Modified Semi-Additive Process Introduction

30 Technical Paper

Journal of the HKPCA / 2018 / Summer / Issue No. 68

Liu Binyun / Ye ShaomingGuang Dong Toneset Science&Technology Co., Ltd.

Abstract:

Key words:

The light, small and multi-functional electronic products have

led to the development of the printed circuit board on which the

lines and vias depend on the high density interconnection, but it

still cannot meet the requirements of the future development.

With the advance of the 4G LTE and the development of the

future 5G, low loss of signal transmission is required, and the

shorter line is better. If the circuit board can achieve the same

density as the IC board, and it is possible to load the IC chip

directly on it. So the concept of the IC substrate like printed

circuits board appears, but the manufacturing of this kind of

board cannot be like the high cost of the IC substrate board

manufacturing. Therefore, the mSAP new technology will be

paid attention and developed. In this paper, the manufacturing

process and requirements of mSAP are discussed, for the

reference of the workers in PCB industry.

Printed Circuits Board (PCB), High Density Interconnection PCB,

Substrate Like PCB (SLPCB), modified Semi-Additive Process

(mSAP).

With the advent of the 5G era, mobile terminals, especially

smart phones, tablet PCs, and wearable devices, have become

smaller and more versatile. Although the size of mobile phones

has not changed significantly, it is clear that components and

as PCBs continue to smaller; the level of integration has

increased significantly to accommodate greater functionality. In

a typical smart phone, most of the space is occupied by display

screens and batteries, and the remaining electronic devices

have been reduced in size and integrated into small areas,

1. Background

resulting in PCB size shrinking, high-density interconnect (HDI)

Printed Circuits Board manufacturing technology continues to

increase. PCB wire width and spacing are further narrowed to

accommodate more components; on the other hand, 5G's

massive processing capacity requires more battery capacity, as

well as more sophisticated processors and components, higher

resolution displays, etc., need to reduce PCB size to the battery

and the display. Therefore, the line width/spacing of the mobile

PCB has dropped to 30/30 m, and is expected to gradually

decrease to 20/20 m . At the same time, the size of the micro-

via/via-pad will gradually decrease from 75/200 m to

50/150 m . With the increase of wireless data transmission

bandwidth and processing speed, the signal trace density is

continuously increasing, if the pitch of the line is less than 65ìm,

it has been very difficult to produce such a line by the traditional

subtractive method. Although the semi-additive process (SAP)

used in the IC substrate board can realize more precise circuit

fabrication, but there is a problem that the manufacturing cost

is high and the production scale is small, this resulted in the

concept of a IC Substrate Like PCB (SLPCB). Therefore, it is

necessary to select a suitable process for fabricating a SLPCB,

this process must be able to meet the requirements for

electroplating and filling holes, and at the same time, fine lines

can be produced and improved. Modified Semi-Additive

Process (mSAP) can achieve this goal.

Apple leads the market in the direction of SLPCB. Typical

products are the SLPCBs used by iPhone 8 and iPhone X. At

present, the minimum line width/spacing manufactured by

mSAP technology is 30/30 m. The typical laser drilling

diameter is designed to be a micro blind hole with diameter of

70 m and thickness of dielectric layer of 50 m. CO laser

µ

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µ

µ

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µ µ

[1 ]

[2 ]

2

31Technical Paper

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drilling will still dominate for a period of time in the future. Status,

in the processing speed, drilling quality and production

efficiency is very advantage . The SLPCB produced by the

mSAP manufacturing process is generally a laminated structure

of micro-via holes or micro-via superimposed. For example, the

process of a ten-layer board is promulgated as follows:

The SLPCB is still a kind of hard board in the PCB, but it is

closer to the IC substrate. Currently, the line width/spacing of

the SLPCB required is becoming smaller and smaller. This

technology will reduce the occupied area of the mobile phone

mainboard, thereby increasing the space of the battery and

making batteries last longer, and also adapted to System in

Package (SiP) technology .

[3 ]

[3 ]

Figure 1. Typical SLPCB Process Design

2. Modified semi-additive process introduction

With the development of electronic interconnection, Integrated

Circuits have been widely used to mount integrated circuit

chips and other electronic components on carrier printed circuit

boards (PCBs), wafers and interposers are required. The

manufacturing of integrated circuit substrate (IC Substrate)

requires interconnection and intercommunication between lines

during assembly, and the size of the pitch varies greatly. The

micro-scale dimensions and application objects are as follows:

