Challenge For Dynamic Flex: Impedance Control

By Happy Holden, Foxconn Advanced Technology

 

One of the advantages of working for Foxconn is that it is now the worlds largest manufacturer of electronics products.  This brings us into contact with the worlds finest OEMs and their designs.  Understanding those designs provides a wonderful and rich learning experience, it also provides the opportunity for us to innovate on our own to improve those products and also lower their manufacturing coats or raise their yields. 

Mobile phones are a big and growing part of our manufacturing.  Thus the problem of “high-speed signaling” across the ‘dynamic flex’ portion of a mobile phone is an important one for Foxconn.  As shown if Figure 1, four types of dynamic hinges are typically used on mobile phones:

§         Clam-shell type

§         Revolve type

§         Universal joint type

§         Slide type

 

This figure also provides some insight into the copper and PI thickness and usual structures for the flex.

 

 

FIGURE 1: Four types of ‘dynamic flex’ for mobile phones with copper thickness, PI thickness and suggested structures

The difficulty arises when very fast rise-time signals are needed between the main control board and other peripherals like the display.  For static pictures, there is no problem, but when video is involved, then transmission lines need to be employed. Here is only a partial list of signal interfaces flex is used for:

 

 IEEE 1394 (110 Ohm)

 PCI-Express Gen1 (100 Ohm)

 USB2.0 (90 Ohm)

 Video (90 Ohm)

 PCI-Express Gen2 (85 Ohm)

 Memory (75 Ohm)

 

 

 Problem is - - most transmission lines require a ‘opposing’ GND reference and this solid ground plane impedes the dynamic flex and will eventually crack.  So two “modified differential-pair” transmission structures are employed.  Figure 2a & 2b show those 2 structures and 2c shows a new “Broadside-Coupled differential-pair” that Foxconn has developed.

 

FIGURE 2: Three novel differential transmission lines for the ‘dynamic region’ of flex circuits  a. Edge-coupled differential microstrip with mesh GND  b. Co-planar differential microstrip w/o opposing GND   c. Broadside Coupled differential microstrip w/offsets

The three structures in Figure 2 are not the only ones, just some of the most popular and useful.  These three are:

§         Edge-coupled differential microstrip with mesh GND (Figure 2a)

§         Co-planar differential microstrip w/o opposing GND (Figure 2b) 

§         Broadside coupled differential microstrip w/offsets (Figure 2c)

Edge-Coupled Differential Microstrip with MESH GND

The  edge-coupled, differential-pair microstrip with the MESH GND, as seen in Figure 3a, is one of the most popular.  The MESH GND provide a Reference Return Path for signals but it distorts the fields set up by the high-speed signals.  To calculate the resulting impedance requires a 3D Field Solver instead of the normal 2D one used in rigid boards.  In Foxconn case, we use the 3D Solver from Computer Simulation Technology, AG (CST) to calculate trace widths, spacings and mesh parameters.  In Figure 3b, a 1 inch transmission line is modeled and seen in Figure 3c.  In this case, this has an E-field this is “Asymmetrical” that can result due to ‘mis-registration’ but also may be intended.  Mesh parameters can create as much as 5 to 6 ohms difference in the resulting differential impedance.

The effect of introducing a ‘MESH’ to the ground reference plane usually results in the differential impedance being increased.  Currently, the MESH GND is a new one of the high-speed models provided by Polar Instruments, just introduced.

 

FIGURE 3: a. Mesh GND differential microstrip  b. a 1.0 inch simulated transmission line  c. Unbalanced E-field from asymmetrical spacing to Mesh  d. Symmetrical E-field from balanced Mesh

Co-planar Differential Microstrip w/o Opposing GND  

More conventional is the co-planar differential microstrip.  This is seen in  Figure 4a.  When used as the PCI-Express transmission line (Figure 4c), Figure 4b shows the resulting eye-diagram.   A 1.0 inch transmission line simulated is seen in Figure 4d. the symmetrical E-field for this ‘relieved’ co-planar diff. microstrip

 

There is no Polar model for the particular structure that Foxconn uses.  Polar has a 2B1A Model, “Diff Embedded coplanar strips with Ground and a 2B1A, “Diff Embedded Coplanar Waveguide” but Foxconn has the ‘opposing’ Guard Ground intrude to the edge of the trace field, or 3*d.  The difference in impedance between these two extremes is nearly 31 ohms.

