Modern electronic products are expected to offer evermore advanced functions, while the products themselves are becoming increasingly smaller. This puts greater demand on the PCB design and the aspects relating to the PCB manufacturing process.
There are two key factors for the successful production of HDI PCBs: first, making the right choices at the design stage, and then carefully choosing the factory that can support the specific technical demands of the project.
Smaller and more complex
Whether it’s consumer electronics, computers, automotive or medical technology, the overall trend is reduction in size. It´s not just a reduction of the actual or finished product size, but also the components themselves are becoming smaller, so the assemblies must be more densely packed and use smaller features.
These more sophisticated electronic products have led to more advanced PCBs becoming more commonplace. The specifications here require high-density interconnect (HDI) solutions with greater number of layers, and more connections both on the surface and inside the PCB, utilizing finer conductor widths and narrower spaces between them. This all leads to a design that is based upon smaller, laser-drilled microvias (blind vias), since normal through-hole vias simply wouldn’t fit into the space available. Therefore, we are seeing manufacturers producing more boards with microvias that also incorporate buried vias. All of which increases the number of interconnections within the board and frees up valuable space on the outer layer for more components to be placed.
The increased number of layers, together with the microvia technology, also requires the use of thinner prepegs and cores than in conventionally manufactured boards which also leads to increased demands upon the factories.
More production stages
Widespread miniaturization is putting far greater demands on the production equipment at the PCB factories. Many of the stages in the production of HDI boards are similar to those used for the manufacture of multilayer boards. However, HDI production calls for considerably more sophisticated equipment to achieve the tiny geometries that are required.
Not only does incorporating several layers of buried vias and/or microvias into the boards require several additional steps, but these also need to be repeated several times and all that increases the degree of complexity and the risk of error. All the geometries are much smaller on HDI boards, which calls for better dedicated equipment designed for high-tech manufacturing. Many factories do have laser drills, and an increasing number of factories possess the appropriate plating equipment and processing experience to enable them to make good-quality, reliable HDI boards. We put a great deal of time and effort into qualifying and verifying a factory before giving it our seal of approval to manufacture HDI boards for our customers.
The first consideration in generating microvias is that of advanced laser drills that can generate blind microvia holes with finished sized of 100 μm or below. The latest generations of these machines are capable of drilling hundreds of holes per second. Following on from that, the transferring of equally critical operation that calls for the highest precision, which the traditional photography-based techniques can’t achieve. Instead, HDI board makers use laser direct imaging (LDI) systems, which images the pattern directly onto the bonded photo-imageable material. This makes for improved quality as no photo tool film is used, thus enabling a much greater accuracy of transfer of pattern features down to 50μm.
Prerequisites: Correct equipment and clean rooms
To ensure the best possible result in the imaging transfer process, it’s vital that it’s performed in special clean rooms with carefully controlled temperature and humidity levels. The clean rooms that have been used for these processes meet the US federal standard 209E Class 10000. This class has constituted the industry standard for many years now and stipulates that the concentration of airborne particles ≥0,5 μm (a human hair is typically 20–50 μm thick) in size should not exceed 10,000 particles per cubic foot.
Today the best factories have clean rooms that meet the Class 1000 requirements. To give another idea of what this means; the air in our normal everyday environment contains 1 million particles, of the same size, per cubic feet. However, good quality clean rooms are expensive, both to buy and to properly maintain.
Producing HDI boards also requires a different type of plating line. For non-HDI boards, one can usually make do with ordinary plating lines, with vertically held panels that use mechanical and air agitation, allowing you to get the plating chemicals to facilitate good copper plating onto the surfaces and into the holes. However, this method isn’t really suitable for HDI boards with blind holes that can measure 100 μm or less in diameter. That’s why most factories use both horizontal plating lines as well as vertical continuous plating (VCP) lines. These methods involve spraying the plating chemicals with high pressure onto the surface, which ensures that the microvias are properly plated.
Positioning the solder mask correctly against the pattern poses a significant challenge, since extreme components, for example 01005 and μBGA circuits with 400 μm or finer pitches, have to provide registration down to 37 μm, or in extreme cases, 25 μm. To achieve this, CCD exposure units are required. PCB makers now have the possibility to use special LDI units to expose the soldermask, as the soldermask manufacturers have developed special soldermask inks, to support HDI designs that require lower energy to polymerize.
