GB2148794A - Method of making a laminate - Google Patents

Abstract

Method of making a laminate wherein an amide or crystalline polyester fabric or mat is impregnated with a resinous epoxy or polyimide resin. Microcracks which form in the laminate as a result of thermal cycling, are reduced by subjecting the fabric or mat, prior to impregnation, to an active oxygen- or nitrogen-containing gas plasma for from 1 to 40 minutes at an RF power to flow rate ratio of from 1 to 4, a gas pressure of from 0.1 to 5 mm, and a frequency of from 100 Hz to 1 GHz. <IMAGE>

Description

SPECIFICATION Method of making a laminate This invention relates to a method of making a laminate. Copper clad laminates for use in making printed circuit boards consist of very thin copper foil bonded to one or both sides of a laminate made of fabric or mat imbedded in an organic resin. Printed circuit boards are made from the copper clad laminates by selectively removing portions of the copper to form electrical pathways, then mounting integrated circuits and other electrical components to the board.

It has been found that failures in printed circuit boards are sometimes due to very tiny cracks ("microcracks") which form in the printed circuit board. These cracks can fracture the copper electrical pathway or permit the entry of moisture which can delaminate the copper or create a short circuit. As the size of the components and the width of the copper electrical pathways have been reduced year after year, printed circuit boards have become more and more vulnerable to failure due to microcracks. This is especially a problem for printed circuit boards used on high flying aircraft, which may alternate between very hot temperatures on the ground and very cold temperatures at high altitudes. This thermal cycling greatly increases the formation of microcracks and the likelihood that the circuit board will fail.

In view of their light weight, the laminates are also used as structural components on aircrafts. The formation of microcracks in a structural part can act as a site for the propagation of the crack, leading to the failure of the part under mechanical stress.

For these reasons reducing the formation of microcracks in laminates is regarded as a problem of considerable importance.

According to the present invention a method of making a laminate comprises impregnating an amide or crystalline polyester fabric or mat with a liquid organic resin and forming said impregnated fabric or mat into a a laminate, the fabric or mat being subjected prior to impregnation to an active oxygen or nitrogencontaining gas plasma for from 1 to 40 minutes at an RF power to flow rate ratio of from 1 to 4, a gas pressure of from 0.1 to 5 mm, and a frequency of from 100 He to 1 GHz.

We have discovered that microcracks in certain types of laminates which are commonly used in printed circuit boards and in structural components on aircraft can be greatly reduced if the fabric or mat from which the laminate is formed is subjected to a gas plasma under certain critical parameters prior to incorporation in the laminate. While we do not wish to be bound by any theories, we believe that exposure to the gas plasma at the critical parameters produces a chemical bonding between the fabric and the resin which results in a remarkable improvement in the shear strength of the laminate. As a result, both the size and the incidence of the microcracks is reduced.

In the method of this invention a reinforcing sheet or substrate, typically a fabric or mat, is treated with a gas plasma prior to being impregnated with a resin and formed into a laminate. The fabric or mat can be formed from any amide or crystalline polyester fiber. These include polyethylene terephthalate, butylene terephthalate, poly (p-phenylene terephthalamide) and nylons such as nylon 66, nylon 6, and nylon 12.

Aramides, such as poly (p-phenylene terephthalamide) are preferred as they are the most commonly used substrate in printed circuit boards which are subject to the microcrack problem. The fibers, which are typically oriented and highly crystalline filaments having a diameter of from 1 to 20 microns, are spun to form yarn which is weaved to form a fabric or is laid to form a mat. The process of this invention is applicable only to fabric or mat, not to fibers, as more material can be treated with a fabric or mat and a treated fiber could not be used soon enough after a plasma treatment to prevent the plasma treatment from attenuating.

The fabric or mat is treated by being placed in a capacitively coupled gas plasma unit. The plasma unit is capacitively coupled rather than inductively coupled as capacitive coupling is believed to form a more uniform radio frequency (RF) field. The plasma may be formed using any inorganic gas that has an active oxygen or nitrogen, such as SOP, NO2, NO, N2, OPI or NH3. Nitrogen, oxygen, or ammonia are preferred, and oxygen is the most preferred as it has been found to work the best.

The ionization of the gas must occur under certain critical parameters if the plasma treatment is to be effective. First, the radio frequency power (in watts) to flow rate (in cc per minute) ratio must be about 1 to about 4 as if the ratio is less than 1 the gas has very little reactivity, and if the ratio is greater than 4 the fabric may be destroyed. The gas pressure should be from 0.1 to 5 mm. Pressures of less than 0.1 mm result in a low reactivity gas and at pressures of over 5 mm the fabric begins to deteriorate. The radio frequency of the gas plasma should be from 100 Hz to 1 GHz because at less than 100 Hz very few ions are produced and at over 1 GHz the fabric is destroyed. The fabric should remain in the gas plasma for from 1 to 40 minutes. Less residence time is ineffective and a greater residence time does not produce any additional beneficial effect.

