What is the difference between solder mask and coverlay?

Have you ever been have trouble in selecting the solder mask and coverlay when you are design a flex pcb? Solder mask and coverlay has the same basic function of insulating, do you know the difference between solder mask and coverlay? Is it possible to make the solder mask and coverlay overlap together for insulating? Do you think the flexible solder mask is the same as rigid printed circuit solder mask?

1)Design: Actually, coverlay is widely used for flexible printed circuit board. Because of most of flex pcb will need to add the PI/FR4 stiffener and EMI shield. But if there are BGA pads and dense solder mask opening area, limits the minimum size of openings and the minimum web of material then can be retained between adjacent openings. I would like to share a coverlay and solder mask overlap together FPCB with you. The coverlay is applied first and the solder mask second with a slight overlap onto the coverlay to prevent any gaps in the combined layer. This kind of design have much higher requirements for our production to ensure your functional and reliable design. Below picture is a coverlay and solder mask overlay together FPCB, hope you will be interested in.

If the coverlay opening is square, the minimum size for coverlay is 0.6mm, and space is 0.25mm. Otherwise you need to print solder mask. If the coverlay opening is round, the mimimum size for coverlay is 0.25mm, and space is 0.15mm. Otherwise you need to print solder mask. Because too small coverlay webs will be easily damaged or broken during manufacturing process, and the limited of adhesive may not allowed for a good lamination.  Coverlay openings may also exhibit a small amount of adhesive squeeze out into the openings. Then the resulting design rules for coverlay are significantly different than those for solder mask. Attached the coverlay square and round opening picture for your reference, please kindly check it.

2)Thickness: Below is the pure green solder mask flexi pcb picture, it do not need to add the stiffener for PCBA (SMT assembly), and there are no BGA area, why it choose to use the green solder mask instead of coverlay? The normal coverlay thickness is 27.5~50um. And the solder mask thickness is 8~12um. Due to the customers need the ultra thin flex pcb. Then we suggest the customer to use the solder mask instead of the coverlay.

3)Color: The most popular coverlay color is yellow, we usually prepared the yellow, white, black coverlay in stock. Solder mask is available in a wider range of colors: green, black, white, yellow, etc. Please kindly note that the flex pcb solder mask is quite different to rigid pcb. The flex pcb solder mask cost is much higher than FR4 PCB.Because the flexible circuit board solder mask the withstand bending and high temperature is better than Rigid FR4 PCB. But the coverlay and solder mask for flex price is the same for us.

If you have any comments, warmly welcome you to contact us. Once we received your news, we will reply to you immediately. Looking forward to receiving your inquiry.

How much do you learn about the loose-leaf rigid flex PCB?

Have you seen loose-leaf (layered) rigid-flex PCB? One end of the multiple flexible boards is fixedly connected to the main rigid board, each flexible PCB will laminate to a fixed connection of rigid part.

Each flexible circuit has a free end, and each flex board can be bent freely, which improves the bending performance of the multilayer PCB. The flex parts can be respectively bent to the required angular positions so that the layered rigid PCB can be connected with components at different angles and different spatial positions.

loose-leaf (layered) rigid-flex PCB

Many FPC have a limited bending radius, flex thickness & width and amount of copper will affect flexibility and bend radius. The loose-leaf approach does allow going below the minimum recommended bend radius. Separating the layers into single flexible printed circuit board within the stack up allows the minimum bend radius to be calculated based on the thickness of the individual layers. But the length of each single flex circuit is made somewhat longer than the one below it, it allows room for all flex parts to bend without undue stress to itself or to the layer immediately adjacent to it.

Best Technology is mainly manufacture FPC board and flex-rigid PCB boards. Below is a ten layers flex rigid PCB with eight different golden fingers sample picture for your reference.

loose-leaf (layered) rigid-flex PCB
loose-leaf (layered) rigid-flex PCB stack up

If you are interested in this designs, please feel free to contact us to know more details at sales@bestfpc.com

What are the differences between the Poyester (PET) and Polymide(PI) material?

The flex pcb raw material and stack up will decide the flex circuit quality. According to the market demand, there are three types of base raw material used for Flexible circuit board. Poyester(PET), adhesive Polymide (PI) and adhesiveless Polymide (PI) . Do you know the difference between the PET and PI material? If not, let us learn about it together now.

The advantages and disadvantages of Polyester(PET) and Polymide(PI).

