Effective measures to solve high frequency PCB manufacturing problems

With that electronic products that are famous for high speed, fewer losses, and high fidelity signal transmission must have compatibility with high consumption conditions for high power operation where PCBA is used to handle the high frequency and high dissipation thermal design with high-quality signal transmission.

The technologies like high frequency, high thermal dissipation and high density interconnect design are making new dimensions in the PCBA industry and will be the main part of PCBA manufacturing in near future. But complex designs and high-density assembly requirements make high-frequency PCBA manufacturing more difficult, forcing turnkey PCB assembly companies to upgrade their manufacturing capabilities. As the first choice for high-speed signal transmission equipment, high-frequency PCBA will encounter many difficulties in the manufacturing process. In this article, FS Technology will explain the common high frequency PCBA manufacturing problems and provide the most professional solutions for you through the past 20 years of manufacturing experience.

Introduction of high frequency PCBA structure and manufacturing materials

Structure Specifications

The figure below shows the internal structure of a high-frequency high-speed PCB. Combining the structure diagram with the basic characteristics of the board is helpful for the reading comprehension of this article.

Internal stacking structure diagram of circuit board	Attributes	Numerical value
	Layer count	18 layers
	PCB dimension	133.2mm*196.5mm
	Aspect ratio	11:1
	Min. Spacing/Trace width (external)	0.13mm/0.10mm
	Min. Spacing btw trace to via	0.175mm
	Min. via diameter	0.25mm
	Properties	High-frequency material, back drilling, high aspect ratio, resin-plugged via, high thermal dissipation via
	Application fields	Communication, Industrial control

Material properties for manufacturing high frequency PCBAs

High frequency has a frequency band value of about 300MHz that can be parted into medium frequency and very high frequency on the basis of wavelengths. An electromagnetic wave that has a wavelength equal to 1GHz is called a microwave. The substrate material of PCBA determines the special functions and properties of the circuit board. That is to say, whether the customer wants to customize the IMS PCBA or the RF board, it is mainly determined by its material properties. FS Technology has to use materials with the following high frequency properties when manufacturing circuit boards for high frequency electronics:

The value of the dielectric constant must be small and have stability.
For signal transmission with quality, dielectric loss also have a small value. If DK is small, signal losses will also small.
To avoid signal losses due to skin effect and mismatching of impedance control, there must be less roughness on copper foil.
There is a low value of hygroscopicity for substrate materials used for high-frequency and high-speed boards. The dielectric constant for water is 70 if the substrate absorbs water, Dk will increase. So changing in the impedance control will disturb the signal transmission.
The copper foil must have a high value of peel strength
The substrate must have a constant size, work well in heat, and have chemical resistance. Also, have good impact strength and manufacturability

All in all, the above properties need to be paid attention to when designing high frequency PCBAs. The following table shows the difference and comparison of circuit board properties brought by different Dk, Df, and Tg.

Item	Material A	Material B	Material C	Material D
D_k	2.1-2.5	2.4-2.7	3.5-3.8	4.0-4.5
D_f	0.0009-0.0017	0.0007-0.001	0.009-0.013	0.018-0.022
T_g	25°C	210°C	185°C-220°C	120°C
Ion-Migration Resistance	A>B>C>D
Ion-Migration Resistance	A>B>C>D
Moisture Resistance	A>C>B>D
Manufacturability	D>C>B>A
Cost	A>B>C>D

Problems during High-Frequency PCB Fabrication Processes and Solutions

Resin plug hole manufacturing problem

Problem Explanation

Resin plugged via manufacturing is used in the PCB industry for PCB products having high density and integrity. Through the use of this technique, faults that cannot be resolved through resin oil plugged via or stacking via are removed. To get the high resin plugged via quality there are some difficulties like resin plugged via itself losses some features and some structural features of boards.

Here we are discussing the 18-layer high-frequency multilayer board with a dimension of 2.65 mm. The maximum layer count conforming to requirement of resin-plugged via is 18 layers and vias are designed into multiple groups with different via diameters: 0.25 mm and 0.5 mm and the highest aspect ratio can be 11:1. As PCBA has a high aspect ratio and numerous via design normal plugging via method and vias of many sizes required different pressure levels with that some problems occur like via mouth sinking, inner cavity bubbles and overflowing, As shown below.

