Take care of thermal distribution for higher-layer PCBs

This is an extension to an earlier piece on the subject. During my various meetings in Hong Kong, I found Johnny Keung, deputy general manager, Circuitone, as a very good resource for discussing PCB services.

He described that immersion tin was economical, complied with RoHS, could replace immersion gold, and go fine line width. Circuitone offers 4µx4µ line width. As for spacing, it can go down to 3µ spacing.

The board size can be limited by equipment. Circuitone has equipment that handles 24x24inch board sizes. It can also offer 0.003” line width (3µx3µ) for high-density PCBs in large volumes. It offers minimum hole-width of 0.2mm, and plans to offer 0.1mm hole-width by Q4-07. This is indeed significant.

There had been some reports in the trade press regarding some Mainland Chinese PCB fabricators offeing 20- and even 40-layer PCBs.

Keung said there were two benchmarks. One, switching from double-sided to four layers, and two, switching from four layers to six layers.

He pointed out that Circuitone could use technology from six layers up to 20 layers. If it went beyond 20 layers, for example, 22 layers, there may be difficulties with thermal distribution within the board.

As I understand from our discussions, for up to 20 layers or so, heat distribution was on the top layer of the PCB, while distribution across middle layer could be uneven. Layers at the bottom could experience higher heat transfer than those in the middle.

Even PCB pressing is done in two stages: one, increase heat so the bonding sheet started to melt, and two, if temperature kept increasing, the glue was transformed into solid. This was the final curing stage.

Commenting on 40-layer PCBs, Keung commented that those boards at the outer layer would likely start melting, and those at the core layer would be in solidstate. When heat was being transferred into core layer, the evenness of distribution changed. The outer layer would remain in solidstate as well. So, expansion/contraction could get uneven, and registration could be a big challenge.

Fabricators should definitely look into this aspect, before designing higher-layer PCBs. I believe, some research work has been done by PCB makers to develop higher-layer PCBs. We discussed the yield rate earlier. That has to rise.

Adding layers on multilayer PCBs

Multilayer PCBs (ML-PCBs) are used for a whole range of applications, such as broadband routers, RF applications, set-top boxes, backpanels, keyboards and power supplies.

However, what is significant today is the number of layers that a PCB comprises. Some other issues include line width, minimum thickness, minimum hole size, surface finish -- immersion gold/silver/tin, lead-free, flash gold, HAL, OSP, etc.

The variety offered by PCB fabricators is indeed stagggering. Consider Introlines. It offers PCBs with line width of 0.08mm and minimum thickness 0.1mm, and minimum hole size 0.2mm.

PCBs come in surface finishes such as lead-free, flash gold, HAL, OSP, etc. Among immersion gold/silver/tin, gold is said to be more popular.

Another point to be noted is how many of the PCBs are RoHS compliant. We don't yet discuss RoHS extensively in India, but those following the industry know exactly how important RoHS has become. About 70 percent of Introlines' PCBs are RoHS compliant.

Main applications for TC Intn’l's ML-PCBs include audio/video, automotive, home appliances, industrial applications and power supplies. The ability to offer low-mix, high-volume boards provides TC Int'l with a competitive advantage.

TC Int’l can offer ML-PCBs with line spacing of 4mil (0.1mm), line width of 0.1mm, finished hole size of 0.2mm and via diameter of 0.2mm. Plating is offered in Ni/Au, immersion gold and lead-free HAL. Immersion silver finish would soon be offered, in fact, from this year onward.

More layers on the PCB, design changes (higher layer count as against lower layer points), and low-mix/high-volume product lines are said to be the emerging trends.
TC Intn’l handles nickel/gold plating, immersion gold, Entek, V-cut, etc. It uses SMT and fine line 4/4mil line width spacing techniques. Surface finishing is taken care of by HAL, lead-free HAL, selective gold plating, OSP or preflux coating (Entek), etc.

TC Intn’l plans to add immersion silver after listing on the HKSE, which is by 2007. The supplier is also targeting CE, automobile and computer peripheral segments.

Tyson produces ML-PCBs up to 10 layers. It maintains line width of 3µ for ML-PCBs. For developing ML-PCBs above 10 layers, it will use special drilling machines with X-ray feature to target the right hole to drill. Tyson’s ML-PCBs are targeted at applications such as GPS, mobile phones, remote controls and Bluetooth chipsets.

United Talent offers SS/DS, rigid and ML-PCBs ranging from four to 16 layers. It provides surface finishes in HAL, Entek, lead-free HASL, and immersion gold/silver/tin.

Coming to the number of layers, TC Intn'l offers ML-PCBs up to 12 layers. It had done trial runs for 10- and 12-layers. Feedback from customers had been encouraging.

TC Intn’l uses FR-1/FR-4 as board materials. For SS-PCBs, it uses base laminates such as XPC, FR1/2, CEM-1/3, FR4, etc. For DS-PCBs, it uses CEM-3 and FR4. For ML-PCBs, it uses FR4. Over 95 percent of its products are lead free.

TC Intn’l has machines for pressing PCBs for higher layers. Mass lamination is used for up to 12 layers, and pin lamination for above 12 layers. Suppliers can also use pin-lamination machines for 24-layer boards.

TC Intn’l offers 4µx4µ line width and spacing. The need for higher-layer PCBs was tight, especially for military, ATC and communications. Yield rate was not very high, and there was more wastage/scrap rate. The 3µx3µ line width was a niche market.

This point has to be noted, especially by fabricators looking to develop higher layer ML-PCBs as the yield rate is of immense importance.