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Main types surface finish of FR4 Printed wiring board

November 4th, 2020

Printed Circuit Board surface finish is a coating between bare FR4 board and components. The main function for surface finish is to protect the exposed copper circuitry and provide a solderable surface when assembling（soldering) the components to a copper clad PCB board.

Most of the Rigid Circuit board company can make several different types of surface finish. Best Technology as a leading custom FR4 printed circuit board manufacturer since year 2006, it could provide following different finish.
HASL
Lead-free HASL
Immersion Tin/Immersion Silver
OSP (organic Solderability Preservative)
Gold
ENIG (Electroless Nickel Immersion)
Hard Gold
Wire bonding Gold

As the surface mounts assembly service became more complex and needs to conform to new regulations like RoHS and WEEE. People maybe face the question how to choose a suitable surface finish for your FR4 Copper board?

Before decide to choose suitable surface of copper conductors circuit board, you may need to take the cost, RoHS, your components type, PCB assembly method, factory circuit fabrication capability, and rigid board circuit testability into consideration.
Basis on above information of copper foil PCB, we would like to make a explain PCB finish in detail, wish this may help when you make decision at circuit board design and manufacturing.

HASL/Lead Free HASL
It is a most popular surface finish, and the cost is low and easy to repairable, it is acceptable for simple SMT. But the surface is uneven, it is not suitable for fine pitch components and not good for plated through-hole(PTH). In other way, it is poor wetting.
Material different with same finish
HASL(standard):Typically Tin-Lead
HASL(Lead Free):Typically Tin-copper, Tin-Nickel, without lead
The thickness will conform to IPC 6012 class 2 standard

Advantages of HASL-LF
Excellent solderability
Inexpensive / Low cost
Widely Available and used
Easy reworkable
Allows large processing window
Long industry experience / well known finish

Disadvantages of HASL-LF
Uneven surfaces for printed circuit board
No good for fine pitch components from Samtec/Hirose/Molex
Thermal shock
Solder Bride for circuit board assembly
Plugged or reduced PTH’s
Not suited for < 20mil pitch SMD & BGA
Bridging on fine pitch
Not ideal for HDI products

Immersion Tin

Advantages
Flat surface
No Pb
Good for fine pitch / BGA / smaller components
Mid range cost for lead free finish
Press fit suitable finish
Good solderability after multiple thermal excursions
Easy reworkable

Disadvantages
Very sensitive to handling – gloves must be used
Tin whisker concerns
Aggressive to solder mask – solder mask dam shall be ≥ 5 mil
Not recommended to use peelable masks
Exposed tin on final assembly can corrode
Not good for multiple reflow/assembly process
Difficult to measure thickness

OSP (organic Solderability Preservative)
OSP(organic Solderability Preservative) same with HASL, lower cost but the OSP have flat surface, and it is not good for PTH components, sensitive and short shelf life, it is very easy come to oxidation.

Advantages
Flat surface
No Pb
Good for fine pitch / BGA / smaller components
Inexpensive / Low cost
Easy reworkable
Simple Process
Disadvantages
Not easy to measure thickness
Not good for circuit copper board plated through-hole(PTH)
Short Shelf Life
Maybe cause ICT issue
Exposed Cu on Final assembly
Handling sensitive– gloves must be used and scratches avoided

The above surface finish of Printed Wiring Board compared with Gold finish, the cost is corresponding cheaper, but in my coming blog, I would like mainly to explain Gold finish for printed circuit board fabricator with 13 years rich-experienced in FR4 PCB custom contract manufacturer.

If you have any question about the printed circuit boards (PCBs). Warmly welcome to send mail or call Best Tech.

Posted in FAQ, FR4 PCB, SMT Technology | No Comments »

What is PP (Prepreg) and CORE in a PCB?

October 30th, 2020

We all know that both Prepreg (PP) and CORE are important parts for the printed circuits board. And they are also very common on the PCB structure.

But what exactly they are? What’s the difference of a PP and Core for a FR4 PCB?

Prepreg, referred to as PP, it is a sheet material impregnated with resin and cured to an intermediate level (B-stage), known an insulating material for the PCB board.
As a prepreg material while the PCB production process, before lamination, it is mainly used as an adhesive material and insulating material for the inner conductive pattern of a multilayer PCB.

After the Prepreg is laminated, the semi-cured epoxy resin is squeezed away, starts to flow and solidify, bonding the multilayer circuit boards together, and forming a reliable insulator.

PP, it is placed between the two copper layers to isolate and make the two copper layers adhere. Below picture for you to see where the PP is on a PCB stack up.

And CORE, it is totally different as the Prepreg.

Core is with certain hardness and specified thickness, and with copper foil in both sides.
It is the basic material for making printed circuits boards, and the multilayer PCB board is actually made by pressing Core and Prepreg.
Sometimes, when people are talking about the Copper Clad Laminate (CCL), they also refer to the CORE.

