Cara Mendesain PCB Multilayer

Saat mendesain PCB multilayer , salah satu hal penting adalah merencanakan tumpukan PCB multilayer untuk mencapai performa terbaik produk. Substrat yang dirancang dengan buruk dan pemilihan material yang tidak tepat akan mengurangi kinerja kelistrikan transmisi sinyal, meningkatkan emisi dan crosstalk, dan juga membuat produk lebih rentan terhadap derau eksternal. Masalah ini dapat menyebabkan pengoperasian terputus-putus, karena gangguan waktu dan gangguan akan sangat mengurangi kinerja produk dan keandalan jangka panjang.

Multilayer PCB stackup

In contrast, a properly constructed PCB circuit board substrate can effectively reduce electromagnetic radiation, crosstalk, and improve signal integrity, thereby providing a low-inductance distribution network. Moreover, from a manufacturing point of view, the manufacturability of the product can also be improved.

The plane in the multilayer PCB can significantly reduce the radiated emissions on the two-layer PCB . According to experience, the radiation of the four-layer board will be reduced by 15 dB compared to the double-sided PCB board.

When choosing a multilayer PCB stack, we should consider the following factors:

• The signal layer should always be adjacent to the plane. This limits the number of signal layers embedded between planes to two and the top and bottom (outer) layers to one signal.
• The signal layer should be tightly coupled (<10 MIL) to its adjacent plane.
• The power plane (and ground) can be used for the signal return path.
• Determine the return path of the signal (which plane will be used).
• Fast rise time signals use the path of least inductance, usually the closest plane.
• Cost (the most important design parameter).

1. Solder mask- impact on impedance

Since printed circuit boards are usually covered with solder masks, the influence impedance of the conformal coating should be considered when calculating. Typically, the solder mask will reduce the impedance by 2 to 3 ohms on the thin trace. As the thickness of the trace increases, the effect of the solder mask is less.

2. Effect of solder mask coating

The figure above illustrates the effect of solder mask coating on microstrip impedance. This example is a commonly used liquid photoimageable solder mask with a thickness of 0.5 MIL and a dielectric constant of 3.3.

The solder mask reduces the characteristic impedance of the microstrip by 2 ohms. The differential impedance is 3.5 ohms. Therefore, if you do not consider solder masks, the calculation may exceed 3% to 4%.

3. Dielectric material

The most popular dielectric material is FR4, which can be a core material or prepreg (prepreg) material.

The core material is a thin dielectric (cured glass fiber epoxy resin), and copper foil is bonded to both sides. For example: Isola’s FR406 material-including 5, 8, 9.5, 14, 18, 21, 28, 35, 39, 47, 59 and 93 MIL cores. The thickness of copper is usually ½ to 2 ounces (17 to 70 microns).

The prepreg material is a glass fiber sheet impregnated with uncured epoxy resin, which hardens when heated and pressed during the pembuatan PCB process. Isola’s FR406 materials-including 1.7, 2.3, 3.9 and 7.1 MIL prepregs, can be combined to achieve the desired prepreg thickness.

The most common stack is called the “foil method” which is a prepreg with copper foil-bonded to the outside on the outermost layers (top and bottom) and then alternated with the prepreg throughout the substrate. The other overlay is called the “encryption method”, which is the opposite of the foil method and is used by old-school military contractors.

Let’s take a look at the most common multi-layer configuration.

4-layer PCB stackup

A typical four-layer board stackup is shown below. Calculate substrate characteristics and differential impedance.

It is common to see four-layer boards stacked evenly. That is, the four evenly spaced layers, with the plane in the center. Although this does make the board symmetrical, it does not help EMC.

In addition, another common mistake is to make the plane tightly coupled to the large dielectric layer and plane between the center and the signal. This will definitely produce good inter-plane capacitance, but it also does not help signal integrity, crosstalk or EMC-this is why we chose to use a 4-layer PCB on two layers.

To improve the EMC performance of capacitors, it is best to place the signal layer as close as possible to the plane (<10 MIL), and use a large core (~40 MIL) between the power and ground planes to maintain the overall substrate thickness ~62 MIL. Close tracking of planar coupling will reduce crosstalk between traces and allow us to keep the impedance at an acceptable value.

