The Shrinking Size of PCB Circuits: Design Challenges and Solutions

Today, the reduction in the size of basic circuit boards will allow designers to shrink their PCB dimensions by half, or even down to a quarter of the original size. The very fine lines that designers were previously unable to use will now become mainstream, with the former minimum line width of 75 microns (3 mils) gradually shrinking to 30 microns (1.2 mils) or even smaller.

For smaller traces and vias, new design rules are required, as the manufacturing methods for PCB boards are now completely different and more advanced. Microelectronics PCB manufacturers cannot reliably produce traces smaller than 75 microns using standard old dry films, flatbed, and etching processes. Photolithography is one method used to generate these extremely fine lines and spaces. Switching to smaller trace widths may catch many “old-school” PCB manufacturers off guard—those who may not even offer 3 mil traces. To remain competitive in the near future, PCB workshops will need to offer at least 50-micron traces and spaces, or even down to 30 microns.

Microelectronics PCB Manufacturing Groups

Manufacturers of fine micro-circuits fall into four groups. The first group, primarily in Asia, has developed unique fine-line processes for mobile phones or iPods, with traces as small as 40-50 microns. The second group is a small number of R&D companies that produce small quantities of highly specialized circuits on Kapton, with traces under 40 microns, though Kapton is expensive, has long lead times (up to three months), and has a very low yield.

The third group is the rapidly expanding mid-sized PCB companies, which provide smaller production quantities with trace widths ranging from 75 to 40 microns and can produce thousands within a few weeks. Finally, the fourth group is for standard fine-line PCB production, with sizes ranging from 125 to 75 microns, capable of large-scale manufacturing and many participants. We place ourselves in the fifth group. We have developed a new manufacturing technology capable of creating 30-micron traces and spacing on FR4 or Kapton.

With the expansion of the microelectronics business and more PCB companies discovering the technologies required to manufacture traces as fine as 40 microns or below, circuit designers will need to familiarize themselves with new design rules, as well as the pros and cons of micro-circuit manufacturing.

How to Design Micro-PCBs

For obvious reasons, very fine 30-micron traces cannot be made with standard 1-ounce copper. As we reduce trace width, we must also reduce the thickness. At Sierra Circuits, we use 18-micron thick copper to create 25-micron production traces, but this is about the upper limit. Unless your design uses higher currents, finer copper traces should not be a problem. In such cases, specific traces can be made wider to handle the higher current. 30-micron traces are solid and reliable, but they do not face many physical limitations, and these limitations can be almost eliminated by using typical solder mask films.

Fine traces may worry many designers, but they need to realize that the 200-micron wide traces that are now being used are being reduced to 25-13 microns with aluminum or gold bond wires to connect the chips to the chip carriers. These fine traces are encapsulated in multi-layer inner layers or through solder mask, which essentially locks them in place. New methods for attaching copper to the PCB surface have been developed to improve the overall adhesion of micro-traces to the surface.

Some of the initial micro designs had large rounded corners on traces and pads down to 30-micron traces. Over time, it has been found unnecessary. Directly routing the traces to pads is much more robust and reliable. Additional “fish-hook” features have been shown to increase exposure time and costs.

Small Vias: Physical Limitations of Micro Vias

There are physical limits to the size of micro vias. Below 50 microns (2 mils), the plating solution cannot properly plate the hole walls, resulting in poor via quality. Our laser can drill holes as small as 20 microns, but we cannot plate them. The thickness of the laminate controls the minimum diameter of vias, and for plating micro vias, the upper limit is 2:1.

For example, in terms of plating, a 3 mil micro via is limited to a 6 mil thick laminate. Our YAG laser also has a depth limitation when drilling vias. As the diameter shrinks, the ability to penetrate the laminate material to form clean holes also decreases. A 3 mil via has a depth limit of 4 to 5 mils in FR4, while in non-glass laminates used for HDI applications, the depth limit is 6 to 7 mils. Not everything about micro vias is bad. While micro vias may not be as small as traces, we can still add “sweeteners” to the pot because the annular ring around micro vias becomes noticeably smaller.