Attached is a form for your reference, hope it will be helpful you. And you can choose the right raw material according the below form.

 Polyester(PET)Adhesive
Polymide
Adhesiveless
 Polymide
Flexibility (2mm radius)BadGoodBest
Tear strength800g500g500g
Strip strength in the air1050N/M1750N/M1225N/M
Eatching>=20%BESTBadGood
Working temperature85~16585~165105~200
ChipBadGoodBest

The advantages of PET material: Production process is less, price is cheaper, lead time is shorter.

The disadvantages of PET material: Polyester material can not pass through soldering flow. Then it can not be working in high temperature environment.

The advantages of polymide (PI): It’s easy to bend. Can work in high temperature.

The disadvantages of Polymide (PI): The price of raw material is much higher than Polymide.

Below picture is the single sided FFC, ENIG 3u’’, 10pin, 0.5mm pitch, and it is made of Poyester(PET) material

BFS17386-B

Below picture is a 1layer flex pcb, 4 mil PET material, print silver ink, carbon finishing mask, insulation( protective coating), carbon resistance less than 500 ohm.

BFS18205

Below picture is 1L FPC, it is made of adhesive polymide raw material. ENIG 1u’’, 1oz Cu, 1mil PI, 1mil coveraly, and the total thickness is 0.13mm+/-0.03mm.

BFS20032

It is a single sided FPC, ENIG 1u’’, 1/2ozCu, 1mil adhesiveless PI, 1/2mil coverlay.

BFS20209

If you have any questions or would like to learn more, warmly welcome you to contact us at sales@bestfpc.com

What’s the difference between rigid-flex PCB and flex PCB with FR4 stiffener?

In the production of FPC, normally we need to add different stiffener to meet the total thickness or hardness, such as PI stiffener, FR4 stiffener and stainless steel stiffener. But the FR4 stiffener after laminated is very similar to the rigid-flex PCB, especially there are also have drilling holes in FR4 stiffener.

Do you know how to identify FR4 stiffener and rigid-flex PCB? Please refer to the below picture. On flex PCB, you can see there are many PTH holes, maybe these boards are need to insert connectors and connect it to other products. So they need the hardness in this area.

From the left sample, we added the FR4 materials on bottom side, there are also have vias on stiffener, but no copper in vias, so this is FR4 stiffener. It can only increase the hardness of flex. From the right sample, the green part are still FR4 materials, but we plated copper for all vias, so it’s a rigid-flex PCB.

FR4 stiffener VS Rigid-flex PCB

Rigid-flex board has many production processes, high production difficulty, low yield rate, so its price is relatively expensive and the production cycle is relatively long than flex PCB. If the FPC with FR4 stiffener can meet your requirements, then it would be a better choice for you.

If you have any other question about stiffener or rigid-flex PCB, please feel free to contact us at sales@bestfpc.com

How to decrease the burr when drilling?

As we all know, the burr will influence the product quality and looks not so beautiful, so today I’ll share you how we to decrease the burr when drilling.

  1. First, the drilling bit must be new.

Usually, the drilling bit can be used for drilling 3000-5000 holes.

To avoid burr, we will change new drilling bit after drilling 2000 holes.

drilling holes
  • The drilling speed must be suitable.

Normally, the drilling speed is around 20W/drilling. The speed shouldn’t be too fast or too slow. If too fast, it might cause holes burning.

The best drilling speed is 16W/drilling.

(Rigid PCB have no requirement).

  • Pile up the boards shouldn’t be too much.

Flex boards pile up shouldn’t exceed 8PCS.

And it will be better to put some clear paper between each two boards.

(Rigid PCB have no requirement.)

If you want to know more about other process, please let us know, we will share with you by email.

How to calculate impedance of FPC?

Due to Flexible boards are suitable for more and more fields with their unique advantages of lightness, thinness and compactness. There are also many boards that need to be assembled components or various signal transmission, so the requirements for impedance are increasing.

Normally there are four factors will affect impedance. 1), DK Value. 2), copper thickness. 3), copper trace and space. 4), dielectric layer thickness (PI&coverlay). You can know more details from below picture.

Er1: DK value for base material, the DK value of different brands’ materials and thickness are not the same, the normal range is 3.15 to 4.2

T1: copper thickness, this is the finished copper thickness, it’s marked 30um in below table, that’s means the base copper thickness will be around 18um.