Looking «under the hood»
We have to carry out a thorough examination of all aspects of a factory’s production processes and equipment when assessing whether it meets the demands for HDI manufacturing. It’s like looking under the hood and servicing the car before buying it. If a factory claims that it has laser drills, and is therefore capable of producing reliable HDI PCBs, it’s rather like saying that all you need to do to become a new Michelangelo is to get yourself a hammer and chisel. We know that laser drilling equipment is not the start and finish when it comes to HDI production; it’s equally as important to have the right kind of plating equipment and the right chemistry, as well as knowing how to handle, control and verify the full plating process. We also look at what kind of chemicals and methods they’re using, image transfer equipment and procedures and this is in conjunction with understanding the numbers behind the factories’ real experience in this field and their performance, both of which are crucial factors.
Our strategy is to ensure that we maintain and develop a best-in-class and secure factory base. We emphasize the importance of the factory being able to not only manufacture HDI PCBs, but also in keeping the number of production errors to a minimum. Producing HDI boards with 3 layers of microvia each side and 4 layers of buried in the center involves laminating, drilling and plating it four times. If they return a 10% rate of failure during each round in the factory, the number of boards they would end up scrapping would exceed the volumes they deliver. In such a case you need to question the quality of the items that make it through to the delivery stage. When you consider that the components on the board can cost more than 100 times the board itself, it’s imperative you can rely on the quality of the board. Otherwise, it can be incredibly expensive, if you’re forced to scrap the product at a later stage.
Get the design right from the start
Yet another aspect you need to prioritize with advanced boards is the design itself. The margins are tiny regarding such factors as conductor widths, isolation distances between copper features, impedance requirements, hole sizes and their relation to capture and target lands. All this poses a considerable challenge at the layout stage. The design rules should be realistic and adapted to volume production right from the start.
There are several pitfalls when only considering prototype factory design rules: One example may be making the inner layer cores too thin to produce a good capacitive coupling. It might work in a prototype factory, where great care is taken to basically hand process these thin inner layer cores. However, it could lead to major problems when the product is in volume production, since they may have different capabilities and in this instance the thinner cores may easily get stuck during the processing through long, volume-oriented etching lines since they are basically too flimsy. We thus recommend working with a dielectric spacing for a microvia layer of 60-80μm as below this may be problematic and our experience tells us that this design guideline works well across all our higher-technology factory base. A few of the factories we work with are more advanced and they can make even thinner cores if needed.
Cost-effective HDI PCB design
If there’s enough space on the board and the component is available with different pitches, we also recommend selecting a component with a larger pitch since it reduces the complexity of the board and saves costs. Smaller components may be less expensive to purchase or more readily available, but this approach might render the board unnecessarily expensive in relation to its end application.
Opting for small components, usually, increases the complexity of the circuitry, and therefore the board will increase in cost also. This is where the customer should work with their PCB supplier to determine if the design needed for such components is a cost-effective one: Is the reduced cost associated with buying more readily available, but more complex components, balanced with a potentially more expensive PCB? Consider if, for example, it is to be used in mobile phones destined for the consumer market, or in low-volume production. We are also seeing more package-on-package (PoP) type components being used in the industry. You should carefully check whether the assembly house is familiar with the technology and the extra costs it might involve. Of course, smaller components are space saving, which could produce a cheaper board, as long as it doesn’t mean making it more complex, with several levels of microvias or adding buried structures etc. One invariably has to weigh up the space contra complexity issues at the design phase.
PCB producers should be involved right at the very start to help customers find the right solution. One must realize that there are real differences between producing prototypes and volume production. If you focus on the wrong things from the start, it could jeopardize the entire project if you find that your design can’t be applied in volume production. We recommend initiating a seamless project together with your PCB producer at an early stage to ensure that the board can be manufactured at a reasonable cost, with the right level of complexity for the design and for reliable yields. Contact us to discuss your projects – we are happy to help you.
Download our design guidelines for HDI PCBs
To prevent getting it wrong from the start, we have put together our design guidelines for HDI PCBs, to use as a checklist.