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Resin Coated Copper Foils For Building Up PCB

Posted date: 2007-2-2 17:12:07 View No.: 274

Resin Coated Copper Foils For Building Up PCB

Eric Chang
Nan Ya plastics corporation, Electronics material Div. Technical Department of Copper Clad laminate

Introduction

The trends of electrical unit is smaller and lighter, (e.g. cell phones, notebook computers etc) and PCB toward to high density. So the Build up process will be growing fast in this field. The high density integrate substrate uses the IVHs (interstitial via hole) to link the layers. That can let the substrate posses lighter weights and higher electrical properties. There are many materials used in Build up process including liquid photovia、Thermount、glass prepreg and RCC. Considering the reliability, the RCC has became the main stream in the Build up process during these years. NAN YA has developed resin coated copper (NPRCC) for use in Build-up process. NPRCC is extremely easy to make IVH by CO2 laser because they do not contain any glass fibers. Furthermore, NPRCC offer the following advantages 1) thin dielectric layer 2) even surface 3) the ability to fill holes 4) high Tg. These excellent characteristics make it easy to produce fine lines for Build up PCB.

Resin Coated Copper

Conventional FR-4 prepreg contains glass fibers serving the function for reinforcement. Glass fibers can also hold the resins from flowing to avoid resin squeeze out in lamination process. RCC doesn't contain glass fiber, so the resin system must be modified by some toughener. The compositions of RCC resin are epoxy resin along with some hardner, accelerator and toughener. The resin will be coated on the HTE copper evenly by Die Coater. During the oven, the RCC materials will proceed the semi-reaction and become the B-Stage. The construction of RCC is shown in Figure 1.

12or18μm copper

60~80μm Resin

Fig1 Resin coated copper

Test Item	Condition	unit	RCC
Copper	-	μm	12 , 18
Resin Thickness	-	μm	60±5 , 80±5
Resin Flow	IPC-TM-650	%	20±5 , 30±5
Volatile	IPC-CF-148	%	Less than 0.6
Table 1 B-stage properties

General Properties of RCC Materials

Using high Tg materials in the build-up process will increase the reliability and, the glass transition temperature (Tg) for Nan Ya RCC material is 180℃. The dielectric constant is between 3.6 ~ 3.8 dissipation factor is between 0.02 ~ 0.04. The heat resistance is over 120 seconds. Cured properties of RCC are shown in Table 2 below:

PROPERTIES		CONDITION	UNIT	NPRCC	A
	Tg	TMA	℃	150～155	120~130
	Tg	DMA	℃	180～185	150~160
	CTE	-	ppm/℃	60～90	50~90
	Surface Flatness	-	μm	2～3	2~3
	Flammability	(UL94)	-	V- 0	V- 0
	Peel Strength	18μm	Lbf/in	7～8	7~7.5
	Heat Resistance	288℃	Sec	120<	120<
	Dielectric Constant	1MHz	-	3.6～3.8	3.8~4.0
	Dissipation Factor	1MHz	-	0.02~0.04	0.02~0.04
	Surface Resistance	A	Ω-cm	1.0×10¹⁴	2.0×10¹⁴
	Volume Resistance	A	Ω	3.0×10¹⁵	3.2×10¹⁵
Table 2 Cured properties of RCC

Figure 2 Dk vs. Frequency

Manufacturing Process of PCB's

Figure 3 compares the new manufacturing process using RCC material with the conventional PCB manufacturing process. Process A use CO2 laser to drill IVH holes. Holes drilled using this method have a much smaller diameter than those drilled by mechanical method of process B. The smaller IVH allows for a high density wiring pattern. Furthermore, the thickness of the outer insulation layer in process B exceeds 50 μm, while process A can control the thickness to below 50 μm. So we can get thinner PCB by using RCC materials.

Fig 3 Manufacturing process of Six-layer PCB with IVH

Lamination Conditions

In general, the press conditions for RCC materials are similar to standard FR-4. However, certain relevant press conditions will need to be modified while pressing the RCC materials. The following are specific changes to the PCB lamination process for use with RCC materials:

	Heating rate of material between70℃and 140℃, 1～3℃/min is acceptable
	Temperature of material over 170℃ must be held for at least 40min to allow epoxy resin to fully cure
	Cooling rate of material should be kept under 2.5℃/min when the temperature of material is over 100℃, in order to avoid introducing twist
	A vacuum of less than 50 torr will be needed to eliminate void

Figure 4 Lamination Conditions

Filling Ability

Nan Ya RCC material also has excellent fill-in-the hole capabilities during lamination.

Figure 5 cross section of 39 mil. 1/1/ core. Hole diameter: 0.3,0.4,0.6 mm

Board Size 406mm’508mm
Hole Number 1200
Inner Layer Cu Thickness 35 μm

RCC	T80
CORE THICKNESS	1.0 mm
HOLE DIAMETER	0.3 mm			0.4mm			0.6mm
HOLE PITCH	1.27	1.9	2.54	1.27	1.9	2.54	1.27	1.9	2.54
FILLING ABILITY	OK	OK	OK	OK	OK	OK	OK	OK	OK

Table 3 Fill-in-the Hole Capability

Process of Laser IVH

The use of lasers contributes to three possible reactions: reflection, absorption, and transmission. Laser drilling is directly related to absorption. Copper foil, epoxy resin, and glass fibers all have high absorption in the UV range of <0.3μm. However, the ability of both copper foil and glass fibers to absorb lasers decreases in both the visual and infrared spectrum. Of these three materials, only epoxy resins can maintain good absorption properties in the three light spectrums. What differentiates RCC material from other prepreg material is its lack of glass fiber reinforcement. This allows CO2 lasers to be used in drilling small IVH in RCC-based material. Drilling in this manner requires that certain steps should be taken:

Conformal Mask

CO2 Laser Drilling

Desmear

plating

Figure 6

A: cross section of IVH

B: SEM of Microvia

C: cross section of IVH & TH

The conditions for laser drilling should be modified according to the set-up of each PCB process, dielectric thickness, desired size of each hole, and any other special factors that need to be taken into account. Typically, CO2 lasers can drill holes with a diameter of 6-8 mils within 3 or 4 shots given a dielectric thickness of 2 or 3 mils.

Desmearring has a large influence on the reliability of IVH. When laser drilling is used, small amounts of resin will be left on the target pad. Furthermore, the surface of the hole will have burnt filings. Both of these can be eliminated through good desmearring technique to ensure better IVH reliability.

The thickness of the insulation will determine the conditions for setting-up the laser drill and may impact the quality of the hole drilled. For instance, should the insulation be too thick, the quality of the hole drilled will be degraded. If the insulation is too thin, the target pad will experience delamination. A good thickness distribution is critical to control the quality of the hole drilled. There are two essential ways that thickness can be controlled: 1) to control the thickness of the raw material and 2) to control set-up of the lamination conditions.

Figure7 distribution of RCC thickness

Characteristic

Reliability of RCC by build-up process is shown table4

Item	Test condition	Characteristic
Thermal shock(hot oil)	260℃,10sec oil?20℃,10sec water	50cycle→OK
Thermal shock MIL	-65℃,30min?125℃,30min	500 cycle→OK

Table 4

Conclusions

There are many advantages to use RCC material in the PCB Build-up process.

1.High Tg and good peel strength provide better reliability.

2.NPRCC material is a resin base that is easy to drill by CO2 laser.

3.Low dielectric constant helps to increase transmission speeds.

4.Even surface improve etching of fine lines and spaces.

5.Filling ability and resin flow is excellent during lamination.

6.RCC can use in field of high interconnect density.

Source: Posted by: peng

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