Q: What Is Halogen-Free FR-4 Material?

In the previous topics, we have talked about “ How to choose the material of PCB? ”, “ Brief Talk about PCB Materials ”, and “ How to Have PCB Material Ready? ”. Today let’s discuss Halogen-Free FR-4 material. What Is Halogen-Free FR-4 Material? Defined by IPC and IEC, only BR(Bromine) and CL(Chlorine) content is less than 900 PPM, and the total of BR(Bromine) CL(Chlorine) content is less than 1500PPM, can be considered as Halogen-Free FR-4. Meanwhile, all materials used must meet the halogen-free requirements when using Halogen-Free FR-4. Halogen-Free FR-4 vs. Lead-Free FR-4 The grade of Halogen-Free FR-4 is greater than Lead-Free FR-4. Halogen-Free FR-4 basically meets the requirements for lead-free assembly. On the other hand, Lead-Free FR-4 that meets the requirements for lead-free assembly does not necessarily meet the requirements for Halogen-Free. As you might know there’re many types of PCB material. And choosing the suitable material will affect the durability and reliability of end products.

Q: Why are PCBs designed stack-up?

PCB stack-up design is a process of arranging the layer configuration of a multi-layer printed circuit board (PCB). This design considers various factors including electrical performance, signal integrity, EMI (electromagnetic interference), and thermal management. The primary purpose of stack-up design is to maximize the performance and reliability of the PCB. PCB stack-ups are necessary for several reasons: Electrical Performance : In high-speed and high-frequency applications, stack-up design ensures stability and consistency in signal transmission. By controlling the distances and arrangements between signal layers, ground planes, and power planes, signal transmission losses and crosstalk can be reduced, thereby enhancing electrical performance. Signal Integrity : In multi-layer PCBs, couplings and interactions exist between signal layers. Proper stack-up design can mitigate interference between signal layers, improve signal integrity, and reduce signal distortion and jitter. EMI Suppression : Rational PCB stack-up design can reduce electromagnetic radiation and external interference (EMI) of the PCB. Proper spacing between layers and distribution of copper foils between layers can enhance PCB's EMI suppression capability. Thermal Management : In high-power applications, stack-up design can aid heat conduction and dissipation. By adding thermal heat spreaders or copper foils at appropriate locations, PCB heat dissipation can be facilitated, preventing overheating and damage. In summary, stack-up design aims to ensure high performance, reliability, and stability of the PCB while meeting specific requirements of diverse applications. The reasonable stack-up design optimizes PCB's design and performance to the fullest extent.

Question 1

What is a PCB? The nervous system of electronic products.

Accepted Answer

The Printed Circuit Board (PCB) is an indispensable component in modern electronic devices.

It functions as the "nervous system" of electronic products, connecting and supporting all electronic components such as resistors, capacitors, and chips.
The primary function of a PCB is to allow electrical current to flow between various components through conductive pathways (copper traces), thereby enabling different circuit designs.

Common PCB materials include FR-4 (fiberglass fabric) and polyimide, among which FR-4 is the most widely used due to its excellent heat resistance and insulation properties.
From mobile phones and computers to automobiles and medical equipment, PCBs can be found in virtually all electronic devices.

Their stability and reliability have a significant impact on product quality and safety.

Question 2

What Are The Material Factors That Influence High-Speed/Frequency Transmitting?

Accepted Answer

More and more designs of PCB is having high speed/frequency transmitting demands. It seems that this is the trend of electronic products and future. Following this trend, do you know what the material factors that influence the signal transmission are?

1) The flatness of copper – It says that better flatness of copper can help decreasing signal loss of transmission.
2) Dielectric constant (DK) – The main factor of determining how fast of the speed and how less of the signal transmission loss, the lower the better.
3) Dissipation factor (DF) – The main factor of signal loss to the near conductor of the transmitting route, the lower the better.

Five levels of signal transmission loss.

	Standard loss	Middle loss	Low loss	Very low loss	Ultra low loss
DK	3.9 ~ 4.5	3.6 ~ 4.4	3.2 ~ 3.8	3.1 ~ 3.6	2.5 ~ 3.2
DF	≧ 0.02	0.01 ~ 0.02	0.01 ~ 0.006	0.003 ~ 0.006	≦ 0.003

Question 3

Distinction Between Traditional Dry Film and Laser Dry Film

Accepted Answer

In a previous article, we introduced inner layer processing and pretreatment. Today, we will focus on exploring the dry film aspect.