W1: copper trace width, and S1 is copper trace space. Trace width and space are important for impedance.

H1: dielectric layer thickness, that is the PI thickness of the base material, PI thickness with adhesive thickness for adhesive materials.

W1&S1: copper width and space.

C1/C2/C3: coverlay thickness. 1/2mil coverlay is 28um, 1mil coverlay is 50um.

CEr: DK value for coverlay, 1/2mil coverlay is 2.45, 1mil coverlay is 3.4

Impedance diagram

Normally customers has required impedance value and total board thickness (stack up). So what can we do to meet the customer’s required impedance?

The first step, adjust copper trace and space to meet impedance, the smaller the trace width, the greater the impedance. Our minimum copper trace and space is 2mil, if we still can not meet impedance when copper trace adjusted to 2mil, then we have to move to second step.

The second step, normally the reference layer of impedance is copper foil, we can change the copper foil to grid copper, because the greater the grid spacing, the greater the impedance value.

The third step, if the above two steps still cannot meet the impedance requirement after adjustment, we need to communicate with the customer to adjust the stack up which included copper thickness, dielectric layer thickness and coverlay thickness.

Finally, we can calculate impedance and adjust copper traces for you if you need our service. If you would like to know more details, please contact us sales@bestfpc.com

What’s the difference between Adhesiveless and Adhesive Polyimide (PI)?

Today I’d like to share what’s the difference between Adhesive PI and Adhesiveless PI.

Item 1: Adhesiveless PI stack up

Adhesiveless PI stack up
Adhesiveless PI stack up

As you can see in above stack up, there is no adhesive connect the Copper layer and Polyimide.

There are four advantages of adhesiveless PI:

  1. Thinner Thickness

As you know, the thickness of adhesive is around 12.5um, 15um, 20um. If you have strict requirement for the whole board thickness, it will be a good choice to use adhesiveless PI.

2.Better Bending

Adhesiveless substrate is thinner than adhesive substrate, so it’s bending is better.

3.  Heat resistance

Due to the adhesive have poor performance in heat resistance, so without the adhesive, the adhesiveless base material have better heat resistance.

Under the same temperature, such as 200℃,the tearing strength of adhesiveless PI change a little. But the tearing strength of adhesive PI material rapidly decrease.

4.  Dimensional stability

The dimension of adhesiveless board change very small when the temperature increase. Even under the temperature of 300℃, it’s dimensional changing rate is no more than 0.1%.

Good dimensional stability will be great help for fine wiring process.

5.  Chemical Resistance

The adhesiveless substrate have good performance of chemical Resistance, it’s tearing strength have no obvious change with time increased.

The adhesive base material has poor chemical resistance, so it’s tear strength decreases greatly with the increase of time.

The only disadvantage for adhesiveless PI is it’s a little expensive than adhesive PI.

Item 2: Adhesive PI stack up

Adhesive PI stack up
Adhesive PI stack up

As you can see, the thickness of  2 layer adhesiveless FPC is 0.19+/-0.03mm.

The thickness of  2 layer adhesive FPC is 0.23+/-0.03mm.

If you need thicker thickness, the adhesive PI might can meet your requirement, the cost will be much lower than increase copper thickness or PI thickness.

There are two mainly advantages for adhesive PI:

  1. Thicker thickness.
  2. Cheaper price.

Anyway, if your project need thinner thickness or smaller line width and space (0.05mm), adhesiveless PI will be the best choice. Welcome to contact us at sales@bestfpc.com if you need to know more details.

What is the Dual access flexible circuit?

Have you ever been heard about the dual access flexible circuit? Why it named dual access flexible circuit? After you read my e-mail, you will find the answer.

Dual access flexible circuit board is similar to double side FPC, but it has different stack up to double side FPC. Why it named dual access FPC? Dual access can access to both side by copper, so it named the dual access flexible circuit. Dual access flex PCB only have 1 layer copper, and coverlayer on both side. They can connect both side by layout through coverlayer opening, and do not need to drill the holes like 2 layers flex PCB to connect both side.

Dual access flexible circuit samples picture:

Dual access flexible circuit sample
Dual access flexible circuit sample
Dual access flexible circuit sample
Dual access flexible circuit sample

Working principle: The coverlayer play the role as an insulator, the copper is a very good kind of conductor. One side is anode, and the other side is cathode. If they the current flow, they form a closed loop circuit.

