Posts Tagged ‘FR4 PCB’

How to prevent the FR4 PCB bended and warped after reflowing?

Wednesday, August 19th, 2020

Everyone may know that PCBs are easy to be bended or warped when get through the reflow process. Do you how to avoid this issue? here are some advices your reference.

  1. Reduce the effect of temperature on PCB board stress

Due to the “temperature” is the main source of stress for board, so as long as the temperature of the reflow oven reduce the temperature or slow the speed of warming up and cooling down in the reflow oven, the board bending and warping issue can be reduced. But it may be occurring other issues, such as short circuit when soldering. See Best Tech make control for the temperature of the reflowing process.

reflow machine

 2. Use high Tg plate

 Tg is the glass transition temperature. The lower of the Tg value, the faster of board starts to soften after finishing reflowing, and deformed of the board become more serious. if use of higher Tg 170 material can increase the ability to withstand stress deformation, but the price of TG170 material is higher. See the stock of high Tg material from Best Tech.

Use high Tg plate

3. Increase the board thickness

If final application allowed, we recommend 1.6mm thickness to prevent the risk of board bending. Best Tech can provide PCB board thickness to 4.5~8.0mm.

PCB board thickness
  • Try to reduce the size of the board and the number of panels during design

 Since most reflow oven use chains to driving the board forward, if larger size of the FR4 PCB, the own weight will be deformed in the reflow oven, so during design try to design the long side of the PCB as a board edge, it can reduce the deformed. And

Same reason, if there are many PCBs in a big panel, PCB will be bigger and heavier, when get through the oven, the speed in the oven will be slowly and it will influence the deformed of the PCB. See following PCB board, we only panel 1 single piece into a panel to keep the size of FR4 PCB panel not bigger.

pcb
pcb

5. Use the reflow carrier/tray

If above methods cannot get a good result for deforming, you maybe can use a reflow carrier/tray to reduce the deformation. The reason reflow tray can fix the circuit board, after the temperature of the printed circuit board is lower than the Tg value, it can maintain the original size.

pcb in tray

6. Use Router instead of V-Cut

Since V-Cut will destroy the structural strength of the panels, try to use Router PCB board or just reduce the depth of the V-Cut will helps the deform issue.

pcb
pcb

If you want to know more about how we control the bended and warped for PCB, you are welcome to contact us.

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What is high Tg PCB (fr4-170TG, TG 130-140, FR4 Tg150)

Thursday, June 11th, 2020

In recent years, there are more and more customers request to manufacture PCB with high Tg, in the following we would like to describe what is high Tg PCB.

fr4 170tg
fr4 170tg

Normally high Tg refers to high heat resistance in PCB raw material, the standard Tg for copper clad laminate is between 130 – 140℃, High Tg is generally greater than 170℃, and middle Tg is generally greater than 150℃. Basically the printed circuit board with Tg≥170℃, we call high Tg PCB. As the rapid development of electric industry, especially for the computer as the representative of electronic products, developing toward the high performance, high multilayer requires PCB substrate material with higher heat resistance to ensure high reliability. On the other hand, as a result of development of SMT, CMT with high density pcb assembly technology, the PCB manufacturing with small hole size, fine lines and thin thickness are more and more inseparable from the support of high heat resistance.

fr4 170tg
fr4 170tg

If the Tg of PCB substrate is increased, the heat resistance, moisture resistance, chemical resistance and stability of printed circuit boards will be improved as well. The high Tg applicates more in the lead free pcb manufacturing process.

Therefore, the difference between general FR4 and high Tg FR4 is, in the hot state, especially in the heat absorption with moisture, the high Tg PCB substrate will perform better than general FR4 in the aspects of mechanical strength, dimensional stability, adhesiveness, water absorption and thermal decomposition.

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How to Choose Thick or Thin FR4 PCB?

Monday, March 30th, 2020

If you’ve ever made a pie with your kids, you know that the thickness of your crust is important. Too thin, and the pie falls apart into a mess of filling. Too thick, and you might as well be chewing on a loaf of bread. Getting the thickness just right is what makes a pie worth eating.

Even though PCB substrate materials are non-conductive and do not carry current, the board material still affects the electrical performance. FR4 PCB substrate is widely used, and a thorough understanding of this material is very necessary.

FR4 PCB
FR4 PCB

FR4 Thickness Design Considerations

If you start your design process by choosing the correct board thickness, you will avoid to redesign later. A number of factors are influenced by the board thickness. Before you start designing your board, think about the following design issues:

Form factor and flexibility: Does your PCB have a strict form factor requirement? Thinner boards tend to be used in smaller devices and can flex more than a thicker board. A more flexible board may be unusable with a pick & place machine. Electrical connections in flexible boards can also break unless they have some mechanical support.

Components and connections: Will the device have any components that require the Printed Circuit Board to have a specific thickness? Components like USB edge connectors and some through-hole components require PCB in correct thickness. BGA connectors generally require thicker boards.