Currently, circuit board manufacturing generally adopts

subtractive method. Subtractive methods for producing fine

lines will face restrictions on copper thickness (base copper

thickness is generally 1/2-2 oz) and copper thickness deviation

Figure 2. Circuit width design and application

Table 1. Comparison of Different Addition Methods and Their Performance

Specific substrate ABF substrate Common substrate

None None Yes

Pattern copper layer Panel plating copper + pattern Base copper layer + Electroless

plating layer Copper layer (possibly with flash

Plating) + pattern plating

0.5 m~6.0 m above 1 m 0.3 m~1.5 m

None Panel plating copper layer Base copper + panel plating copper

layer (possibly with flash plating)

adhesion is poor Resin surface roughening or The resin is connected to the base

Molecular interface technology copper and has good adhesion

to improve adhesion

Explosive plate is easy to There is a risk of stratified Good reliability, can meet

delamination, poor blasting and prone to poor the requirements

thermal stability anodic ion migration

No etching, less pollution Micro etching, less waste Low etching, low wastewater

water discharge discharge

Low High Medium

µ µ µ µ µ

Technology AP SAP mSAP

Substrate

Base copper

Copper layer

Electoless Copper

Etching thickness

Adhesion

Reliability

Environmental impact

Manufacturing cost

32 Technical Paper

Journal of the HKPCA / 2018 / Summer / Issue No. 68

(uniformity of plated copper plating). Base copper, electroless

copper and electroplating copper is needed to be etched away.

The thickness of the etched copper is generally more than

20 m, so the line side etching is relatively large. It is very

difficult to produce a line width/space below 50 m, and the

yield rate is low; to meet the needs of high-speed signal

transmission and miniaturization, if the PCB line is 30 m or less,

then the IC substrate can be omitted directly. This poses a

challenge to the traditional PCB manufacturing process and

requires the introduction of new additions process.

It can be seen that the selected semi-additive method

(hereinafter referred to as mSAP) material is available, the

process is similar to the existing PCB manufacturing, the

product reliability is good, the manufacturing cost is moderate,

based on the above analysis, mSAP method is selected for

manufacturing, and the fine line SLPCB is the best choice for

PCB development.

The so-called mSAP has no fixed standard process, diverse

process can be choosing due to different orders, equipments,

and raw materials. The most typical process flow and

requirements are as follows:

Because 3 m copper foil is more expensive, and some

factories directly try to use 1/3 OZ copper foil and then reduce

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µ

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µ

to 3 m, so that the manufacturing cost is lower, so this article

will introduce the mSAP process and requirements according to

process 1, the flow diagram is shown As in Figure 4.

The first UV laser drilling rig launched in 1997 was a 355 nm

wavelength Nd:YAG. By the mid-2000s, industry-leading PCB

manufacturers began to develop CO laser drilling, which first

reduced the thickness of copper to 5 m to 12 m, and use the

black oxidation or brown oxide before the drilling to roughen

and darken the copper surface and directly drill the copper foil

with a CO laser. Due to the need to make fine PCB traces for

the SLPCB, if we use 1/3-1/2 oz base copper, before laser

drilling, it is necessary to reduce the thickness to 3 m, laser

drilling has the requirements of the aperture and roundness, so

the uniformity of copper reduction is particularly high.

The technical advantage of this laser direct drilling formation is

the reduction of the copper window etching step and the cost

is significantly reduced, this is the main method used today for

the mass production of the blind vias. However, the

disadvantage of this method is that the processing window is

narrow and cannot be reworked. From the quality point of view,

it is a huge challenge to stabilize the production of micro blind

vias with a volume of less than 70 m. Because defects such as

copper overhang, glass fiber protrusion, and resin residue will

lead to quality problems in the subsequent desmearing and

µ

µ µ

µ

µ

3.1 Roughening and laser drilling

2

2

3. mSAP process requirement and control

Figure 3. Typical mSAP flow

Figure 4. mSAP process flow diagram

33Technical Paper

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electroplating processes, these micro blind holes smaller than

100 m must be optimized for the hole shape, remove the

copper over the hole, and eliminate Glass fiber protruding and

resin residues and other defects.

For the current chemical copper reducing technology, it is

difficult for the electrolytic copper foil to obtain a uniform thin

copper and a better hole type. First of all, it is difficult to

manage the uniformity of copper thickness 3.0 0.5 m by the

normal copper reducing chemical, the best way is to use the

brown oxide process to directly reduce the copper, and then

with the uniformity design of the device, copper thickness after

the reduction of copper can be controlled within 0.5 m. At the

same time, the conventional browning has a specific surface

area of about 1.5. However, the 3 m ultra-thin copper foil may

have a specific surface area of 2.2-2.4 directly treated by a

low-etch LDD brown oxide process, and it also matches for

subsequent etching, smaller side etching, the side wall of the

circuit is very vertical, not cause under-cut, can get the good

line profile.