Traditional Dry Film Process

Use a photomask to transfer the design pattern onto the dry film via exposure.
Perform the development process to remove unexposed areas, leaving the desired pattern.
Proceed with etching or electroplating.

Application Scenario: Suitable for mass production of PCBs with moderate specification requirements.

Laser Dry Film (Laser Direct Imaging, LDI) Process

No photomask is required; the laser directly draws the design pattern onto the dry film.
After laser imaging, perform development to remove unexposed areas.
Continue with etching or electroplating.

Application Scenario: Ideal for high-density PCBs and semiconductor package substrates (e.g., BGA, CSP) requiring precise patterns.

Advantages and Future Trends

Advantages of Laser Dry Film:
- Eliminates the labor and material costs associated with photomasks.
- Accurately meets the increasing demands for high-density and high-performance modern electronics.
- Better suited for rapid design iteration and customization needs.
Future Trends:
As LDI equipment costs decrease and the technology becomes more widespread, laser dry film technology will gradually become mainstream, particularly in the production of high-end electronic products and semiconductor substrates.

Question 4

How is the Number of PCB Layers Counted? What is the Difference between Double Sided PCB and Multi-layer PCB?

Accepted Answer

“PCB Layer Count”
The number of PCB layers depends on its copper layers. For instance, if the design contains only one layer of copper then it is a single sided PCB; if it contains two layers of copper then it becomes a double sided PCB. If there’re more than two layers of copper, we call it a Multi-layer PCB. Please refer to the image below.

“Multi-layer PCB Production Process”
How do PCB manufacturers produce Multi-layer PCBs? Take below six layer board for example, PCB manufacturers has to first complete inner layer; then stack up the completed inner layer by placing prepreg in between. Once all layers are in correct position, then laminate all layers into one.

“Double sided PCB vs. Multi-layer PCB”
The difference between double sided PCB and Multi-layer PCB is that double sided PCB has no inner layer and lamination process.

“Single sided PCB vs. Multi-layer PCB”
On the other hand, there’s no inner layer and PTH process in single sided PCB.

Question 5

Can ENIG Replace ENEPIG for Wire Bonding?

Accepted Answer

When it comes to wire bonding, the first choice could be ENEPIG. ENEPIG seems to be the most stable one for wire bonding, however, the production cost is much higher than other surface finish. Here is one solution of using ENIG instead of ENEPIG by increasing the thickness of Au to 5u”.
See below pulling test condition and result of showing performance of using ENEPIG and ENIG.

Test condition:
To use 1.0 mil gold wire and pull strength to be above 3 grams force. (as TM-650 2.4.42.3)
PCB A is as ENEPIG finish with Ni:200u”; Pd:2u”; Au:2u”
PCB B is as ENIG finish with Ni:200u”; Au:5u”

Result:
PCB A: 5.091 ~ 11.186 (g)
PCB B: 4.432 ~ 12.030 (g)

To sum up, both ENEPIG and ENIG are ok for wire bonding.

Question 6

Introduction to CCL

Accepted Answer

CCL is primarily divided into three categories.

The first is high-frequency, the second is high-speed, and the third is standard.
There are three main standards to distinguish this categories:

Df (Dielectric Loss): The loss of electrical signals within the material.
Dk (Dielectric Constant): The delay of electrical signals within the material.
Tg (Glass Transition Temperature): The heat resistance of CCL.

Basic standards are as follows:

Df < 0.005 is high-frequency.
Df between 0.005 and 0.01 is high-speed.
Df > 0.01 is Standard.

The higher the grade of the CCL, the lower the values of Df and Dk, resulting in less signal loss and delay. This means the technical requirements and the added value will be higher, naturally leading to higher profit margins and unit prices. If we want to divide it further, there are a total of six categories. Among them, automotive use is related to safety, so it will be classified as high-frequency CCL with 5G, high-speed CCL is used in servers and switches, and standard CCL is generally used in consumer
electronics.

Major Advantages of High-Frequency CCL

High efficiency
Fast speed
Good control
Strong durability

In the manufacturing process, resin, catalyst, and curing agent are first mixed, then applied to fiberglass cloth. This is bonded with PP and covered with copper foil to form the so-called copper-clad laminate.

In terms of cost proportion, CCL as a whole accounts for about 67% of the overall proportion of PCB, and the cost proportion is relatively high.

Question 7

What Is Halogen-Free FR-4 Material?

Accepted Answer

In the previous topics, we have talked about “How to choose the material of PCB?”, “Brief Talk about PCB Materials”, and “How to Have PCB Material Ready?”. Today let’s discuss Halogen-Free FR-4 material.

