Registering Multilayers for Lamination: The Challenge of Registering New Materials on High-Density Interconnection PCBs

Estimated reading time: 18 minutes

Editor's Note: This article originally appeared in the March 2012 issue of The PCB Magazine.

Many people think of imaging as the first step in the manufacture of a PCB. Imaging on the PCB raw substrate is, however, the culmination of much planning; it is also the realization of only one step in the design of a product, and one aspect of information processed into reality. The connections made (or avoided) on a PCB are the physical embodiments of an electronic engineer’s scribbles on a schematic. The designer of the product must bring together many images in order to complete his circuit design. Obviously, if the board is multilayered there are as many images as layers. Then, there are the two solder mask images, a legend, a layer of solderable material added to the pads (a kind of pseudo-image), not to mention the needles of the electrical test machine and the solder paste mask prior to assembly.

One can’t forget the holes which combine the layers into a working circuit. A modern circuit board contains multilayered holes. Consider that there are through holes, blind holes and buried holes. When considering buried holes, there are those made by drilling through the inner layer and those made by drilling through a laminated assembly, which is then laminated again into a sequentially pressed multilayer. When considering blind holes, there are those made through sequential lamination, those formed by laser from the surface down and those mechanically drilled--there are even holes created by plasma (dry) etching. All holes are like layers of image that need to be brought into the proper relationship to the images of a circuit to make it work correctly.

The Importance of Registration

When we discuss imaging in PCB production there is nothing more important than registration. The process of registration is no less than the orchestration of all images, features and processes such that each occupies the proper relationship in space with all the other images, features and processes of the PCB. This orchestration begins in the design phase with the design of a data set synchronized to a given Cartesian (X, Y, Z) co-ordinate set of dimensions.

From data, the image is fed to CAM software where the data is elongated, or foreshortened and fed as an instruction set to plotters, LDI machines, silk-screening machines, electrical test machines, AOI machines, ink-jet machines, drilling and routing machines, registration punches and any other machines responsible for putting features into or onto substrates at specific locations in space, relative to other features of the PCB.

The registration system is there to follow the images through all their changes. Through wet processes, temperature and humidity changes and scores of processes, the registration system makes certain that the holes meet the pads correctly; that the solder-mask openings are where they are supposed to be; that the conductive part of the solderable surface finish doesn’t build unwanted connections between pads and tracks, and the legend designates the location of a component correctly.

In short, the registration system makes all the images usable or not.

The early days of PCB manufacture involved much eye alignment of images to other images or circuit features. The use of such alignment is still practiced today to make highly accurate and professional circuits. The practice of using diazo film is slowly fading away, but among small prototype companies in the West and for many under-financed companies in Asia, diazo film is still used despite its disadvantages. The distortions to the images introduced by the very production of the diazo image has made the practice of aligning diazo images to circuit features unworkable for most modern factories.

Among the earliest professional registration systems was a system developed by Multiline Technology in the late 1970s and involved the use of a machine for punching slots in artworks and a punch machine (positioned in the yellow room between the dry film laminator and the exposure process), which placed four slots into the inner layers. The slots in the films were then matched to the slots in the laminate during the exposure process using pins set into or upon the glass of the exposure machine. As the film had been punched to match the drilled holes of an especially created drilled master panel, it was assumed that any panels containing slots in the same positions as the film would align to the drilled image that came much later in the process. After imaging, developing, etching, stripping and black oxidation, the inner layers were pinned together for the lamination of the layers into a multilayer panel with the use of the same slots that had been used to register the images to one another. Later, the same slots were used to align the panels on the drilling machines with the use of so-called hard tooling or soft tooling plates. The drilled holes were, in this way, aligned to the image in the artwork punching process. The relationship between drilled holes and the four-slot tooling holes was created early on and carried through the inner layer process.

The problems of using such a system became apparent as layers became thinner and the personality of every layer could express itself more freely. The material was hardly ever homogeneous. The materials absorbed moisture, were heated and cooled, were subjected to mechanical pressures and stresses which caused localized anomalies, which in turn created localized mechanical differences. Nothing could be counted on to be where it was supposed to be after going through the complete process of inner layer manufacture.

Manual and automated exposure processes had difficulties working with the four-slot pinning system because of its relative inflexibility to various panel sizes on the same piece of glass. As a result, a series of other pinning solutions such as edge-tooling, T-tooling and L-tooling became popular pinning systems for exposure. Today, most manufacturers with the financial resources to do so align images for exposure using some kind of automatic optical alignment means.

