What we do to meet your impedance control?

Have you designed the flex pcb or rigid flex required the impedance control? Recently, there are more and more customers are required the impedance controlled circuits throughout the industry in a wide range of applications. And also need a minimum bend radius. It gives a big change to improve our production capability.

Attached a picture for 7 layers HDI rigid flex pcb and impedance control required picture for your reference, please kindly have a look at it.

Rigid flex with impedance control

Line widths and spacing, and the copper thickness of the flex layers interact to achieve the impedance values required. Normally, we will adjust the trace spacing and width to meet our customers impedance control requirements. Thinner copper allows for a thinner line width/spacing and a thinner flex core thickness. This results in a thinner flex area, which will have the highest degree of flexibility and the tightest min. bend capability. The minimum trace width and spacing we can make is 2/2 mil for flex pcb. If the customers are required 2/2mil trace width and spacing, we will use 1/3 oz Cu. The following are typical line width and spacing for the more common impedance values when using 1/2 OZ copper:

50 Ohm Single Ended: 0.004” line.

90 Ohm Differential Pair: 0.004” line/0.0055” spacing.

100 Ohm Differential Pair: 0.004” line/0.006” spacing.

120 Ohm Differential Pair: 0.0037” line/0.0075” spacing.

If 1 OZ copper is required, usually due to a higher current carrying requirement on non-impedance lines, the above line widths are not valid as a 0.004” line width is below the manufactured limit of 1 OZ copper. This requires the line widths/spacing as well as the flex core thickness to increase, which negatively impacts flexibility.

Polymide flex materials are very well suited for impedance-controlled designs. The material is homogenous, has a low DK (3.2-3.4), is very uniform, and has tightly controlled thickness. And we usually use the polymide base material and coverlay to start production. There are many factors will affect the impedance control.

If you would like to learn more details,please feel free contact us at sales@bestfpc.com. We have impedance control tester in our factory. And we can calculate the impedance control details for you for free.

Impedance control tester

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How to avoid fake exposed copper for Flex-Rigid PCB?

Did you meet the situation? When your Flex-Rigid PCB product produced, but PCB vias has fake exposed copper phenomenon, which might caused worse problem. (Such as below picture show)

Detailed picture for fake exposed copper

Today we’ll share some info with you, wish it can be helpful to know more about it.

Why fake exposed copper happens? There are two main reasons:

  1. Reason one: Just use screen to plug via holes in the solder mask process(have not used aluminum pluged), which caused the solder mask poor in via holes and the hole edge have fake exposed copper.

2) Reason two: When the boards printed not so well on first time, need to be washed and perform a second printing. If the solder mask in holes didn’t clear up, it will cause the solder mask cannot plug through the holes, plug holes not full, then fake exposed copper occurs.

fake exposed copper

How to improve this situation?

1) Suggestion one: Plug vias with resin, and then do the solder mask. It can avoid fake exposed copper.

2) Suggestion two: Can considering change the stack up, such as change 4 layer flex-rigid PCB(1L PCB + 2L FPC + 1L PCB), replace the TOP/Bottom PCB layer with FPC. And use coverlay, the coverlay can cover the vias, which can help solve this problem.

If you have any further question, pls feel free to contact us.

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