Finally, the treated fabric is preferably incorporated within the laminate within about 1 month after treatment with the gas plasma as the effectiveness of the treatment may tend to diminish after that time.

A A sheet suitable for a laminate is produced by impregnating the treated fabric with a liquid organic resin.

Typically, the sheet, and laminates made from a plurality of sheets, have a resin content of from 35 to 60%.

The resin may be liquefied by heating, or it may be a liquid at room temperature, or it may be a solid dissolved in a solvent. Resins which have a fusible B-stage are most frequently used. Resins which are applicable in this invention include epoxy resins and polyimide resins. Polyimide resins include polymaleimides, amide-imides and polyamide-imide resins. Epoxy resins are preferred as they are used most frequently in printed circuit boards which are subject to microcracking.

The laminate is ordinarily produced by solidifying a plurality of the resin impregnated sheets, using heat and pressure. While the laminate can be produced in any shape, thinner laminates are more subject to microcracks and receive a greater benefit from the process of this invention. If the laminate is to be used to form a circuit board, thin sheets of copper are bonded onto one or both sides of the laminate. Printed circuit boards used for very high speed integrated circuits (VHSIC; are particularly benefitted by the treatment of this invention as the components on these circuit boards are very close together and they are very vulnerable to microcracking. It should be understood that a single sheet of a resin impregnated fabric or mat may be employed instead of a plurality of laminated sheets.

The invention will now be illustrated with reference to the following Examples:.

Example 1 Pieces of a 352 weave style fabric, supplied by Clark-Schwebel, under the trade designation "Kevla r CS800" believed to be poly (p-phenylene-terephthalamide), were tested accoding to ASTM D-1876. Two pieces of the fabric were bonded together using a polyamide-cured epoxy resin supplied by Hughson Chemical under the trade designation "Chemlok 305". At least eight replicate samples were evaluated for each treatment condition. A capacitively coupled oxygen plasma unit supplied an oxygen plasma at 0.7 mm, 100 watts of RF power, and an oxygen flow rate of 30 cc/minute. The fabrics were exposed to the oxygen plasma for various times then bonded together with the resin which was then cured for one hour at 1 00'C.

The samples were allowed to cool and equilibrate at ambient conditions for at least 16 hours before testing.

The two pieces of fabric were then pulled apart and the peel strength was measured. The following table gives the results: Duration of Exposure Peel Strength Sam pie No. to Plasma (minutes) Ubs in) 1 0 6.24 + 0.93 2 10 7.06 - 0.79 3 30 8 24 0.88 4 4 60 8.16#0.70 The above table shows that adhesion increased with exposure to the oxygen plasma up to about 30 minutes.

Example 2 An epoxy resin was formulated from 100 pbw diethylene-triamine and 100 pbw of a liquid epoxy resin sold by Dow Chemical Company under the trade designation DER 332, believed to be pure diglycidyl-ether of bisphenol A having an epoxy equivalent weight of 172 to 176. The Kevlar fabric described in Example 1 was exposed to the oxygen plasma described in Example 1 for 30 minutes per side and then was impregnated with the epoxy resin formulation. Fifteen plies of the impregnated Kevlar " cloth were then stacked on top of each other and laminated in a press at 188#F, 100 psi, for 10 minutes. The laminate was allowed to cool to room temperature and equilibrate for 16 hours at ambient conditions.Test samples 1.75 inches by 0.250 inches were then cut from the 0.250 inch thick laminate and tested according to ASTM D-2344. A set of control samples, which had not been treated with the oxygen plasma, were also tested under the same conditions. The test consisted of placing the pieces of laminate, between two beams and measuring the force required by a third beam pressing in the middle to break the laminate. The following table gives the results: Oxygen Plasma Control Treated I Ipsil (psi) 3690 4872 3400 5050 3580 4940 3555 4810 3000 4810 4110 4850 3570 4700 3850 4890 3290 4724 3780 4630 Ave t (r 3583 t 9% 4828 I 3% The above table shows that those samples treated with the oxygen plasma had a significant increase in short beam shear strength.

Example 3 The Kevlar fabric described in Example 1 was treated with the oxygen plasma described in Example 1 using 100 watts RF power for 30 minutes and 30 cc/minute flow rate. After treatment the fabric was dipped in an NEMA Grade FR4 epoxy resin employed by Westinghouse for commercial laminates. The treated fabric plies were then B-staged in a hot air oven at 303 F for 7.5 minutes and were laminated together to form a composite board. The same procedures were followed using untreated Kevlar fabric. Prior to resin impregnation each ply of Keviar fabric, both treated and untreated, was weighed and its weight recorded.

After each laminate was cured it was weighed and the resin content calculated using the following equation: n Weight of Laminate - s (Fabric Weights) 1 x 100 Laminate Weight The resin contents of both cases are given in the following table: KevlarQ' Treatment Resin Content O2 Plasma Treatment (30 ply) 45.6% Untreated (30 ply) 37.7% The above table shows that there was an 8% increase in resin content for the laminate prepared from the oxygen plasma-treated fabric when compared with laminates prepared from untreated Kevlare fabrics.

