Why MuoN SFPs feel hot, but still are the coolest
product aroundby Renaud Lavoie and Yannick ValléeIntroduction
The goal of this white paper is to provide a basic understanding of thermal analysis to broadcast equipment users and to give some insights
into the thermal specifications of semiconductor packages. To this end, this paper first presents the basic principles of thermal systems
on a high level. Then it explains why you may believe a product is too hot when touching the case. This is a comment that has come up
with MuoN SFPs in operation, as they do feel hot indeed. But, while MuoN SFPs do radiate a considerable amount of heat, they are actuallyrunning at a very low temperature.

1.

Definitions

1.1 TJ : Junction TemperatureThis is the temperature of the semiconductor ‘silicon die’ and it is often assumed that the die temperature is homogenous across thesemiconductor surface. CMOS processes can usually reach a maximum junction temperature of 175°C. This maximum junction temperature
is not be exceeded, otherwise the silicon die can be damaged or life expectancy can be greatly affected.

1.2 TC and TA: Package (or Case) Temperature and Ambient Temperature
The TC (package temperature) is measured at the interface between the package and the heat sink. For an optimized engineering design,
it should be the hottest spot on the package surface. Note: for SFP+/SFP28s such as the MuoN SFPs, TC represents the case temperature,
because SFP+/SFP28s go without any molded package (please see figure 3). TA (ambient temperature) refers to the air temperature ofenvironment where equipment is used.

1.3 PD: Power Dissipation
Power dissipation is the source of heat flux and is measured in Watts. As you can imagine, the higher the power dissipation, the higher theheat expressed in Watts. Current processing semiconductors, such as CPU, FPGA or GPU can easily dissipate more than 100 Watts of powerin an area of just a few cm2!

1.4 RΘJC : Junction to Case Thermal ResistanceThis is one of the most important factors to ensure that the silicon die is running correctly. If the resistance between the junction and thetop of the package (or the case for an SFP+/SFP28) is too high, the heat will not be dissipated correctly, the component will overheat, andperformance will be affected. The other important parameter is RΘJA.

As you already know, if the RΘJC of a device is too high, the heat dissipation will not be efficient. By design, we want the RΘJA minimized,
meaning that the resistance between the junction to the ambient is low and heat is flowing easily from the junction (silicon) to the ambient(air). Usually, the heat sink represents the biggest impact on this resistance1. In general, higher heat sink designs are preferable, as they canprovide more air contact. In the case of the MuoN SFP, the heat sink is installed on top of the SFP+/SFP28 cage.

TJ = TA + (RΘJA × consumed power)

Figure 1. SFP+/SFP28 heat sink____________________
1

There is a significant amount of literature on heat sinks (forced air or convection), and a detailed account is not within the scope of this paper.

Riedel Communications GmbH & Co. KG | Uellendahler Str. 353 | 42109 Wuppertal | Germany

2021-01 | Subject to change without notice. Errors and omissions excepted.

1.5 RΘJA : Junction to Ambient Thermal Resistancewww.riedel.net

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2. Let’s do some math: TJ with different kinds of packages
Now, let’s see how these parameters respond to a silicon semiconductor in a heat spreader-equipped flip chip package – one of the best
and most common packages – and in our MuoN SFPs.

2.1 Thermal
The Flip Chip
Package (FPGA, CPU, GPU)
spreaderThe flip chip package is a package consisting of a silicon die with something called “C4 bumps” connected to a substrate. This is the first levelof connection. The substrate is then connected to the mainboard (just like a modular processing card installed inside a frame) or the PCB ofa 1RU ‘pizza box’. This is the second level interconnect. To ensure that the die junction is as close as possible to the ambient, heat spreaders
are used, or the bare silicon die is attached directly to the heat sink. When a semiconductor die draws a significant amount of power, thistype of package is recommended.

0 CA
0 JA

Heat Spreader

TJTC

0 JC

Silicon Die

1st Level Interconnect

Substrate
2nd Level Interconnect
Figure 2. Flip chip package thermal specifications
As a reference, a flip chip package (FC-PBGA in literature) has a RΘJA of between 12-17 °C/W. Our reference here is a flip chip on substrate
without exposed die or spreader, and without airflow. Please note that this figure can change based on the manufacturer. It represents a
23mmx23mm package, which corresponds to the MuoN package top area. The FC-PGBA with heat spreader has an impressive RΘJC of 0.2°C/W, therefore we consider this package as one of the best.
FC-PBGA TJ without heat sink

TJ = 25°C + ( 12 °C / W  × 8W )TJ =121°C
Which is close to the maximum limit of the TJ.

FC-PBGA TJ with heat sink + air flow

TJ = 25°C + (6.9 °C / W  × 8W )TJ =80.2°C
Which is far from the maximum limit of the TJ.

To improve the thermal transfer, a heat spreader can be added, which results in a RΘJC of 0.2 °C/W instead of 3.9°C/W. If we add heatspreader, heat sink and air flow, then:
FC-PBGA TJ with heat spreader +heat sink + air flow

TJ = 25°C + (3.2 °C / W × 8W )TJ =50.6°C

Which is even further from the maximum limit of the TJ.

