PCB Tech - High-density circuit board HDI

Obstacles to HDI implementation

There are several possible dilemmas to use HDI technology, so that the use of this technology faces risks. The typical dilemma is as follows.

1 Predictability

Customers need to know the HDI stacking status, the number of holes and the price, which must be known at the beginning of the project and design. Manufacturers often have to make a quotation after the product structure design is completed. There is almost no relevant data to refer to during the previous work, which makes the design and use of customers feel like a blind man. If the concept of HDI plate microwells is unclear, the correct design may not be made, leading to waste. These problems are gradually improving, and when enough experience is accumulated, a certain degree of estimation can be made.

2 Design model

If there is an accurate winding model, the basic component data, geometric relationship and circuit board size can be imported to generate stack structure and design criteria analysis, and then the product performance status can be roughly understood. At present, only a few relatively large-scale manufacturers have the technical ability to simulate the final product of this kind.

Since HDI boards have become more common and computer aids available have gradually matured, if you can learn more about the characteristics of HDI boards, you will have the opportunity to make good designs. New product design requires regular stacking structure, routing channels, and large-area wiring guidelines. Small-area layout may be relatively simple, but the planning of complex products cannot be figured out with simple computer-aided tools.

3 Signal integration

To use HDI structure, one must understand the electrical improvement effect it can bring. Otherwise, designers who are accustomed to traditional circuit boards may still prefer to use through-hole design.

4 Mass production

Most manufacturers that mass produce HDI boards will pay more attention to mobile phones and consumer products. However, to be more widely involved in new products, manufacturers should also pay attention to HDI products that are in small demand.

5 New materials

HDI introduces a lot of new materials that some users are not familiar with, such as: resin-coated copper skin, vacuum lamination of dielectric layers, etc. The characteristics of the substrate are becoming more and more important for the performance of the circuit board, and the low-loss substrate and low dielectric constant are both key. High heat resistance is also a necessary condition for the lead-free process, and new materials require a relatively high material decomposition temperature, which can be measured with a thermogravimetric analyzer (TGA-Thermal gravimetric Analyze), which is a test method specified by ASTM D 3850. In fact, even if the material only has a weight loss of 2-3%, especially in the face of multiple thermal cycles, there may still be a serious decline in reliability.

Other important substrate characteristics include: uniform glass fiber reinforcement is beneficial to laser processing, thin glass fiber is beneficial to electrical characteristics, thin and high dielectric coefficient materials can be configured with more capacitance between the power/ground plane, Adding additional substrates can produce embedded passive component layers and so on.

6 Assembly problems

Many assemblers are not accustomed to the hole-on-pad (VIP) structure, and think that this structure will share the amount of solder joints, but in fact, the amount of solder paste occupied by thin plates and small holes may only be 1 to 3%. Forcing the circuit board design to use full fill-in is sometimes unnecessary, which may increase the production cost of the circuit board by more than 10%. If the dogbone (dogbone) layout is used on the HDI board, it will consume a lot of area and increase the inductance of the circuit (~25nH per inch). The selection of these structures will directly affect the smoothness of assembly and product cost and performance. Figure 9 shows the unfilled and fully-filled cross-sectional structure.

Using holes on the pads, blind holes, and the back of the circuit board, there are no general through-hole test points available, and there is almost no space to accommodate 50mil test pads as test points with high density. The ability to reduce the size of test points and access (Access) is an important task of HDI. In theory, there are many high-density test tools and methods available, but in practice, it may be difficult to match products. DFT-Design For Testing (DFT-Design For Testing) design allows testing engineers and circuit board designers to plan together. They can predict possible failure conditions, plan test strategies, understand the scope of failures, and weigh the planning of test proximity before circuit board layout/winding design.

This aspect is very important for mass production, because it involves comparing product testing costs. Some software can predict the possible failure types of each contact, component, and board signal, so that the test mode with the best coverage can be planned. List the necessary test pads, which can provide the best test coverage and sequence, and the designer can effectively determine the test method based on the limited board surface proximity.

7 Design and cost estimation capabilities require models

To effectively design circuit boards with HDI technology, we must pay attention to many possible stack structure changes, hole structures and design criteria. At present, the industry has developed some estimation methods based on its experience, so that the design work can choose the best stacking method and structure according to the plan.

The minimum aperture, hole circle, circuit width, etc. used in the design have a significant impact on the yield performance, while the material thickness, stack structure, line hole number, hole density, etc. will also have a significant impact on the cost. Other cost factors, such as: final metal surface treatment, blanking, allowable tolerances, etc. will also affect the production cost.

8 Design tool-CAD

Although the development of electronic design automation (EDA) tools for HDI board design is slow, there are already many mature products, and their functions have improved with demand, but the high price is more troublesome for small design companies. Compared with traditional through-hole automation design tools, the important differences and functions are added as follows:

1) Staggered (adjacent) structure, stacked (opposite) and embedded structure with blind microvias

2) Fully stacked layer (Any Layer) and symmetrical layer stacked structure

3) Blind/buried hole gap problem

4) There are holes in the pad (Via-in-pad) structure and the parts can be arranged on it

5) Multiple winding angles

6) Configuration automation of BGA fan-out

7) Dynamic hole position and part line configuration

8) Pushing and displacement of the hole

9) Automatic winding optimization function is required to deal with blind/buried holes

10) Link with electrical, thermal and FPGA simulation tools

11) Design criteria inspection system with HDI structure

12) There is a local area criterion in the component placement area

The typical design of HDI, the complex layout of the BGA string out (Escape) and the improvement of the state of the later entering the wiring channel are the more noticeable parts of this type of issue.

9 Electrical performance and signal integration

Adjacent to the signal, power integration, HDI layout tools, it can support further HDI design, so that the output structure has excellent electrical properties. In the face of advanced ICs that require faster rise-times, the parasitic noise of the carrier that has been neglected in the past must be considered. These parasitic noises include: power/ground plane capacitance, inductance, package capacitance, inductance, and circuit board effects. The capacitance and inductance of the connector, the capacitance and inductance of the backplane or cable, the capacitance and inductance of the connection between the circuit boards, and the capacitance and inductance of the power/ground plane should also be considered.

The electrical influence of the hole in the high-speed network cannot be ignored. The through hole has relatively high capacitance, inductance and other parasitic noise, which may become an obvious interference to the signal performance. Almost all the structures tied around the vias have more than ten times the amount of parasitic noise from the microvias.