Automatic optical aligning came to the printed circuit industry in the form of the post-etch punch (and the automatic artwork punch) in 1983. The post-etch punch automatically positioned a panel with respect to a four-slot punch using closed circuit video technology, measured the relationship between two targets on the panel and then punched the panel very accurately. Post-etch punching works as a multilayer registration process because it holds the panels flat and mimics the lamination process before punching the panels. During the process of punching, the panel is held in position and has a punch inserted in much the same way as a pin will be inserted later for the lamination process. The process is successful because it reproduces the very act of lamination. Every panel was inspected, measured, aligned and punched with speed, precision, accuracy and careful handling. This leap in technology yielded the following benefits:

• The process freed the registration process from reliance on the eye-hand coordination of workers.
• It allowed the creation of process data on the size of images, up to punching process and through all the wet processes. For the first time one could know how every single panel had reacted to the production processes preceding punching.
• All process tolerances were automatically split to the middle of the panel and the four slots preserved this split into the lamination process.
• Accuracies of inner layer registration to one another improved by one order of magnitude. Previously, layer-to-layer accuracies of 2, 3 and 4 mils (up to 100 microns) were possible; today, punches are specified for layer-to-layer accuracies of 10 microns.
• The exposure process became quicker as the layers needed only to be aligned top-to-bottom and no more pinning of layers was required for exposure. The introduction of optically aligned artworks for top-to-bottom alignment made the inner-layer exposure process faster, more reliable, cleaner and cheaper.
• Inner layer productivity soared. Today, there are factories that are capable of producing up to 50,000 highly accurate inner layer panels per day using automatic exposure and post etch punching!

Most PCB professionals today agree that registration of multilayers is best done by post-etch punching and pin lamination. Regarding pin lamination (pin-lam), most people understand a process that involves the use of lamination tools (press-plates, caul plates or lamination plates) with holes (most often contained in bushings) that are either round, oval, slotted or slot-shaped (flatted round) separator plates that also contain similar holes and pins held in place by the lamination plates and are used to align the various layers of the to-be-laminated panels. The most popular pin-lam system is the four-slot “multiline” system which guarantees a good center-zero lamination of the panels. The benefits of four-slot pin-lam are well known, but a couple of disadvantages keep the industry on the lookout for a possible alternative. The disadvantages can be summarized in the following two statements:

1. Pin-lam (and particularly four-slot pin-lam!) is a process which allows prepreg epoxy to migrate up, down and out of  the pin-holes to cause possible contamination to the outer-layer copper surface, which can then cause imaging defects during outer-layer imaging. In any case, the prepreg flow causes the necessity of expensive and difficult maintenance of the plates and process.
2. Four-slot pin-lam uses more panel area than some manufacturers would like to use because of its requirement of having a slot in the middle of each edge of the panel.

An Introduction to State-of-the-Art Multilayer Registration and Lamination

While most people concede that the best registration is achieved using post-etch punching and four-slot pin-lam, mass-lamination (mass-lam) techniques have become popular for manufacturers of multilayer PCBs who are not seeking the most accurate of layer-to-layer registration systems because of the relatively economical panel utilization and the relatively pristine outer-layer surface offered by mass-lam.

Accepting that there are advantages and disadvantages in each of the two main methods of aligning inner layers into multilayers, the difference is basic. Pin-lam requires tooling pins (round or slot) during the lamination process, whereas, in mass-lam, the pins are not present during lamination because the layers were registered and brought into alignment previously, and maintain alignment through some kind of attachment means.

1. Pin-lam: Tooling pins are present during lamination.
2. Mass-lam: The registered multilayer is laminated with no pins during lamination.

Our goal now is to describe the different mass-lam methods presently used in the industry in order to compare them and show the advantages and disadvantages of each. In this way we will be able to offer a roadmap to the best performance for today’s high-tech materials and high-density PCBs.

Mass-Lam Registration Systems

In most mass-lam methods, the inner layers of a multilayer PCB are aligned using a high precision template with tooling pins as in the pin-lam method. The templates can use two or three round pins, as well three- or four-slot pins. If the inner layers were properly image scaled, drilled or punched with an accurate image compensation system, the registration template tooling is highly accurate, and the operator is careful during pinning of the inner layers, then it is possible to achieve, before lamination, tight alignment tolerances (within a range of 15µm) from one layer to another, as shown in Figure 1.

Figure 1: Displacement of L3 to L17 in a 20-layer PCB.