____________________
2We used the same heat sink specs for the MuoN heat sink inside a VirtU 32.

Riedel Communications GmbH & Co. KG | Uellendahler Str. 353 | 42109 Wuppertal | Germany

2021-01 | Subject to change without notice. Errors and omissions excepted.

If we add heat sink2 and air flow, then:www.riedel.net

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2.2 The MuoN Package
The MuoN package is a custom SFP+/SFP28 package designed to support HD-BNC connectors. Reflecting the evolving needs of IP toIP functionalities, the new MuoN SFPs are also offered without connectors. To protect Riedel’s intellectual proprerty and competitive
advantages in terms of this crucial thermal management design, we will not disclose the information in this white paper, but we will provide
the specifications of the package.

ΘCAMuoN Body

TJ

ΘJATC

ΘJC

Riedel Proprietary Components
Printed Circuit BoardFigure 3. MuoN package thermal specifications
Let’s compare the RΘJC of our MuoN (0.5°C/W) with the measurement we did on the FC-PBGA with heat spreader (0.2 °C/W). Assuming atheorical 8W load for both, we get a difference of 2.4 °C, which is almost neglectable.
The MuoN package has delta of 0.3 °C/W on RΘJA compared to a FC-PBGA with a heat sink with the same area as that of an SFP283. Thistranslates to a 6 °C/W in VirtU 48-S and a 3.5 °C/W in VirtU 32. This means the dual UHD JPEG-XS to ST2110 code (8W) would create adifference of 48°C between the junction and the ambient in a VirtU 48-S or 25GE switch and 28°C between the junction and the ambient in a
VirtU 32.
VirtU 48-S

TJ = 25°C + ( 6 °C / W × 8W )TJ =73°C
Which is far from the maximum limit of the TJ and better than the FC-PBGA without heat spreader.

VirtU 32

TJ = 25°C + (3.5 °C / W × 8W )TJ =53°C

So we can establish that the thermal performance of the MuoN package is comparable to that of an FC-PBGA package.
The difference between the junction temperature in an FC-PBGA and a MuoN running with the same load is about 2.4 °C.
Please note that the MuoN thermal transfer and the RΘJC figures contain all semiconductors (other semiconductors, memories, orpower supplies) inside the MuoN. It is impossible to accurately calculate the added heat generated by individual MuoN components, but
we can estimate that without other sources of heat, the MuoN die RΘJC will be even closer to the die RΘJC of the FC-PBGA with heatsink.

____________________
3

Because the average 1W SFP+ cage without heat sink is not efficient enough to manage the thermal dissipation of SFP28s, SFP28 cages feature heat sinks as shown in Figure 1.

Riedel Communications GmbH & Co. KG | Uellendahler Str. 353 | 42109 Wuppertal | Germany

2021-01 | Subject to change without notice. Errors and omissions excepted.

Which is far from the maximum limit of the TJ and on par with the FC-PBGA with heat spreader.

www.riedel.net

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3. CONCLUSION
Our study of different heat dissipation and package methods (FC-PGBA and MuoN) demonstrated that MuoN’s thermal specifications are
pretty much on par with the most advanced package on the market. MuoN’s thermal design might create the impression that the device
is running hot or overheating, but, as this white paper aimed to demonstrate, this is far from the truth:
With a Junction Temperature (TJ) of 53°C, the temperature of MuoN’s silicon die is nearly the same as that of the best FC-PBGA on
the market (50.6°C)
So why is it that measuring the temperature of the MuoN SFP head may lead to the assumption that the MuoN is running hot, whenin fact, it is operating at a very low temperature? In a nutshell, the main reason is that the Riedel design team perfectly optimized thedevice’s thermal resistance, creating an extremely low resistance between the junction and the ambient. Therefore, the heat is flowing
easily from the silicon to the outer body of the MuoN.
When comparing MuoN’s heat generation to that of other CMOS processors, you should keep in mind that with MuoN, you are almost
holding the device’s silicon die in your bare hand! This would be comparable to directly touching the silicon chip installed on normal
modular cards.
To take a simple, real-world example, consider a powerful motorcycle and an equally powerful car. You know that both vehicles generate
extreme heat within their engine blocks to create propulsion. But, while it would be no problem at all to touch the hood of the car, you
would deeply regret getting too close to the actual engine of the motorcycle. But does that mean that the motorcycle engine itself ishotter? No!
And this is just the same with MuoN. Here, just like with an open motorcycle engine, the hottest part of the construction is practicallyexposed. While Muon’s core operates at a lower temperature that comparable processors, the heat it generates is directly transported
outwards, without heat spreaders or additional protective layers. So you can rest assured that your MuoNs are not overheating.

vs.

Riedel Communications GmbH & Co. KG | Uellendahler Str. 353 | 42109 Wuppertal | Germany

2021-01 | Subject to change without notice. Errors and omissions excepted.

Because MuoN SFPs feel hot, but are, in fact, the coolest product around.

www.riedel.net