Stack up of dual access flex circuit:

Stack up of dual access flex circuit
Stack up of dual access flex circuit

Dual access circuit advantages:

  1. It can be connected to both side through copper, and do not need to make a through hole. If it is 2 layers FPC. You have to make a through hole to connect both side.

2. The appearance will be more beautiful than 2 layers flex PCB. Because we can conduct by trace, it will be more beautiful than through holes to conduct.

Dual access circuit disadvantages:

  1. Complex production process: The production process is similar to 2 layers flex circuit, but it will be more difficult and complicate compared with 2 layers flex PCB. It is easy to scrap. And many factories can not make dual access circuit board due to production technique.
  2. Easy to wrinkle. Because the dual access is very thin. Only one copper and two coverlayers, it will be easy to tear up. When it comes to laminate process, is also very easy to laminate wrinkles and blister.

If you have any questions or would like to learn more details, please feel free to contact us.

14-Year Anniversary Celebration of Best Technology-Thank you!

Recently, Best Technology has completed 14 years of its glorious journey. Every small step is worth celebrating. On June 26, 2020, our founder, Peter, Emily, group leaders and all the employees spent this unforgettable festival in union.  

Best Technology Team
Best Technology Team

On Best Technology successful journey, Peter stressed- “These past 14 years have been a period of growth and a learning experience for us as we went through several trials and errors till we deliver successful business. Today we are proud of the fact that our employees can make great progress day by day after training with us and feel confident taking on tough project-related challenges,thanks for all the endeavour from employees and thanks for the great support from our suppliers and customers. Best Technology’s continuous development can not do without them.”

We have come a long way from 2006 when we started off with just 1 people(Peter). Today we are proud to announce that we have nearly 100 employees and many sister companies spread all over China mainland and also have customers in over 150 countries around the world.

number of employees
number of employees

From above, it is evident that there has been a staggering growth in the number of employees in the span of 14 years. This enhanced employee strength has built the roadmap to Best Technology’s success.

annual sales volume
annual sales volume

The above graph presents our company’s growth against the years 2006 to 2020. It can be seen that the growth rate is very high from 2006($8000) to 2019($6560,000). At this year 2020, Best Technology still keeps growing with increasing sales volume in spite of COVID-19 outbreak.

Best Technology’s unique journey has been a brick-by-brick progress and has entailed years of  efforts. Our company’s commitment in becoming world class distinctive solution of printed circuit board and metal domes in 2025 has paved the way to the series of milestones achieved. These are equally appreciated by the correct leadership of our chairman, Peter.

The primary aim of our company has been to equip the professionals with all possible accoutrements to tackle on-job challenges. Today’s ever changing face of businesses demands deeper knowledge, quality, and on-time delivery. Customers are always looking out for continuous and improved delivery. The best means to achieve the same is to keep improving.

14-year anniversary
14-year anniversary

We will continue our journey towards innovating the new ways of learning, delivering excellence to the customers and lending a hand to achieve professional goals.

What’s more, Best Technology’s 14th-year anniversary celebration is a solemn procession of efforts and dedication of the trainers and staffs who have spent their quality time to provide the best services and meet more expectations of the ever evolving PCB world.

How to improve the bending effect of multilayer rigid-flex PCB?

Normally the rigid-flex PCB need to use in a small space area, and the flex area needs to be bent repeatedly or better bend radius. So many customers are concerned about the bending effect of the flex area, do you know how to improve the bend effect when you design? Here are a few suggestions for your reference.

The first one, the thickness of flex PCB board. The thinner the board, the better the flexibility. Such as below 6 layers stack up with 2 layers flex, we can used 1/3 oz copper instead of 1/2oz or 1oz, and 1/2mil PI instead of 1mil PI, then total thickness will be around 0.1mm.

The second one, adhesiveless material instead of adhesive material. The adhesive in base material will also affect the flexibility. Such as below stack up of flex part, there is no adhesive between Copper and Polyimide, but the coverlay must be glued and then laminated.

The third one, design a loose leaf (air gap) for flex area. Loose leaf can reduce the total thickness of flex part, especially for multilayer flex circuits in rigid-flex PCB design. Such as below sample pictures, no matter 2 layers or 4 layers in flex part, many customers will add one air gap. If you think the flexibility is not very good when we add one air gap, it’s okay to add two or more air gaps for multilayer flex PCB.

If you would like to know more details, please send your questions to our email sales@bestfpc.com