In addition, it is common for resin cleaning to be incomplete. FS Technology recommends 1 to 2 grinding jobs to ensure a complete PCBA cleaning job. For the grinding process, FS Technology’s point of view is to set the number of grindings according to the needs of the project. Excessive grinding can lead to various common PCBA problems, such as: yield drop, board deformation, insufficient copper thickness, via fracture, etc.

Solutions

Prior to resin plugging, the circuit board has to be dried to guarantee non-existence of moisture inside via, which aims to stop separation between via copper and resin due to moisture inside via.
Before being used, resin has to be stirred and go through defoaming in order to eliminate internal bubbles inside resin and to reduce resin viscosity. Under such circumstance, opportunities will be created for resin-plugged vias with a high aspect ratio.
When vacuum plugging machines are being applied for resin plugging, small vias with high aspect ratio have to be fully plugged to stop bubbles from being generated so that quality of resin-plugged vias can be ensured.
With rein-plugged vias completed, pre-solidification has to be carried out on resin based on stage baking by abrasive belt prior to grinding. Specific baking parameters are summarized into the following table.

Temperature	Baking time
80°C	20min
100°C	20min
130°C	20min
150°C	30min

The parameters discussed in the table must be followed, some quality problems can be avoided like the separation between the resin and copper and cracks on the resin. With that, some favorable circumstances can be made for resing grinding resulting in incomplete solidification of resin with some problem’s removal like board deformation and insufficient copper thickness. In the figure below, we can see good resin plugged via having a smooth via mouth and no bubbles or cavity in via and qualified resin grinding.

Stacking Problems of High Frequency PCBA

Problem Explanation

A laminated board is commonly used in PCB-based projects. Their feature of lead-free soldering technology made use in HDI boards.

The type of PCBA we are using in this post is an 18-layer HDI PCB having two-time stacking. Through the use of a high-frequency board, numerous groups of resin-plugged buried via were created with one to eighteen layers having 0.26 mm via spacing. As a result, a weak binding force is created among the vias. Moreover, binding force between resin and prepreg sheet is so weak that lamination will be caused in the area where resin-plugged buried vias are densely placed after high-temperature soldering.

Solutions

Many components cause the lamination in places where resin-plugged buried vias have high density. To solve this problem, some solutions are material selection, PCB manufacturing, and manufacturing techniques.

The first parameters to consider existing in compatibility among the plugging oil, substrate material, and Tg and CTE. When a relatively larger distinction occurs among them, plugging oil and substrate material will get their resultant Tg temperature intervals and different levels of expansion will result in similar heating time and temperature rise speed. So the solution that exists in the optimal finding of resin for plugging based on substrate material Tg and CTE.

The limited binding forces among plugging resin and prepreg less effective adhesive in prepreg of buried via area and incomplete resing grinding, resultant lamination will result of bad binding forces among the layers. To enhance resing plugging method pre-solidification must be done prior to grinding to produce resin fully grinding prior to net solidification the resin will stop leaving. Prepreg stacking has to be designed again and prepreg containing a high content of adhesive should be applied in resin-plugged density area in order to ensure sufficient stacking flow adhesive and heat resistance of end products.

If there is high density via and the board margin gets affected by the rough drilling and milling then mechanical stress causes the lamination. Then new drilling blade and resin aluminum cover were used on the high density via points. Drilling and stacking count must be decreased, and a bit bouncing will be used with the baking board after the application of drilling. Mechanical stress must be decreased and mechanical drilling must be improved to decrease the effect on the board through the structure. Tool vias count must be shrunk and the operating life of the milling blade and stacking should also be controlled.

PCB boards will absorb moisture through the manufacturing process and absorbed moisture will be evaporated through high temperatures and get expands due to copper with high pressure. As binding forces and prepreg and copper layers are weak which caused peeling and also lamination. So throughout manufacturing process moisture absorption should be controlled and monitored regularly.

