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.

Question 1

Solder Mask Defined (SMD) vs. Non-Solder Mask Defined (NSMD) Pads in PCB Design

Accepted Answer

In printed circuit board (PCB) design, choosing between Solder Mask Defined (SMD) and Non-Solder Mask Defined (NSMD) pads is crucial, especially for surface mount technology (SMT) components like Ball Grid Arrays (BGAs). Each approach has distinct advantages and applications depending on factors like mechanical strength, solderability, and manufacturability.In printed circuit board (PCB) design, choosing between Solder Mask Defined (SMD) and Non-Solder Mask Defined (NSMD) pads is crucial, especially for surface mount technology (SMT) components like Ball Grid Arrays (BGAs). Each approach has distinct advantages and applications depending on factors like mechanical strength, solderability, and manufacturability.

What are the differences between SMD and NSMD Pads
Solder Mask Defined (SMD) Pads
In SMD pads, the solder mask opening defines the pad size by covering the copper pad, leaving only a portion of it exposed for soldering. This method restricts solder flow and provides better mechanical strength.
• Advantages:

• Improved mechanical strength, making it ideal for high-reliability applications.
• Helps prevent pad lifting, especially for larger components.
• Reduces solder spread, ensuring controlled solder deposition.

• Disadvantages:
• Reduced copper pad area due to mask coverage, which can impact electrical performance.
• Higher stress concentration at the solder joint, increasing the risk of failures under thermal cycling.

Non-Solder Mask Defined (NSMD) Pads
In NSMD pads, the copper pad is fully exposed, with the solder mask opening being larger than the pad itself. This approach allows for better solder flow and stronger solder joints.
• Advantages:
• Larger copper pad area improves electrical performance.
• Better solder joint reliability due to improved solder fillet formation.
• More flexibility in PCB manufacturing and repair.

• Disadvantages:
• Less mechanical strength since the solder joint relies solely on the copper pad adhesion.
• Increased risk of pad lifting, especially for larger BGA components.

The choice between SMD and NSMD pads depends on the specific requirements of the PCB design. While SMD pads offer better mechanical stability, NSMD pads provide improved solder joint reliability and electrical performance. By carefully considering the application, pitch size, and reliability factors, PCB designers can select the most suitable pad type for optimal performance and manufacturability.

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

Micro-section Inspection of PCB

Accepted Answer

Micro-section Inspection is a key quality analysis method used to evaluate the interlayer connections and conductor shapes of printed circuit boards. This inspection method is usually performed on the finished PCB to ensure that the conductors between each layer comply with the design requirements and production standards, or when a product defect occurs, sampling and analysis can be done at specific location to find cause of the abnormality.

When performing micro-section inspection, a sample of a certain size is first cut from the finished PCB, and then its surface is ground to make the internal structure visible. A microscope is then used to observe and take images of micro-sections, which show the vias, conduct layers and dielectric layers of the PCB.

Through micro-section inspection, PCB manufacturers can evaluate several key factors:

Conductor Integrity and Connections: Check whether the shape, thickness and position of the conductors are consistent with the design to ensure that the circuit connection is reliable.
Quality of interlayer dielectric: Check the dielectric layers are even and free of cracks or air voids to prevent short circuit between conductor layers or insufficient insulation strength.
Copper plating quality of PTH holes: Check whether the copper plating of the through holes is sufficient and evenly distributed to ensure the conductivity and reliability of the product.

Micro-section inspection is an important step in quality control during the PCB manufacturing process. It can detect potential manufacturing defects early and make timely adjustments and improvements. This not only helps improve PCB manufacturing efficiency and product reliability, but also reduces the cost and time investment in subsequent testing and repair.

Question 7

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 8

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 9

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 10

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.

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