Low Halogen Flame Retardant Stack-Up Considerations; Low Halogen Background; Choosing A Low Halogen Material; Electrical Limits Of Low Halogen Material Properties - Intel Quark SoC X1000 Design Manual

Stack-Up and PCB Considerations—Intel
Table 1. Platform Stack-up Parameter Values (Microstrip) (Sheet 2 of 2)
Microstrip Units
Trace Width
mils
(Bottom)
Trace Width
mils
(Top)
2.2

Low Halogen Flame Retardant Stack-Up Considerations

2.2.1

Low Halogen Background

FR4 epoxy has been used in the construction of PCBs for decades. Consequently, its
electrical properties, which are influenced by brominated flame retardants integrated
into the molecular structure of the resin, have been studied extensively. As an
environmentally friendly alternative to the halogenated flame retardants in FR4,
several new low halogen (LH) formulations were developed by different material
suppliers. Unfortunately, each new formulation has a unique electrical performance that
differs from FR4. This leads to the current problem: The critical electrical properties of
many LH dielectrics currently on the market make it difficult to design high-speed
buses without increasing the cost of the system. The range of supported Er values
therefore limits the amount of cost added.
The most apparent problem lies with the increased permittivity of the LH dielectric
materials compared to FR4. Measurements show that several LH PCB materials on the
market can have permittivity values around 5 at 1 GHz (using 1080 glass), while FR4
has permittivity values in the 3.6 - 3.9 range. Increased permittivity requires thicker
dielectric layers to achieve the equivalent impedance as FR4. In turn, the thicker
dielectric layers lead to an increase in crosstalk which reduces bus performance. If the
trace-trace spacing and line widths remain constant (consequently board area
consumed remains constant), and the dielectric thickness is adjusted to maintain
constant characteristic impedance, the bus performance is reduced for high permittivity
values due to increased crosstalk.
Conventional means of reducing crosstalk is to isolate traces as much as practical to
reduce the electromagnetic coupling of energy. Unfortunately, modern motherboard
designs are often real-estate constrained, requiring extra layers (and therefore cost) to
provide additional space for crosstalk compensation when using high permittivity
dielectrics.
2.2.2

Choosing a Low Halogen Material

Choosing an LH dielectric material for a specific design requires a compromise between
performance and cost, especially when a single design is required to work with both LH
and FR4 PCB materials. Consequently, it is impossible to define universal requirements
for LH dielectrics that will be adequate for all products on the market. Generally
speaking however, the dielectric materials used to design printed circuit boards (PCBs)
with high-speed digital interfaces perform better with low permittivity and low losses.
Low permittivity tends to reduce crosstalk noise for given impedance and low loss
tangents reduce signal attenuation.
2.2.3

Electrical Limits of Low Halogen Material Properties

Although it is impossible to define electrical limits to universally select "good" materials
versus "bad" materials because each design is unique in its implementation, general
limits can be chosen that will be adequate for most applications. Simulations were
June 2014
Order Number: 330258-002US
®
Quark™ SoC X1000
- Manufac- - Design/
Min
turing Material
Value
Tolerance Tolerance
3.1 0.40
2.1 0.40
+ Design/
Typical
Material
Value
Tolerance
0.5 4.00 0.25
0.5 3.00 0.25
+ Manufac-
Max
turing
Value
Tolerance
0.40 4.65
0.40 3.65
®
Intel Quark™ SoC X1000
PDG
19