually place the convection reflow oven in-line with the SMT assembly equipment, allowing for a relatively high throughput, without additional handling. However, each product requires its own reflow profile and the engineering team has to create this before start of production.

Convection reflow ovens usually have a large footprint, and therefore, consume a large amount of floor space in the factory. Although each heating zone has its own temperature control, engineers usually face a challenge when reflowing densely populated circuit boards in a convection reflow oven, as it is not possible to control the temperature at individual component level.

Also Read; Factors Affecting the Longevity of Copper Bond

Vapor Phase Reflow Oven

Unlike convection, this type of ovens uses condensation or vapor phase for soldering. The vapor comes from boiling perfluoropolyether, an inert heat transfer liquid. In contrast with the convection reflow oven, the vapor phase reflow oven has a much smaller footprint, and the PCB assembly moves vertically up and down instead of sideways.

The vapor layer transfers heat to the PCB and associated components as the assembly sits within it. The heat transfer rate is high, achieves good wetting, and requires much less power input. The process produces very little temperature difference between components of different thermal mass on the PCB. This makes the process very suitable for densely populated PCBs.

In a vapor phase oven, limitations of the physical temperature reliably prevent overheating of any part in the soldering process. As the vapor has a higher density, it is heavier than the surrounding air. This allows the soldered parts to remain sealed inside a neutral atmosphere. For instance, the inert fluid boils at 230°C and creates the vapor layer above it at 230°C. The air over the vapor phase however, does not heat up more than 50-80°C.

The heat transfer fluids used in modern vapor phase reflow ovens, such as perfluropolyether do not contain any CFC or other harmful ingredients that could place limitations for transportation and storage of these liquids. Its main properties are is excellent chemical and thermal resistance, very high electric insulation properties, non-toxicity, low viscosity, and no flash or fire point.

Control of Heat Transfer Through Adjustment of Heat

Engineers adjusted the temperature gradients in earlier vapor phase machines by regulating the power to the heating elements. Greater the power transferred to the heaters, more are the vapors produced, and more the heat transferred to the PCB assembly.

As the rising vapors create an inert atmosphere, the process heats up the boards and the soldering process takes place in an oxygen-free atmosphere, which reduces the oxide formation and improves wetting. However, the slight time delay in the creation and subsistence of vapors with the heater controls prevents the creation of sophisticated temperature profiles. This has led to Soft Vapor Phase (SVP) type of reflow machines.

rushpcb

Also Read;  Fiberglass Fabric Styles Used in Laminates

Control of Heat Transfer through Adjustment of Level

To realize any temperature gradient with a vapor phase reflow machine, engineers now follow the patented process of the soft vapor phase mode. The benefit of the SVP process is engineers can control the immediate temperature gradient as a function of the height level of the boards above the liquid surface.

In the SVP process, as the PCB moves into the vapor, its temperature increases. Holding the board at a certain depth realizes the pre-heating of the board. As the depth increases, the board reaches the liquidus temperature. By preselecting and controlling the soldering time automatically, engineers can create any thermal profile necessary. Once soldered, the process moves the board up to the vapor boundary to lower temperatures and finally out of the vapor to cool down. The SVP process does not require additional mechanisms to control overheating.

However, void formation is an unavoidable risk diminishing the electrical and thermal conductivity of the solder joint. For countering this, engineers prefer a vacuum controlled process, capable of outgassing such voids.

Reduction of Void Formation through Control of Vacuum

By adjusting the pressure over the liquid, engineers assure stable conditions, as a reduction of pressure lowers the boiling point of the liquid and vice-versa. The use of vacuum extends the time above the liquidus by about 30 seconds and this effectively reduces the number and formation of voids, as it is necessary to conduct the void reduction process in the molten state of the solder.

Advantages of Vapor Phase Reflow Soldering

Vapor phase soldering transfers heat at about 100-400 W/m2 K, which is considerably higher than 10-60 W/m2 K of heat transferred by convection reflow soldering. Moreover, the heat transferred by vapor phase is uniform on all components of the PCB, reducing stresses during soldering. The ability to optimize the thermal profile with extended peak times makes the process suitable for all types of electronics. Additionally, the vacuum controlled process takes care of outgassing of the voids, leading to more uniform and reliable joints.

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Fiberglass Fabric Styles Used in Laminates

Written by Rush PCB Inc on . Posted in PCB, PCB Fabrication

Eminent PCB fabricators such as Rush PCB Inc. use different types of prepregs and laminates for their PCBs. The manufacturers of these materials offer a number of fiberglass fabric styles. They base their selection on the thickness that the finished laminate or printed circuit board will take. It also depends on the amount of resin that will be present for filling and bonding. They make the specific choice depending not only on building up the thickness, but also on secondary properties such as cost, dimensional stability, CTE control, dielectric constant, and stiffness.