What Is Halogen-Free FR-4 Material?
Defined by IPC and IEC, only BR(Bromine) and CL(Chlorine) content is less than 900 PPM, and the total of BR(Bromine) CL(Chlorine) content is less than 1500PPM, can be considered as Halogen-Free FR-4. Meanwhile, all materials used must meet the halogen-free requirements when using Halogen-Free FR-4.

Halogen-Free FR-4 vs. Lead-Free FR-4
The grade of Halogen-Free FR-4 is greater than Lead-Free FR-4. Halogen-Free FR-4 basically meets the requirements for lead-free assembly. On the other hand, Lead-Free FR-4 that meets the requirements for lead-free assembly does not necessarily meet the requirements for Halogen-Free.

As you might know there’re many types of PCB material. And choosing the suitable material will affect the durability and reliability of end products.

Question 8

PTH Killer – Some Common Plating Void Factors

Accepted Answer

PTH Killer – Some Common Plating Void Factors

You may know that the most serious problem of PTH is void at where copper should be plated on. But do you know why this is happening? The common factors could be on material, drilling process, or plating process.

1) Material: The CTE of material is not suitable for the plating process. Compare to leaded process in the old times, the recent lead free process is having higher CTE requirement of material. If the material was still suitable for only leaded process, it might lead to void when it applies to lead free process.

2) Drilling Process: After drilling process there will be remaining dust on the material. When the vacuuming machine doesn’t function well that dust is still in the hole, there might be having high chance of leading to void after plating process.

3) Plating Process: When the board enters the plating liquid, bubbles will occur in the hole. Although the plating machine will vibrate to shake off the bubbles, the bubbles may still exist due to not precise parameter or some other reasons. And the bubbles, of course, will lead to void after plating process just the same idea as drilling dust remaining.

Question 9

PCB Vias with epoxy filling before plating.

Accepted Answer

You may have heard of the PCB plugging process. Why use epoxy resin to block PCB vias? What are its pros and cons?

Improved Thermal Conductivity:
Epoxy filling enhances the thermal conductivity of the via, allowing for better heat dissipation within the PCB. This is particularly beneficial for high-power applications or designs with components that generate significant heat.
Enhanced Signal Integrity:
Epoxy filling helps to reduce signal loss and electromagnetic interference (EMI) by improving the integrity of high-speed signal transmission. It minimizes signal reflections and crosstalk, leading to improved overall signal quality.

Improved Electrical Isolation:
Epoxy filling helps to enhance electrical isolation between adjacent layers of the PCB, reducing the risk of short circuits and ensuring proper isolation between different signal or power domains.

It's worth noting that while epoxy-filled vias offer these benefits, they also introduce some challenges. For example, they can be more expensive to manufacture compared to non-filled vias, and the process may require additional steps and specialized equipment. Additionally, if rework or repair is necessary, epoxy-filled vias can be more difficult to modify. Therefore, the decision to use epoxy-filled vias should consider the specific requirements and constraints of the PCB design and application.

Question 10

Why are PCBs designed stack-up?

Accepted Answer

PCB stack-up design is a process of arranging the layer configuration of a multi-layer printed circuit board (PCB). This design considers various factors including electrical performance, signal integrity, EMI (electromagnetic interference), and thermal management. The primary purpose of stack-up design is to maximize the performance and reliability of the PCB.

PCB stack-ups are necessary for several reasons:

Electrical Performance: In high-speed and high-frequency applications, stack-up design ensures stability and consistency in signal transmission. By controlling the distances and arrangements between signal layers, ground planes, and power planes, signal transmission losses and crosstalk can be reduced, thereby enhancing electrical performance.

Signal Integrity: In multi-layer PCBs, couplings and interactions exist between signal layers. Proper stack-up design can mitigate interference between signal layers, improve signal integrity, and reduce signal distortion and jitter.

EMI Suppression: Rational PCB stack-up design can reduce electromagnetic radiation and external interference (EMI) of the PCB. Proper spacing between layers and distribution of copper foils between layers can enhance PCB's EMI suppression capability.

Thermal Management: In high-power applications, stack-up design can aid heat conduction and dissipation. By adding thermal heat spreaders or copper foils at appropriate locations, PCB heat dissipation can be facilitated, preventing overheating and damage.

In summary, stack-up design aims to ensure high performance, reliability, and stability of the PCB while meeting specific requirements of diverse applications. The reasonable stack-up design optimizes PCB's design and performance to the fullest extent.

PCB Knowledge