Let’s consider, for the purpose of this system comparison, this highly accurate alignment result of around 15µm from layer-to-layer as a constant given. Our consideration of various mass-lam methods will then focus on how the mass-lam attachment means hold the registration through lamination that was achieved at the beginning of the alignment process. We will consider three popular and usable mass-lam systems:

1. Riveting (metal or plastic).
2. Thermal bonding.
3. InduBond®.

Riveting Mass-Lam

This mechanical system is the oldest mass-lam system we will consider. After many years it is still used in numerous companies in spite of its inherent limitations. The most popular rivets are made of brass but they can also be made of high-temperature plastic. Basically, the process of riveting begins by aligning the inner layers to one another on a tooling template. Then the layers are drilled as a package (generally a total of four holes or more) along both long edges of the panel, which most often corresponds to the warp direction, including the prepreg insulation layers (Figure 2, a). The rivets are then inserted into the drilled holes and compressed using a high-pressure riveting tool (Figure 2, b). The uncontrolled mechanical expansion of the rivets caused by the force of the riveting tool produces internal stresses and movements. The result is misalignment of the individual inner layers one to another within the stack, especially when the inner layers are thin (Figure 2, c).

Figure 2: Riveting process and results (a, b, c).

Thermal Bonding Mass-Lam

This bonding system principle relies on the prepreg layers bonding to the inner layers by means of a resin polymerization effect. To polymerize the prepreg resin, the thermal bonding system uses high-temperature electric heaters similar to soldering irons, which transfer heat from the outside to the cores of the stack. The heaters are elevated to a temperature of approximately 300ºC (572ºF). Between three and eight bonding points are typically used on each long edge of the panel (Figure 3, a).

An inherent drawback with the design is the inefficient heat transfer through the substrate material to the internal layers in order to initiate polymerization of the prepreg. Due to space restraints, the bonding heads must be as small as possible (as small as 4mm diameter). The system needs to apply high pressure to overcome the thermal insulation properties of the laminates. This excessive bonding pressure causes deformation around the bonding area (Figure 3, b).

At the start of the bonding cycle, the outer surfaces of the bonding points are subject to extreme temperatures (of around 300ºC) while the center of the multilayer remains relatively cold. To assure good polymerization it is required to compromise in order to come to a balance among cycle time, pressure and temperature settings. Obviously, this attempt at balancing of parameters becomes much more critical in high-layer count PCBs (>10L). Two possible negative scenarios can result:

1. Once the center of the thickness achieves the right polymerization temperature, the outer layers (top and bottom surfaces) in touch with the bonding heads can become too hot and possibly burn, causing poor adhesion as a result of carbonization of the epoxy resin.
2. Conversely, the surfaces in contact with the heads are at the right temperature while the center of the thickness is too cool, resulting in poor polymerization of the prepreg resin.

The combination of high temperature and compression applied by the small surface area of the heaters can create a crater/volcano effect, causing resin migration away from the bonding points (Figure 3: c, d).

Figure 3: Thermal bonding process and results (a, b, c, d).

InduBond® System

The InduBond is a registered trademark of a patented technology (US7009157 B2) that utilizes inductive bonding heads that heat up etched copper patterns (heater circuits) on both sides of each layer of the multilayer assembly stack. These etched copper pads are heated by means of a high-frequency magnetic field that passes across the panel thickness producing high-energy eddy currents. Each copper pattern is penetrated by the same magnitude of the magnetic field. The result is that all copper patterns on each of the layers are heated with the same level of energy and polymerization of the resin is uniform throughout the stack. No matter the thickness, the heat is uniformly generated throughout the panel build-up (Figure 4).

Figure 4:  InduBond head draw, function principle.

The core of the inductive bonding head is made with a specially selected magnetic material (ferrite). The generated high-frequency magnetic field only creates heat through induction of magnetically excitable materials lying perpendicular to the magnetic field, so the inductive bonding heads remain cold at ambient temperature of the environment (Figure 5, a, b).

This technology produces uniform temperature in all thicknesses because each copper pattern heats the resin of every prepreg above and below the inner layers to cure the prepreg in the area of the heater circuit (Figure 5, c). The result is that the inductive bonding point remains a flat bonding point, maintaining the same thickness as the complete multilayer circuit after pressing.

The created bonding points are robust and flat. They are capable of withstanding the expansion and contraction of the materials during the hot-press cycle. Each bonded point provides maximum assurance to the linear movement of all layers in a multilayer stack. An added benefit is minimal internal tensions (stress) that cause warping or other deformations of the panel during pressing and possible misalignments between inner layers (Figure 5, d).

Figure 5: Inductive bonding (InduBond®) process and results (a, b, c, d).