As can be seen from the figure below, the 3 m ultra-thin

copper foil with low-etch mSAP brown oxide is used together

with the low etching, the copper surface is villous fluffy, while

the copper surface treated by traditional browning solution is a

honeycomb structure.

Making hole conductive of mSAP uses generally a horizontal

PTH, it can deposit a thin electroless copper layer to achieve a

good coverage of the insulating layer in the blind via, which

µ

± µ

± µ

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3.2 Electroless Copper

reduces the under-cut of the PTH layer during subsequent

etching and reaches the sidewall of the circuit vertical purpose.

The current blind via diameter is about 50~80 m, and the

dielectric thickness is 30~50 m. There is enough metallization

ability for the current horizontal PTH, but if the via pattern of

laser drilling is not good, or there is excessive biting by the next

steps, the PTH is required to have sufficient coverage to

prevent voids under the over-hang and crab legs, as shown in

Figure 7 below.

However, it should be noted that high-frequency materials are

more difficult to remove the drilling smear, increase the amount

of bite and fear of wicking problem, which will be fatal to signal

transmission, and therefore need to have a better desmearing

µ

µ

Copper foil type & B/O Etching rate of Specific

Brown Oxide surface area

Ordinary copper foil with 1.4~1.8 m 1.4 ~ 1.6

normal LDD B/O

3 m ultra-thin copper foil 0.6~0.8 m 2.2 ~ 2.4

with LE mSAP B/O

µ

µ

Table 2. Comparison of effects of different browning treatments

Figure 5. SEM results of low etching brown oxide of mSAP

Figure 6. SEM results of browning after general brown oxide treatment

34 Technical Paper

Journal of the HKPCA / 2018 / Summer / Issue No. 68

effect. On the other hand, quality of the board surface after

electroless copper is the key. If the depositing copper layer is

rough and there are residues or copper nodule on the surface,

it will directly affect the effect of electroplating, which is a fatal

flaw for the fine lines. In addition, the low stress copper is

generally used. Series potions to maintain a good adhesion of

the copper sink layer to the substrate.

Flash Plating, also known as "Cu Strike," is mainly for the

purpose of protecting the electroless copper layer from the

pattern transfer process, and can be microetched and

roughened before the subsequent pattern via-filling plating to

improve the copper layer inter-binding force.

The choice of flash plating solution and the thickness of copper

plating are all important. Generally, the solution with common

through holes and blind via plating capacity is selected. On the

one hand, the throwing power of the blind hole is better, and

the bottom of the blind via can be coated with arc-shaped

effects. "Crab legs" (incomplete copper plating or thin copper

thickness at the bottom corner of the blind via) are not allowed;

at the same time, the throwing power ability of the through hole

can be taken into account, in particular, the knee of the through

hole cannot be cut.

Copper thickness control is an important part of the mSAP.

How much copper is plated and how much copper is etched in

each stage must be accurately calculated to achieve the goal of

3.3 Flash plating

final copper thickness. Accordingly, the thickness of the flash

copper plating layer is generally 1-3 m or 5-7 m or so, and

can adjust according to requirements.

Graphics transfer includes pre-treatment, dry film image,

development steps.

Pre-treatment generally uses chemical micro-etching on flash

plating cooper, and common acid rinse is mainly used to high-

build electroless copper. Flash plating generally uses medium

roughening (flash-plated copper layer thickness is 1-3 m) or

super-roughening (the thickness of flash-plated copper is

5-7 m). The medium roughening can use hydrogen peroxide

type or composite salt series of micro-etching agents, and

there are also selective CuCl super-roughening agents for the

thickness flash copper. The overall point of view is the control of

adhesion and total copper thickness.

Fine pitches and hole rings require more rigorous control of the

graphics transfer process. For fine lines, rework, repair, etc.

cannot be accepted. If we want to get a higher yield, we must

pay attention to the quality of the graphics tools. The use of

laser direct imaging (LDI) to replace contact film exposure mode

is increasingly introduced for the manufacture of SLPCB.

Although LDI is inefficient and costly, but it has the advantage of

allowing each PCB board of different increases and decreases,

which will reduce scrap due to misalignment.

The development solution is nothing special, it can be prepared

with Na CO . The equipment has particular attention, to achieve

such a fine resolution, how to avoid the "pool effect" is the key.

The industry has a vertical developing machine, which has a

good development effect.