A good impedance range (Zo) is 50 to 60 ohms. Keep in mind that lower impedance will increase dI/dt and significantly increase the current drawn (not good for PDN), higher impedance will emit more EMI, and make the design more susceptible to interference from outside influences.

6-layer PCB stackup

The six-layer board is basically a four-layer board with two additional signal layers added between the planes. This greatly improves EMI because it provides two buried layers for high-speed signals and two surface layers for routing low-speed signals.

The plate thickness (62 MIL) consists of a thicker central core. There is always a trade-off between trace impedance, trace width, and prepreg/core thickness, and it is best to use Rayming recommendations to provide a quick “what-if” analysis. Rayming Stackup Planner calculates characteristic impedance plus edge-coupled and broad-side coupled differential impedance. The latter applies only to embedded dual stripline layers. Differential pairs are becoming more and more common in high-speed designs, using differential mode signals to reduce noise.

8-layer PCB stackup

Untuk meningkatkan kinerja EMC, tambahkan dua bidang lagi di tumpukan enam lapis. Direkomendasikan untuk tidak memiliki lebih dari dua lapisan sinyal yang berdekatan di antara bidang, karena ini akan menciptakan diskontinuitas impedansi (perbedaan impedansi lapisan sinyal ~ 20 ohm) dan meningkatkan crosstalk antara lapisan sinyal ini.

Dalam kasus berikut, dua lapisan planar ditambahkan ke tengah substrat. Hal ini memungkinkan kopling erat antara bidang tengah dan mengisolasi setiap bidang sinyal, sehingga mengurangi kopling dan dengan demikian meningkatkan crosstalk secara signifikan. Konfigurasi ini umumnya digunakan untuk sinyal berkecepatan tinggi dalam desain DDR2 dan DDR3, di mana crosstalk karena kabel yang ketat menjadi masalah.

Penumpukan PCB 10 lapis

Ketika enam lapisan kabel dan empat bidang diperlukan, papan sepuluh lapisan harus digunakan - dan EMC layak mendapat perhatian.

The above demonstrates a typical 10-layer layer overlay. This superposition is ideal because of the tight coupling of the signal and return planes, the shielding of high-speed signal layers, the presence of multiple ground planes, and the tightly coupled power/ground plane pair in the center of the board. High-speed signals are usually routed on signal layers buried between planes (3-4 layers and 7-8 layers in this case). However, care should be taken to properly route these signals. Others, avoid coupling (crosstalk) between adjacent layers.

12-layer PCB stackup

12 layers is the maximum number of layers that can usually be easily manufactured in 62MIL thick plates. Occasionally you will see 14 to 16 layers of circuit boards made of 62MIL thick circuit boards, but the number of manufacturers that manufacture them is limited to those who can produce HDI circuit boards.

High-level counting boards (more than a dozen) require thin dielectrics (usually 5MIL or less on 62MIL thick boards), so they are automatically tightly coupled.

With proper stacking and wiring, they can meet all our high-speed requirements and will have excellent EMC performance and signal integrity. The above twelve layers provide shielding on six inner layers.

Determine the number of layers

The technical rules are based on the minimum pitch of the SMT components used, which is basically the largest trace, clearance and through holes allow, while minimizing PCB manufacturing costs. Complex high-speed designs using ball grid arrays (BGA) usually require 4/4 MIL (trace/gap) and 20/8 MIL (pad/hole) via technologies. However, if required, use a lower size, which will reduce costs and increase manufacturing yield.

Setelah aturan ini dibuat, hitung impedansi karakteristik yang diperlukan (Zo) dan superimposisi yang diperlukan (Zdiff) sesuai dengan lembar data komponen. Umumnya, 50 Ohm Zo dan 100 Ohm Zdiff digunakan. Perlu diingat bahwa impedansi yang lebih rendah akan meningkatkan dI / dt dan secara signifikan meningkatkan arus yang ditarik (tidak menguntungkan bagi PDN), dan impedansi yang lebih tinggi akan menghasilkan lebih banyak EMI dan membuat desain lebih rentan terhadap gangguan eksternal. Oleh karena itu, kisaran Zo yang baik adalah 50-60 ohm.