Basically, all the mass-lam methods attempt to hold the registration of layers achieved in the laying up on a precision template without the requirement to use pins in the press. The goal is to achieve the same registration results achieved by a four-slot pin-lam system without the disadvantages of the pin-lam system. All of the systems discussed here have their pros and cons, but the one that delivers similar registration results to pin-lam while avoiding pin-lam disadvantages is the InduBond® technology (Figure 6). Note the following summarized results of InduBond® processing:

• Tight alignment tolerances similar to pin-lam as a result of using the same tooling for pinning.
• Good registration and flat panel during lamination process similar to pin-lam.
• Robust, inductive bonding points that hold the registration during handling and pressing, similar to pin-lam.
• Robust, inductive bonding points that let the different mechanical expansion of each layer in their natural axis as pin-lam.

Figure 6: Layer-to-layer alignment on a random sample of 28L PCB on InduBond® process.

Table 1: Mass-lam systems comparison of pros and cons. (CLICK HERE)

CCD Camera Alignment (Pinless Mass-Lam)

The pinless mass-lam concept is a very attractive system for manufacturers of PCBs who wish to implement a mass-lam concept because it eliminates the need for layers to be punched or drilled for alignment purposes. The process aligns every inner layer by means of the image, using CCD cameras to measure, compensate any error and align the layer. Once the system aligns all the layers to one another, a system to mechanically maintain the alignment is still required. It is possible to use any of the mass-lam attachment methods to maintain alignment to and through the lamination press. An important consideration is whether the pinless mass-lam process truly delivers pin-lam-like registration results with all the advantages of mass-lam.

The existing pinless mass-lam technology is based on a bottom vacuum plate that holds the first inner layer (L1) of the stack. A 2- or 4-CCD camera system measure the fiducials’ distance position of this first layer and maintains this positional relationship in memory. The first layer (L1) is the reference layer for the rest. The successive layers (L1+n) are then placed on the alignment unit where it is picked up by a top vacuum plate, which is typically smaller than the inner layer. The CCD camera alignment system measures the fiducial targets of the new layer (e.g., L2) and compares the relative position of targets to the memorized data from the reference fiducial positions of L1. The unit calculates the best fit position that guaranties the best alignment between L1 and L2 and the motorized top plate moves according to the calculated results. Once the top plate achieves the best-fit position, the top plate moves the layer down into contact with L1, maintaining a holding pressure while a side mechanical clamping system clamps (L2) against the reference layer (L1). The process repeats for each inner layer to be added (Figure 7).

Figure 7: Pinless functioning block diagram.

In theory, the concept seems wonderful but it is well known how fragile and mechanically unstable the very thin cores of high-layer count multilayers are. It is extremely important to assure that the thin cores remain flat while being measured, aligned and registered to the stack using rivets, heat or inductive bonding. The lack of flat handling of the layers, combined with the repeated mechanical disruptions of clamping and unclamping the layers, disturb the registration process. Errors and additional tolerances can be attributed as follows:

1. The equipment is measuring the targets while the new inner layer is not completely flat. The thinner the inner layer, the bigger the error of measuring can be (Figure 8, a).
2. Once the system considers that the alignment between the reference layer and the new one is good, a lateral clamping system moves in to clamp both long edges of the panel in order to fix the position of the layer with respect to the already aligned layers. During the clamping process, the thin cores are not pressed completely flat. This lack of flatness, along with the movement of the clamps in and out, produce uncontrollable and random small movements of the cores (Figures 8, b,c).
3. This operation of clamping and unclamping the stack is being done with the addition of every new layer. It is easy to understand that thin cores without enough internal rigidity, working under these conditions, will succumb to random movements while the wish of the industry is to have an accurate and, moreover, repeatable results to achieve a reliable and controllable process.
4. This sequential laying up, waiting for alignment, unclamping and clamping needs time to proceed through all the routines and steps. Productivity of such a system is low when compared to punching and drilling layers and pinning them to a template in a typical mass-lam process.

Figures 8: Pinless functioning, clamping sequence (a, b, c).

As per our tests and analyses we have made in cooperation with several PCB companies, we conclude that the pinless systems of today, in addition to low productivity, can only achieve alignments of layer-to-layer in the range of 60µm when the equipment is handling thin cores.

Victor Lazaro is the inventor of the inductive bonding technology (InduBond®) applied to the inner layer registration process as well as the procedure for bonding by induction the constituent layers of a multilayer PCB. He has worked at Chemplate Materials, S.L. since 2001. Contact Lazaro at v.lazaro@chemplate.com.

Paul R. Waldner has spent most of his professional life in Germany and considers himself a proud citizen of the world of printed circuits, which he entered via a summer job as a circuit designer in 1974. Later responsibilities included the development of several medical devices and the development of processes and equipment used in the production of advanced printed circuits. Waldner is the owner and managing director of Multiline International Europe and may be reached at Paul.Waldner@mie.de.