The pattern plating of mSAP is a pattern via filling plating

µ µ

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3.4 Graphic Transfer

3.5 Pattern Plating

2

2 3

Figure 7. Horizontal electroless copper plating coverage

35Technical Paper

www.hkpca.org

technology. This via filling process is a high-tech process, it

breaks the regularity of the distribution of the traditional

electroplating current field, and changes the difference in current

distribution between the plate surface and the hole, the copper

plating speed in the hole is fast. On the surface of the board, the

recessed holes are filled. For blind vias with a diameter of about

50~80 m and a thickness of 30~50 m, microcavity-free in the

filled via,the dimples and protrusions of the filling holes should

be less than 5 m, and it must control the copper thickness after

filling, the scope of the requirements, rather than the thinner the

better. On the other hand, the inhomogeneity of the graphics will

result in inconsistent distribution of the current density in the

plane direction, to make the surface copper plating uniform, the

requirements for equipment and chemical are even higher. The

thickness difference between the dense circuit area and the

sparse area of the SLPCB is guaranteed within 5 m, and there

is no phenomenon of film clamping. Moreover, the surface of the

circuit needs to be flat and straight, and cannot assume an arc

shape of the trace line surface. The arc rate of the line surface

requires 20%.

In addition, the quality of this copper platting is strict, because

the line width is only about 30 m, plating can not have any

pitting & dent, wrinkle and rough, even a pinhole on the circuit

will cause scrap. Special attention is paid to the fact that the

microcavity caused by subsequent etching is not allowed,

maybe the plating is too large crystal.

µ µ

µ

µ

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3.6 Film stripping

3.7 Quick etching

The traditional NaOH cannot meet the requirements of fine line

film stripping. The need to remove the dry film between fine

lines requires the use of a special stripper, usually an organic

amine or solvent type, which can effectively strip the 20 to

30 m pitch dry film. There must be no residue of dry film debris

on the board after stripping. In addition, there is a higher

requirement for film stripping equipment and filtration effect,

and prevent the film from sticking to the surface of the board.

Figure 9 shows the effect of removing the film with organic film

stripper:

The quick etching is also called "Flash Etching", the etching

adopted by the mSAP process is completely different from the

traditional one. It does not have dry film or tin plating as the

resist layer. The whole board of the SLPCB after the film

stripping is only copper, but the height difference is not the

same, the copper thickness of the circuit surface is more than

is more than 25 m and the copper thickness between the lines

is only a few microns, so the differential etching method is used

to quickly etch away the copper layer between the lines, as the

same time, the copper on the line surface also erodes the same

thickness copper, it just comes back need to control the finish

line copper thickness.

µ

µ

Figure 9. The effect of removing the film of fine line

Figure 8. mSAP via filled by pattern plating

36 Technical Paper

Journal of the HKPCA / 2018 / Summer / Issue No. 68

Hydrogen peroxide-type etching solution are also commonly

used in mSAP, and also known as "flash" agents. The main

reason is that the vertical downward etching rate is controllable,

and the line side etching is smaller, and the ratio of down

etching to side etching is larger than 1:0.6. Strictly control the

line type, the circuit etched out must be straight, the top line

width and bottom width of the entire production board is almost

the same, to ensure the required signal impedance and

insulation resistance, so that the transmission loss and

crosstalk are relatively low, the signal to noise ratio is high, the

signal integrity is good.

With the rapid development of mobile communications, the

development of compact and multifunctional portable and

wearable devices is still the mainstream, resulting in the

development of traditional high-density interconnect (HDI)

boards toward the Substrate Like PCB (SLPCB). The result is

mSAP technology application and development.

This article through the typical SLPCB manufacturing process

elaboration, mSAP process and requirements have a clear

outline, but after all, different SLPCB have different designs and

requirements, mSAP process will also be different; mSAP also

due to the current technology base of different companies, from

resources and with the continuous development and progress

of various processes, will continue to improve and improve in

the mass production. It is hoped that the majority of workers in

PCB industry will continue to work hard to improve the mSAP

process capability, to improve the quality and yield of SLPCB

4. Conclusions and Prospects

Figure 10. The geometry of the conductors after quick etching

products and pave the way for the early realization of new

technologies such as Internet of Things and 5G.

[1] Meny Gantz, mSAP: An Indispensable New PCB

Manufacturing Technology for 5G Smart Phones [J]. Hong

Kong Circuit Board Directory, 2017/Winter, (66): 16-18

[2] Erkko Helminen, Tatja Rapala-Virtanen, Miniaturization

Influence on Electrical Functionality of High Density Printed

Circuit Board in press ECWC13, 2014

[3] Wu Jinhua. The Challenge of the Next Generation of Ultra-

thin HDI Printed Circuit Boards [J]. Printed Circuit

Information, 2015, (6): 45-49.

[4] PCB Info, an article to understand PCB technology

changes and market trends [EB/OL].

http://www.pcbinfo.net/InfoShow.asp?cls=100&ID=208578,

2017-06-02

[5] PCBTech.Net, a paper that reads the status quo of major

PCB countries in the world [EB/OL].

http://www.pcbtech.net/view/analysis/19567.html, 2018-

06-07

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