High density interconnect represents the cutting edge of the printed circuit board industry today, due to demand for more compact PCBs and stronger signal integrity discipline.
Previous entries in the Becoming a PCB Design Master series have focused on designing for assembly and achieving faster time to market. This entry will hone in on the finer points designers must know in order to master HDI PCBs.
What is HDI?
HDI PCBs are printed circuit boards that feature higher wiring density per surface area than standard PCBs. HDI PCBs possess:
- Lines and spaces of less than or equal to 100 micrometers.
- Vias of less than or equal to 150 micrometers.
- Capture pads of less than or equal to 400 micrometers.
- Capture pad density of more than 20 pads per centimeter square.
HDI PCBs can be classified as one of six types, based on specific construction criteria:
- Type I 1(C) 0 or 1(C) 1
- Type II 1(C) 0 or 1(C) 1
- Type III 2>=(C)>=0
- Type IV >=1(P)>=0
- Type V coreless
- Type VI constructions
Increased interconnection density allows for enhanced signal strength and improved reliability. Additionally, thanks to design efficiency and space maximization, HDI PCBs make it possible to manufacture smaller, more powerful electronic devices.
“Increased interconnection density allows enhanced signal strength.”
Becoming an HDI Master
Higher density means HDI designers must constantly keep layout and spacing in mind, among other factors. The following are four tips on avoiding common HDI PCB design mistakes.
1. Make Space a Priority
Considering space during HDI PCB design means more than squeezing in as many components as possible. First, designers must ensure boards can be properly maintained in the future. Determining the amount of space between specific components and opting for extra room is a requirement. This can also make things simpler during PCB manufacturing. Via diameter, pad diameter and trace width should all be taken into account before implementation begins. Otherwise, PCBs may have to be completely redesigned, requiring further time, money and energy.
2. Understand Implications of Blind and Buried Vias
HDI PCBs incorporate blind and buried vias in order to make the most of limited space. Blind vias connect outer layers to inner layers but do not go through the entire board. Buried vias connect multiple inner layers but do not go through outer layers. While practical and functional, blind and buried vias differ from traditional through holes and greatly influence the overall design of PCBs. Designers must keep the vias in mind as they not only affect how boards need to be constructed, but can impact signal performance based on their location and placement.
3. Avoiding Solder-related Issues
If designers aren’t careful, solder can cause multiple HDI PCB issues. For instance, via-in-pads may siphon solder away from specific components, leading to shoddy connections. One workaround to this issues is avoiding the use of via-in-pads. Unfortunately, depending on the HDI PCB design and what it’s being used for, sometimes this is impossible. In cases like these, covering via-in-pads with soldermask can eliminate the issue. However, using soldermask can lead to its own complications. Soldermask with large openings can allow substantial amounts of solder to reach the board, resulting in tombstoning or or disconnected pins. It’s vital for designers to monitor potential solder issues before designing begins.
4. Incorporate Design Reviews
This applies to all PCB design projects, yet deserves special focus within HDI. It can be difficult for designers to step away from their own work and spot small errors. This becomes doubly hard when designers are working with an even higher density of components. Incorporating advice and perspective from other designers, technicians, and more is a strategic way to fix small mistakes before they become larger, more costly problems. It can be tempting to skip design reviews to meet tight deadlines, but ultimately, such reviews are the ideal way to save time and money.