And here is the major difference for the PP and Core:

  1. Prepreg is stayed with a semi-solid material in a PCB, similar to cardboard. But core is different, core is hard, similar to copper;
  2. Prepreg is similar to adhesive + insulator; while Core is the basic material of PCB, they have completely different functions;
  3. PCB Prepreg can curl, but PCB Core cannot bend;
  4. Prepreg is non-conductive, and the core has a copper layer on both sides, which is the conductive medium for a PCB board.

As an PCB manufacturer with more than 15 years experiences in China, Best Technology insists on using the best Copper Clad Laminate ( CCL) materials to make the PCB, no matter Prepreg or CORE, we will select the best FR4 materials for the PCB manufacturing, to make sure all the PCB boards we supplied to customers are with qualified materials and to be your best PCB suppliers in China.

Posted in FR4 PCB, PCB News, PCB Technology | 2 Comments »

What’s the Countersink and counter bore on a PCB drawing?

September 30th, 2020

Firstly, do you know what’s a countersink and counter bore?

By definition, a countersink is a stepped hole, which can allow the fasten parts head to sink fully to the parts.

With counter sink on your design, the tapered head of a screw can sit flush with the top of the laminate, commonly countersink is used to install bolts or other connecting parts.

Below is the example for the hole drawing and picture for a countersink:

Picture: counterbore holes attributes

To drill your countersunk holes accurately in our PCB fabrication plant, the following information is need to be defined when you supply the PCB drawing for us to make the PCB:

  • Which side of the board is the countersink hole should be drilled, on top or bottom?
  • Is the sink and shaft to be through plated or non-plated?
  • Taper angle or countersink angle. 82°, 90°,120° etc.
  • Countersink diameter of top
  • Countersink diameter of bottom (body)
  • Depth of the countersink is to be drilled（the height of the screw to determine the countersink depth.

By comparison, a counterbore makes a flat-bottomed hole and its sides are drilled straight down. This is usually used when a fastener such as a bolt or cap head screw is required to sit flush with or below the level of a surface.

Below is the drawing and picture for a countersink.

Same as the countersink, the counterbore holes attributes drawing is needed to supply with your PCB drawing when fabricate the counterbored holes. But for counterbore on PCB, because the sides of the hole are always parallel, there is no need to specify the angle.

With below drawing, believe it will be more helpful for you to understand the difference for a counter sink hole and counterbore on the printed circuits board.

If need to have any question regarding the counter sunk or counter board on a PCB drawing, please contact us feel free.

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What is an HDI FR4 PCB?

September 16th, 2020

HDI abbreviation means High Density Interconnect. HDI PCBs have extremely density trace spaces and lines, micro vias under 0.25mm(10mil), smaller pads and higher connection pad density. See the photo of HDI board laser drill& prepreg thickness ratio.

It is very helpful in enhancing electrical performance and HDI PCB use for high-end products and HDI PCB is regularly used in 4G network communications, medical, Military and Aerospace. See the HDI board which Best Tech made for customers.

HDI PCB is the better option for high-layer count and costly laminated boards. Due to the increasing complexity of design structures laminate, the Blind vias and Buried vias are increasingly used in high-density circuit boards (HDI PCB board). Best Tech always improved manufacture capability for HDI, following production capability of the comparison for 2020 and the capability which Best Tech want to achieve at year 2021.

Following is the advantages to use an HDI PCB

The common reason for using HDI technology is a significant increase in packaging density. In addition, overall space requirements are reduced will result in smaller board sizes and fewer layers. 

1.Denser trace

2.More stable performance

3.Reduce interference inductance and capacitance effects

4.Improve signal integrity in high-speed design

5. Reduce frequent relocation of components

What’s mean for a blind via hole?

It is a hole runs from an outer layer, but not through the entire PCB. These holes can be drilled mechanically or using laser technology.

And what’s mean for a buried via hole?

This is a hole that runs between one or more inner layers. They are normally mechanically drilled.

By the way, we would like to make some explain for the through via hole, see attached photo to know the detail.

Do you want to know the different types for HDI PCB boards?

The following photos mainly shows the different laminate structures available from Best Tech for HDI PCB for prototype and mass production.

What is the minimum pad size of hole on the outer and inner layer?

This is different from manufacturer to manufacturer, but in general you can say that the majority of manufacturers can produce them as follows:
A = 0.10 mm
B = 0.15 mm
C = 0.20 mm

Finally, here is an 8L HDI FR4 PCB which we made for our customer at June. See the stack up information.

Surface finish: ENIG(2u”)

Features

1. 3+（2）+3  HDI FR4 PCB board

2. L1 L2、L2 L3、 L3 L4、 L5 L6、 L6 L7、 L7 L8 laser blind hole, micro via size:0.15mm

3. L3、L6 mechanical blind hole, hole size:0.25mm, through hole:0.3mm

4. minimum trace width and space is 3/3mil

5. Finished board thickness: 1.3±10%

6. following is the stack up information

For more information for HDI PCB, welcome to send any query to Best Tech for advice.

Posted in FR4 PCB, HDI PCB, Our News, PCB News, PCB Technology | 1 Comment »

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

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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