When we produced our first micro PCB, the first thing we noticed was that vias were centered in the pads. The design used 9 mil pads and 3 mil vias, which was tight by traditional PCB standards. New, more precise laser manufacturing methods will allow pads as small as 5 mils with 3 mil vias, saving a significant amount of board area.

(9 mil pads and 3 mil vias with 40-micron traces)

Using new micro-circuit design technologies instead of conventional PCB techniques can save a lot of space. Today, the best spacing for typical 75-micron wide traces is about 0.5 mm, but with 75-micron traces and 250-micron (10 mil) pads, it’s reduced to 75 microns (3 mils). The spacing between pads is 225 microns (9 mils), and only one 75-micron trace is allowed between pads. For most shops, this minimum specification is difficult to achieve.

Micro PCB Design Guidelines

Using 3 mil vias, 5 mils, and 30-micron traces and spaces, micro-circuit technologies can produce a layout with 0.2mm spacing. Compared to standard 3 mil PCB layouts, micro-circuit technology can reduce the used area by five times. In later articles, we will discuss ideas for reducing component area requirements. But even using the same components, switching to 30-micron traces and smaller pads will significantly reduce board area. When routing, use the same techniques, but try to angle the traces at corners instead of using 90-degree turns. The angled traces at corners distribute stress over a larger area.

1. Micro Vias
   Micro vias can be used to connect thin layers in multi-layer boards, or as HDI technology layers in all HDI multi-layers. Holes with a diameter of 5.9 mils (up to 60 mils) can be drilled, or laser-drilled holes as small as 2-3 mils, but only on 2-4 mil thick HDI laminates. Remember, compared to laser-drilled holes, drill hole drift is significant, which limits the size of pads relative to vias. For drilled holes, use 12 mil pads and 6 mil vias, while for laser-drilled micro vias, 5 mil pads with 3 mil vias can be used.
2. Hole Size
   Although it seems obvious, it’s worth repeating: every element of traditional PCB design needs adjustment for the smaller micro sizes. For PCB layout engineers familiar with traditional designs, this may present a challenge. The most common mistake in this area is using holes that are too large. In reality, micro PCB designs should use laser-drilled micro vias to interconnect layers of the substrate. If the holes in the design are too large (which is often the case), the micro PCB will not perform optimally or even fail.
3. Copper Thickness
   Standard 3 mil fine-line circuits use 1-ounce copper, while micro-circuits use oz for every 30-micron width. Conventional pattern electroplating is used for micro-circuit manufacturing, meaning traces don’t need to be drawn from the circuit to the electroplating bus, but rather the electroplated pattern connects the entire circuit, while the lead bonding electroplating is chemical or electrical.
4. Reliability
   Most conventional PCBs can be used for HDI or micro-circuits, but with limitations. Micro single-sided and double-sided circuits can be made from rigid FR4 laminates, but they need to be thin to accommodate micro vias.
5. Electrical Testing
   Currently, the limit for flying probes or even rigid probes (bed-of-nails) technology is 2-3 mils. We expect this to decrease over time as smaller platforms become necessary. If your micro-circuit has small pads (such as edge connectors), it is advisable to extend the circuit lines outside the pads to 3-4 mil pads.
6. Solder Mask
   Unfortunately, the imaging technology that allows us to manufacture 30-micron traces has not yet been transferred to solder masks. The positional accuracy and image resolution for 75 microns remain the limit.
7. Identification Marks
   Typical silk-screen image accuracy is too large for micro-circuits. Sierra Proto uses very fine inkjet printers, which produce extremely small identification mark resolutions.
8. Safety Markings
   Very small barcodes can be imaged onto solder masks to properly identify PCBs. The barcode is so small that it is almost invisible to the naked eye.
9. Final Finish
   Normal PCBs can use soft gold, immersion tin, or silver for soldering. Most micro-circuits use soft gold, immersion tin, or silver finishes for soldering.
10. Understanding Manufacturer Capabilities
    So far, whether for smaller 3D or conventional circuit boards, manufacturers will use smaller vias and solder masks for micro-circuits. Today, manufacturers should be prepared to meet the new guidelines for micro-circuit production.