The problem of dense heat dissipation holes of high frequency PCB

Problem Explanation

Heat dissipation can not be ignored as high-frequency and high-speed multilayer boards need high frequency, high-density high integrity, and high accuracy. While comparing to ordinary multilayer boards, high density, high accuracy, and high integrity design consist of numerous components assembled through high density. High frequency, high speed, and high function design of HDI PCB famous for high power. Fewer spaces and high power will make it challenging for heat dissipation of final projects and also have an effect on the reliability of boards. On the basis of structure parameters and high frequency and high-speed operation, high-density heat dissipation hole designing must be considered. Heat dissipation holes compared to high-density metalized holes, have an effect on thin copper conduit which faces the PCB thickness direction factor, in results components heat flow to the backside of board and with speed move to other dissipation layers.

High-density heat dissipation holes face a simple process, but it is not easy to give attention to quality insurance in the PCB production process. For instance, when high-frequency and high-speed multilayer boards have a margin of high-density heat dissipation hole area with more than one thousand holes having a dia of 0.50 mm and spacing 1.2 mm, for this process, a normal drilling technique cannot be used since drilling cutting will not be removed in a respective time interval and heat generated through drill not get dissipated that will cause melting drill that has connected with the hole wall. When they get cool a huge amount of glue refuse will be created has an effect on the hole’s walls’ quality. The larger glues refuse will bock the holes and has a very bad effect on PCBs since not easy to remove these blockages.

Solutions

Normally drilling will be replaced with new drills to make drilling to minimize problems such as rough hole walls and concentrating heat produced by not good drill length, drill abrasion, and not proper cutting elimination. The value of vacuuming and suction pressure must be 0.0014MPa to 0.02MPa, which increases the quantity of drilling cutting. The resin cover is used over the normal aluminum cover, which has the ability to absorb heat produced in the drilling process, decrease drill temperatures, create drill lubrication, reduce the size of drill contaminants and enhance drilling ability.

Another solution that is good for a bit bouncing technology for high-density small hole production has drill heat dissipation time and cutting elimination time increment and problems created like cutting blocking, concentrating heat, and rough hole wall

Back-drilling problems and solutions for high-frequency PCBA

Problem Explanation

Transmission return circuit related to high-speed and high-frequency signals normally depends on copper tracing and graphics PCBAs. When copper gets punctured through the thru-hole technique return circuit will be managed with the signal falling into a mess.

For example, when a signal sends from the upper layer to some outer layers there is the creation of an additional stub that provides the electric connection. High-speed signals will be parted into 2 parts, which will be reflected to the back point when goes in the lower layers, and the other portion follow the normal path to the inner circuitry.

Different related to phases of 2 categories of signals cause the production of the resonance due to interference at frequency points. Resonance increases insertion losses which are ambient to resonance frequency and has an effect on signal transmission. If the stub is longer the larger capacity will create a lower resonance frequency that will reduce the lower transmission quality of the signal. In response to this problem, FS Technology can provide three solutions: reducing the thickness of the substrate material, placing high-speed signals on the bottom layer of the circuit board, and using back drilling.

Till now, we learned that back drilling is a low-cost high frequency and high-speed PCBA construction technique that has the ability to fulfill the required features. Everything has advantages and disadvantages. In the actual High frequency PCB manufacturing process, due to the limitation of the back-drilling structure, manufacturers will have quality issues such as inner hole metal, hole plugging, and cutting drilling in production.

The ordinary procedure pre-process→board plating→external graphics→graphic plating→external etching→back drilling→post-process tends to call for issues like internal via burr and copper wires. During hole drilling electro-copper at the hole, wall causes weaker binding forces than RA copper for substrate material having copper on the surface, hole copper causes peeling in the procedure of hole drilling, consequence of via burr and copper wires.

Solutions

There is the use of a drill having a good angle and manufacturing requirements related to drilling angle that results to reduce burr occurring in the internal side of the hole due to insufficient angle, unstable rotating speed, and rough cutting feature. So there is a need to use this process, pre-processing, board plating, outer graphics, graphics plating, back drilling, outer etching and post-process. In the below figure, you can see backing the drilling sample.