Ultra thin HDI flex pcb Design Guidelines

Surveys show that the market for high density interconnection technology (HDI) is growing at a faster pace than conventional PCBs. HDI PCB boards usually have a pitch of less than 8 mils and an aperture of less than 10 mils. The demand for HDI PCBs is driven mostly by computer and mobile devices. Other applications include hard drives, flat panel displays, and chip packaging.

HDI flex pcb boards require smaller plated through-holes and finer lines, so thinner substrates and conductor layers are needed in the production process.

When designing ultra thin HDI flex PCBs, there are a few manufacturing challenges that must be overcome. These challenges include:

● Smaller components that are placed in close proximity to each other
● Tighter workspace area
● Using more components on either side of the board
● Long traceroutes that result in higher signal flight times
● More traceroutes are needed to complete the HDI PCB

However, the right set of routing tools can result in the creation of powerful HDI flex PCBs with extremely high interconnect density.

What’s Unique About HDI flex PCB Design Guidelines?

The HDI Flex PCB production process is very similar to the traditional board manufacturing process. However, there are a few key differences that should be noted. For one, the use of smaller vias, thinner traces, tinier components, and more layers are accommodated. Most manufacturers take advantage of automation for greater reliability.

Material selection also needs to be based on the following questions:
● Will the dielectric meet thermal requirements?
● Will the dielectric survive thermal shock when exposed to multiple reflows and accelerated thermal cycles?
● Will it be compatible with the chemistry of the core substrate material?

The most popular dielectric materials used in ultra thin flex HDI boards are:
● Photosensitive dry film dielectrics
● Polyimide flexible film
● Photosensitive liquid dielectrics
● Heat cured liquid dielectrics
● Heat cured dry films
● RCC foil (reinforced and dual-layered)
● Thermoplastics

Routing density of the HDI flex pcb board depends on part placement, design rules, architecture, and the overall stackup. An important component of HDI PCBs is the use of microvias, which must be placed accurately to facilitate routing between layers.

Design Guidelines for Ultra Thin HDI Flex PCB

There are various steps involved in the flex PCB manufacturing process, however, ultra thin HDI flex PCB manufacturing uses additional steps that are not used with other boards. These steps include:
● Figuring out the layer count to route all signals, usually with the help of a BGA component on the PCB board
● Consulting with the vendor of the PCB board to determine the right stackup (based on dielectric data)
● Determining the vias used to route signaling through the layers of the board (based on the thickness and layer count)
● Verifying that the materials will not erode interconnectivity during the assembly process
● Figure out design rules based on reliability requirements (need for trace widths, clearances, tear drops, and others)

The most important aspects of designing ultra thin HDI flex PCBs are the stackup and design rules because they will determine the routing and reliability of the product.

Design Guidelines for HDI Flex PCBs

The board designer must gather the following data before proceeding with the assembly:
● Annual ring and aspect ratio limits
● Spacing limits and trace widths
● The material utilized in the flexible circuit for controlling impedance in the desired stackup

Types of Vias Used in Ultra Thin HDI Flex PCB for Routing

The vias used in ultra thin HDI flex PCB design have a low aspect ratio of less than 1. Although it is common for some manufacturers to provide reliability claims of aspect ratios higher than 2. At the center of the HDI PCB stackup is a buried via that facilitates connections through the core layer.

Once the designer has determined the dielectric thickness and layer count, they can start designing within the limits of the aspect ratio. It is extremely important to stay within the limits of the aspect ratio to ensure reliability, especially if the ultra thin HDI boards will pass through reflow, or when subjected to continued thermal and mechanical shocks.

Sequential Buildup

The sequential lamination process is especially important for ultra thin HDI boards. This is because of the formation of high density, ultra thin dielectrics in each layer around a thicker core, so lamination needs to take place in multiple steps for building the stackup. This sequential building process comprises of the following steps:

Exposure of Photoresist: Thai process is used to highlight areas that need to be etched in order to leave behind a flex circuit pattern on the laminate.
Etching: The most popular etchant is ferric chloride solution.
Formation of Vias and Drilling: High density via holes need to be defined using laser drilling.
Via Metallization: Next, the high density vias are metalized to form an uninterrupted conductive interconnect.

Forming Vias in Ultra Thin HDI flex PCB

Ultra thin HDI PCBs often require interconnects that lie within the lower limit of the via size. This process can be difficult once the via holes start getting smaller than 6 mils. Filled plated microvias are a common feature on ultra thin HDI PCBs and can be used in via-in-pad designs to increase the overall destroy. It is often easier to use via-in-pads and they also allow more components in the board while providing a direct connection to the internal layer.

Metallization
The four most common metallization processes used in ultra thin HDI PCBs are:
● Conductive graphite
● Electroplating and electroless copper
● Conductive pastes or inks
● Fully additive and semi-additive electroless copper

It is possible to drill larger vias into the flexible PCB, but the process can quickly start getting more expensive. Laser drilling is a more convenient method to form microvia holes, however, it is not the fastest process for via formation. Designers prefer to use chemical etching because it is the fastest method and allows them to etch 8000 to over 12000 vias per second. Photovia formation and plasma via formation also offer similar via formation speeds.

Wrapping Up

In today’s constantly evolving market, it is clear that ultra thin HDI flex PCBs will lead the trend. Ultra thin HDI flex PCB design guidelines are more complicated than conventional rigid PCB processes, but the rules are mostly the same. Reliability mostly boils down to choosing the right materials and the manufacturing process.

Get in touch with Hemeixin PCB for more information on ultra thin HDI flex PCB design.