 

FIGURE 4: Co-Planar Transmission line without bottom GND   a. Structure  b. eye-diagram for the PCI-Express  transmission line in c.   d. simulation of the symmetrical E-field for this ‘relieved’ co-planar diff. microstrip

Broadside-Coupled Differential Microstrip w/Offsets

In order to avoid too thin of a dielectric and the resulting close spacing of signal traces to the lower ground plane that results in lower trace impedance, Foxconn removed the lower ground plane and routed the P/N signals on the upper and lower plane respectively.  This closely simulates a conventional ‘twisted-pair’ scheme, see Figure 5a. .

This new design scheme allows the changing of the offset distance (d) of the P/N signal lines on different layers  to match the requirement of the target differential impedance (Figure 5b). P/N signals continually exchange the routing layers at a specific distance, which mimics the twisted-cable-like routing.  Based on the proposed broadside-coupled, differential signal design, this extended design scheme is called “twisted differential-pair signals”. 

On the same layer, the current flow direction through Section 1 and Section 2  is inverse and can further suppress EM radiation (Figure 5c).

d (mil)	Zdiff (Ohm)A	Zdiff (Ohm)B
5	105	120.847
4.5	99	116.744
4	92	112.339
3.5	86	107.789
3	80	103.115
2.5	75	98.337
2	69.6	93.298
1.5	65	88.198
1	60.5	82.83
0.5	55	77.564
0	50	72.096

 

 

The only ‘downside’ is the “Via Effect” results in higher resonant frequency and a peak at 165 psec of +14 ohms  Given 100-ps TDR rise time，it can be observed the differential impedance of the twisted differential-pair signal is higher than the conventional broadside coupling one @ 3.5GHz.  Only over some frequency bands is the sdd21 of twisted broadside coupling scheme better.  Compared with differential mode return loss, common mode return loss shows larger reflection, which means more mismatch exists in terms of common mode impedance.  For Far-Field Radiation over 3-meters-Twisted differential-pair signal scheme shows better differential mode radiation than the conventional broadside coupling scheme.  Twisted dps scheme is a little worse than conventional broadside coupling scheme under 1GHz.  Twisted dps scheme is better than conventional broadside coupling scheme above 1 GHz.

 

Table1: Comparison of different offset distance (d) with resulting differential impedances for Polar calculation (A) and 3D Field Solver from CST (B).

 

Polar’s recommendation for simulation is to use a “Broadside-Couples Stripline 3S but change ‘Substrate-3’ dielectric thickness to be “Large (>40 mil)” and to change it’s dielectric constant to air (Dk=1.0).  This removes the ‘outside’ reference grounds from having a field effect and substitutes air for one of the plastics.  The Polar simulation can be seen in Table 1 compared to the 3D Simulation from CST.

 

FIGURE 5: New design with offset broadside-coupled differential transmission lines.  a.  structure  b.  relationship of offset distance (d) and resulting impedance (Zdiff)  c. new design scheme：Change the offset distance (d) of the P/N signal lines on different layers  to match the requirement of the target differential impedance.

 

 

	Conventional Mesh Ground	Twisted Broadside Coupling Diff. Pair
EMI Suppression	Acceptable	Good
Measured TDR Impedance	Bad	Good
Common-mode Noise Suppression	Bad	Good
Routing Feasibility	Acceptable	Good

Table 2: Comparison of different design schemes to fix low impedances in dynamic FPCs

 

The proposed twisted broadside coupling differential pair, which combines the concept of the previously-published broadside coupling differential scheme, not only can precisely control  Transmission Line Impedance but also can further suppress EMI effect. As seen in Table 2, it has a number of advantages  over the conventional MESH GND approach.  This new proposal only adjusts the routing procedures without extra cost, and is therefore considered a cost-effective and novel solution.

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