Construction of Fiberglass Boards

Manufacturers begin with fiberglass fabric on a warp beam that contains several thousands of individual strands of yarn rolled over a master beam. These yarns constitute the machine direction the fabric will take, which is also called the warp direction. They then slash the warp yarns, or run them through a solution of lubricants or sizing agents and this protects them from damage during the weaving process.

The actual weaving process begins by mounting the warp beam on the back of a loom, with the fill yarns being inserted as the warp yarns pass through from the back of the loom to its front. Earlier, there were the Draper looms, where a wooden shuttle with the fill yarn would proceed back and forth from one side of the loom to another to insert the fill yarns, while the alternating warp yarns moved up and down in a mechanical frame as it created the traditional plain weave, resulting in a woven or drapered edge.

The newer looms operate on air or water jet, where the jet carries the fill yarns across the loom. Cutting off the fill yarns individually leaves a fringed edge. In the modern looms, a single warp beam can contain several thousand meters of warp yarns that represent as much as a single loom can weave in a week.

Conductor Surface Roughness

After the weaving is over, the electrical grade glasses need to be scoured with an aggressive water rinse, which removes excess sizing from the warp yarns. To remove the balance, they are then heated in an oven at elevated temperatures for a long period. The weaver then treats the fabric with finishing agents such as organosilane that provides a surface that resin systems can wet and bond.

Glass intended for polyimide manufacturing requires application of a high temperature finish such as amino-silanes. This makes the bonds tough enough to withstand the use conditions the polyimide will go through.

Although the finish on the fiberglass fabric is only a very tiny amount of material, it vitally affects the way resin will wet the surface during the prepregging process. Poor scouring, heat cleaning, or inadequate silane treatment can leave the prepreg with repellent streaks or spots that often show up when heated. Glass weavers offer a variety of finishes, and the correct choice of finish for each resin determines its performance critically.

Also Read; How to Choose Professional PCB Prototype Assembler

Green Epoxy

Several epoxy resin systems are green in color. This started with fiberglass finishes that had chromium chemistry as their base with the trade name of Volan, which produced a green finish. Now, very few manufacturers use Volan, and use organosilanes instead. However, the complaints from end users about the change in appearance of their product have forced the suppliers to start dying their FR-4 products green to make them look same as before. That explains the reason for green being so common a color for epoxy printed circuit boards.

Smooth resin rich surfaces offer a better fill for internal etched copper patterns. This is often a result of lightweight fabrics with high resin content. Using heavier fabrics result in lower cost, while offering enhanced dimensional stability and permit building up greater thickness at lower cost per mil. However, the use of heavier fabrics usually affects drilling characteristics and surface smoothness. Although thicker and heavier fabrics result in low-cost rigid laminates, they can deflect small drill bits causing them to break.

Warp, Fill, and Direction of Weave

For a woven fabric, the term warp indicates the direction of the length of the roll, while fill indicates the direction of the yarns that fill in from side to side in the weaving process. In commonly used fabrics, the tensions and the number of yarns are not evenly balanced and that has a varying effect on stability and subsequently on registration. For a PCB fabricator, it is necessary to know the direction of warp and fill so they can orient them similarly each time and adjust the processing and compensate for predictable effects.

For instance, for a laminate measuring 36 x 48 inches, the warp is normally parallel to the longer dimension. Unless the customer makes a special request, or it is necessary to cut otherwise, the warp usually follows the longest dimension of a piece of cut panelized or prepreg laminate. When warp is not along the longest dimension, or for square panels, the fabricator marks the warp direction clearly by an arrow on the package or on the material.

Also Read;   PCB Testing: Why is it Important?

Weave Distortion

Under normal handling conditions, warp yarns are under tension and remain straight, while fill yarns should remain at right angles to them. However, for some reason, if some of the fill yarns in the fabric move away from their 90° position, the laminate or multilayer may develop a ripple or twist. For the fabricator, it is very important to have raw fabric with undistorted yarns. However, even for fabric with undistorted yarns, warpage can still occur unless the laminator takes care to align the yarns in one sheet of prepreg relative to another.

Summary

It is not easy to specify or measure the dielectric constant of the laminate, as it depends not only on its intrinsic properties, but also on the method of testing, conditioning of the sample before and during the test, and the test frequency. Moreover, dielectric constant tends to vary with temperature.

At Rush PCB Inc., we determine the characteristic impedance of a PCB based on the laminate thickness, its dielectric constant, and the height and width of the etched line height. Impedance matching and control are critical to linked functional modules when dealing with high-speed devices and designs.