Cavity PCB manufacturing is an evolving process. The cave PCB manufacturing industry includes quite number of bucket factories, so called because most of their processing is administered in small portable buckets containing etchants, solvents and other mysterious solutions. Using at the most problematic devices and methods, they need come up with a really basic, low-cost, low-complexity cavity PCB. Their business and environmental practices are often questioned.
The good news for circuit card customers is that the barreled workshop may be a thing of the past. Manufacturers that are still viable can repeat all the essential production steps on computer circuit boards, which were considered very complex just a couple of years ago, with little chance of scrap. However, not every vendor has the power to perform high-end processes.
It is important to make sure that the boards to be ordered are handed over to PCB manufacturers which will handle a spread of inauspicious design features at an equivalent time. Otherwise, your order could also be cancelled or appear on your dock thanks to undetectable manufacturing errors, which may complicate assembly, installation, or functionality. the subsequent articles explain a number of the harder processes in order that you’ll determine the proper manufacturer for your order.
Cavity PCB manufacturing-Control impedance
There was a time when many small and medium stores couldn’t support impedance testing, but that day passed. Today, controlled impedance PCB design is very common, so the ability to check impedance results has become an essential requirement for maintaining business. Now, orders can be tested no matter where they are built. If you provide the necessary information at the beginning of the project, your board should attach a report with the results. Expect to pay a small power on charge for each PCB. The additional cost covers the cost of setting up and performing tests that are not performed on the same machine as the machine used to check basic continuity. Although there are no other manufacturing errors, it also covers a potential small increase in boards that fail impedance tests.
Cavity PCB manufacturing-Surround edge plating
Sometimes it is desirable to form a connection by wrapping the coating around the edge of the PCB rather than through the standard through-hole so that the edge can be used as a conductor. The processor performs additional major milling operations immediately after drilling to complete the packing plate. The purpose of this wiring step is to expose the PCB sidewalls so that they can be coated with electroless copper (or equivalent) in the same way and at the same time when applied to drilling.
After the photographic circuit image is applied and developed, the electroless coating provides a conductive surface to which thicker, more durable electrolytic copper can adhere. If the artwork includes mats or other shapes that define the wiring area as plating features, electrolytic copper plating will cover the wiring channels to form the required connections. Although this process adds steps and adds some complexity to the array setup and process planning, it is not difficult to achieve good results.
Cavity PCB manufacturing-Electroplating of tooth edge
In connection with surround coating, tooth edge plating involves coating a series of drilled holes so that the center point falls along the outline of PCB. The holes are then passed through to leave the plated half holes with space between them. Castles are most commonly used to facilitate the installation of pre installed modules on smaller or larger boards. The half holes in the cast plate are usually spaced to align with the surface mounting pads on the larger plate. When the two plates are joined together, the plating in the barrel of each half hole provides an additional mating surface for welding. The assembly solder core absorbs a half hole barrel, making the joint stronger than the basic butt joint.
Cavity PCB manufacturing-Electroplating cavity
The cavity is a concave opening that allows the assembly to be placed on a layer other than the top or bottom – essentially a cut that does not completely pass through the circuit board. Although cavities can be used for complex RF applications and help with thermal management, they are most commonly used to save physical space. The components installed in the cavity can effectively reduce the height of PCBA. The advantage is that the assembled board can now be placed in a thinner shell.
Other more advanced cavities can fully embed the component. After the assembly is installed in the cavity, other materials are laminated over its top to seal it in a multilayer structure. Placing components inside a circuit board frees up space on the outer layer, allowing the placement of other components or circuits, or reducing the footprint area of the circuit board.
Cavity formation requires experience in laminate process and material selection. The more uniform the layer thickness, the easier it is to control the cutting depth. Accurate machining of the cavity is also critical. Considering the inevitable changes in material thickness and CNC machine settings, as well as the roughness of copper surface after mechanical milling, it is usually necessary to replace mechanical milling with more controllable laser ablation steps.
Laser ablation can completely remove the dielectric material, and there is no shock mark, so that the copper surface of the inner layer is clean and smooth enough to install the components reliably. If the layers to be exposed are deep within the PCB structure, the initial CNC milling steps may still be useful. CNC channel reduces the thickness of dielectric material to shorten the laser cycle time. The laser then removes the residual dielectric material to obtain a clean surface. In each step, good results depend on the board manufacturer’s equipment, skills and experience.
Cavity PCB manufacturing-Micropore and sequential lamination
Micro vias are very small holes that are drilled from one layer to the next to form an interconnection, which does not pass through the board from top to bottom like ordinary electroplated vias. Their small size (usually 0.003 inch to 0.004 inch) allows many holes to be placed in very small areas, allowing high density circuit wiring, especially near high pin count components such as ball grid array (BGA). In most cases, they are formed by laser drilling. If the micropores are only connected from the outer layer to the adjacent inner layer, these blind holes can be formed after all lamination is completed. However, for more complex HDI boards, additional laminating steps are required.
Sometimes, micro through holes are connected between different pairs of layers at different locations (for example, some holes in L1-L2 and others in L2-L3), while sometimes they can be drilled at the same location at different times to form “stacked” micropores. An example of the latter is a structure designed to connect L1-L3.
The challenge is that laser microholes cannot penetrate very deep. Because of their small diameter and V-shaped cross-section, the aspect ratio between hole diameter and dielectric thickness should be less than 1:1. Otherwise, the bottom of the hole will not remove all the dielectric material required to expose the copper surface of the adjacent layer, and the hole cannot be reliably electroplated. It is then necessary to stack multiple laser micropores together at the same location to connect multiple pairs of layers, while using the smallest diameter holes in the same location (or using elongated capture pads to bring the holes very close to each other). The most common function of driving such HDI structures is the dense high pin count BGA, in which there is no space for escape wiring on the external layer alone.
Therefore, sequential lamination is needed, in which several layers are laminated together to form a substructure and drill blind holes. Then, they are covered with the next set of layers, laminated, and then blind again. For example, a 10 ply board may have a sublayer consisting of l3-l8. The daughter board is laser drilled from both sides and plated to connect L3-L4 and l8-l7. Then l2-l9 was added, laser drilling was performed to connect L2-L3 and l9-l8, and then plating was performed. Finally, L1 and L10 were added for laser drilling and electroplating. There are now connections from L1-L4 and l10-l7. If necessary, the circuit or solid-state injected copper can now be added in the same position as the via holes on layers 5 and 6 because there is no via structure that takes up space.
In order to perform the steps required for a successful HDI / sequential laminate product, all equipment must be well calibrated and all processes must be strictly controlled because of the high cost of scrap and restart. With the increasing density of circuit boards, it is important for manufacturers to have the right equipment and experience to produce today’s complex multilayer structures.
Cavity PCB manufacturing-Through hole in pad
Most HDI designs require the through hole to be drilled into the actual component pad so that additional circuits and pads do not have to be simply run to place the via. The problem is that the drilled through holes cannot be retained in the pad center on the finished PCB. If the hole remains open, the surface area of the weld is small or not, and the applied solder will only be absorbed through the open through-hole core, resulting in poor welding. The solution is to fill the through hole and then coat it to restore the pad area for SMT welding. This process is sometimes referred to as planarization on clad plates (vippo), and the resulting through holes are called IPC VII vias, also known as “active pads.”.
Before other plated through holes, drill through holes separately to fill only through holes. Through holes are plated and then filled with conductive or non-conductive epoxy resin. The panel is then sent through the planarization device to smooth any areas where the epoxy resin may protrude from the surface, as components such as BGA must be mounted on a flat, uniform surface