FR4 PCB
FR4 PCB

Impedance matching: Traces on adjacent layers in multi-layer boards form a capacitor, and the layer thickness and its dielectric constant determines the equivalent capacitance. Impedance matching is critical, and the capacitance must be taken into consideration when designing the board. This is especially true in HDI PCB.

High-speed devices: If you are working with a high-speed device, FR4 is always not the best option, and some other material should be used instead. FR4 PCB could be used in high-speed devices when the layers are coated with high-speed laminates. These laminates offer significantly improved electrical performance over standard FR4, and generally a combination of FR4 and a high-speed laminate may be preferable to an alternative material.

RF losses: FR4 tends to have higher losses than other PCB materials that are specialized for RF applications. Traces on FR4 will have larger attenuation at RF frequencies than other specialized materials for a given board thickness. The board thickness also changes the effective dielectric constant of the board, which then changes the required impedance matching in RF circuits.

Thermal and Reliability Considerations

All materials expand at higher temperatures, and the thermal expansion coefficient must be taken into consideration when choosing the board thickness. The thermal expansion coefficient of FR4 changes drastically above the material’s glass transition temperature (140℃). The board can become electrically and mechanically unstable when the board operates above the glass transition temperature.

The reliability of the trace materials, laminate materials, and solder joints on a PCB laminate are determined by the resistance to thermal cycling. When the thermal expansion coefficients of the different materials are mismatched, fatigue occurs after repeated thermal cycling. Copper plating in vias and solder balls are especially vulnerable to damage under thermal cycling.

This is a greater problem in thick FR4 PCB with high aspect ratio vias. Plating damage in through-hole vias, lifted pads, and cracked surface resin can occur under the stress caused by thermal cycling. A thicker board will have a larger expansion magnitude for a given via aspect ratio, resulting in more damage to the board and the electronic components. Thermal stresses can be huge when an FR4 PCB runs above the glass transition temperature.

Volumetric expansion is also critical in rigid-flex FR4 PCB. Thermoplastic adhesives with low glass transition temperatures and high Z-direction expansion coefficients can exhibit very large volume expansion at high temperature. Z-direction expansion in these situations can be as large as 500 ppm/℃.

Even though a thicker FR4 PCB has greater thermal mass and can dissipate more heat from electronic components, there is also the potential for more damage due to thermal expansion. Therefore, how to choose between a thick and thin FR4 PCB that satisfies all other requirements, the thinner board may be a better choice if the FR4 PCB will undergo thermal cycling frequently.

If you have any comments. queries or suggestions on this, welcome to contact us.

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Soldermask on Hole (Via) Without Opening

Thursday, October 6th, 2016

Soldermask opening always is a very important thing for various PCB, esp. for rigid FR4 PCB. In previous article, we already discussed why soldermask opening & tented was not the same like original gerber file.

If you really want soldermask opening for hole (Via) less than 0.60mm, you should added special notice when placing order.

At the same time, there’s another method you can use: To increase the soldermask opening diameter, that means add an “Annular Ring”. Normally you will need to increase 0.10mm (4mil) on each side, so that there’ll be a 0.1mm “Annular Ring” around the hole/Via, that Annual ring will stop soldermask into the hole/via of PCB.

Because in a lot of design, if there’s Via/Hole, there’ll no opening in its soldermask file, maybe engineer think the drill will remove the soldermask and then there’ll no soldermask on these Via/hole of board. As mentioned before, if Via/Hole in Printed circuit board was less than 0.6mm, or around 0.65mm then it’ll be a problem.

See from bellowing pictures of PCB board:

0.635mm Drill hole (Via)

0.635mm Drill hole (Via)

(0.635mm Drill hole (Via)

This is the original PCB Gerber file, the diameter of drill hole was 0.635mm and there’s no soldermask opening on GTS (top soldermask) Gerber file. (Purple color was GTS, Yellow: GTL (top layer); Blue: Drill (PTH) file), and no Annular Ring.

So final circuit boards will be like belowing:

0.635mm Via covered/tented by soldermask

0.635mm Via covered/tented by soldermask

 

 

 

 

 

 

 

 

 

 

 

So you need to add at least 0.10mm (4mil) soldermask opening on each side of hole, so that there’ll be a 0.1mm Annular Ring, like bellowing:

Add 4mil Annular Ring on PCB Via

Add 4mil Annular Ring on PCB Via

 

 

 

 

 

 

 

 

 

 

Then that hole will become like bellowing:

Hole/Via without soldermask (soldermask opened)

Hole/Via without soldermask (soldermask opened)

 

 

 

 

 

 

 

 

 

 

We believe there’re a lot of other handling method about that soldermask opening, please leave your comments, or contact us to discuss more about that, and believe you will be happy to receive with the PCB with correct soldermask opening!

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