A112"x A -80~Cto150'Cfor30cycles,inorderto evaluate the effect of oxygen plasma-treated fabric enforcement on the microcracking of epoxy, Kevlare laminates normally encountered with the FR4 resin system. Scanning electron microscopy was used in the study to evaluate the number and severity of microcracking. At 110 times magnification no cracking of the oxygen plasma through the Kevla laminate was detectable, but the cracks were clearly visible on the untreated laminates. At 550 times magnification some microcracks were evident in the treated laminates as well as in the untreated laminates, but crack formation was just beginning in the plasma-treated laminate.

Also the extent of cracking in terms of size and number was not as severe in the oxygen plasma-treated laminate as in the untreated laminate. Figures 1 and 2 are scanning electron micrographs at 550 times magnification for the laminate prepared from the untreated and treated fabrics, respectively. The lines designated A and B in Figures 1 and 2, respectively, are the microcracks.

Example 4 Kevlar# laminates are notorious for their hydroscopic nature. Because of this, Kevlar laminates usually undergo severe degradation of mechanical properties in humid environments. In order to test the effect of oxygen plasma treatment of Kevlar on the mechanical properties of Kevlar epoxy laminates, laminates molded using both oxygen plasma-treated and untreated Kevlar as described in the previous Examples were subjected to 5-day water-boil test. After this severe exposure, the laminates were tested for interlaminar shear strength using procedures given in ASTM D-2344. A control sample set for each laminate (no water boil) was also tested at this time.The following table gives the results: Shear Strength Kevlart' Treatment Water Boil Epsi + tr 2 Plasma Treatment No 4814 + 7% O2 Plasma Treatment Yes 5030 I 6% Untreated No 3892 I 8% Untreated Yes 3240 I 27% The above table shows that a severe degradation in shear strength due to the weakening of the Kevlar" fiberiresin interfacial adhesion is evident in the untreated Kevlark ' reinforced laminate but not in the oxygen plasma-treated Kevlar~/epoxy laminates.Further evidence of a weakening of the interlaminate adhesion after the water boil is the tremendous increase in data scatter, as measured by the standard deviation, which was noted for the laminate without treated Kevlar~ but was not observed for the treated laminate.

Example 5 To evaluate oxygen-plasma treatment of Kevlar~ for large-scale use in production operations, the beneficial effects of plasma treatment must be long-lived so as to provide a wide processing latitude for resin impregnation of the fabric. This is because the treated fiber may not be impregnated with resin for many hours after plasma treatment. Kevlar fabric was treated with oxygen plasma as before, however it was stored in a dessicator containing Drie-Rites (calcium carbonate) and atmospheric air for 44 hours prior to sample preparation. Bond strength of treated and untreated Kevlar~ as well as treated Kevlar@ that was aged were measured using the T-peel test, ASTM D-1876. Chemlok 305 epoxy resin was used as the resin bonding agent.The results of these test are given in the following table: Peel Strength of Plasma-Treated Peel Strength SamplesAfterAging Before Treatment OHrs. 4Hrs. 44Hrs.

Lbslln. Width Aging Aging Aging 6.24 1 0.93 8.3 1 0.83 8.57 1 1.0 8.71 1 0.88 The above table shows that there was no difference between the oxygen plasma-treated Kevlar~ samples that were aged in the desiccator prior to bonding, and those that were bonded immediately after plasma treatment. In both cases the bond strengths were measurably greater than for untreated samples. Electron spectroscopy for chemical analysis confirmed these results by showing a large increase in the oxygen/carbon and oxygen/nitrogen ratios on the Kevlar~ fiber surface. This indicates that a chemical change in the surface chemistry of the fiber occurred, which should not degrade immediately.

Oxygen-plasma treatment of Kevlar@ fibers is believed to produce oxygenated derivatives of poly (p-phenylene terephthalamide) molecules on the fiber surface.

Claims (6)

1. A method of making a laminate which comprises impregnating an amide or crystalline polyester fabric or mat with a liquid organic resin and forming said impregnated fabric or mat into a laminate, the fabric or mat being subjected prior to impregnation to an active oxygen or nitrogen-containing gas plasma for from 1 to 40 minutes at an RF power to flow rate ratio of from 1 to 4, a gas pressure of from 0.1 to 5 mm, and a frequency of from 100 Hz to 1 GHz.

2. A method according to claim 1, wherein the gas plasma is formed from oxygen, nitrogen, ammonia or mixtures thereof.

3. A method according to claim 1 or 2, wherein the resin is an epoxy or polyimide resin.

4. A method according to claim 1,2 or 3, wherein the fabric or mat is formed from poly (p-phenylene terephthalamide).

5. A method of making a laminate as claimed in claim 1 and substantially as described herein with particular reference to Examples 2 and 3 of the foregoing Examples.

6. Laminates when made by a method as claimed in any of claims 1 to 5.