Wiring substrate and process for producing it

1. A wiring substrate comprising:

an electrical insulator layer,

a first conductor layer formed on a first surface of the electrical insulator layer,

a second conductor layer formed on a second surface opposite from the first surface of the electrical insulator layer, and

a plating layer formed on an inner wall surface of a hole which opens from the first conductor layer through the second conductor layer,

wherein the electrical insulator layer has a heat resistant resin layer containing a heat resistant resin and a resin powder,

the resin powder is formed from a resin material containing a melt-formable fluororesin having at least one type of functional group selected from the group consisting of a carbonyl group-containing group, a hydroxy group, an epoxy group and an isocyanate group,

the content of the resin powder is from 5 to 70 mass % to the heat resistant resin layer, and

the electrical insulator layer has a dielectric constant of from 2.0 to 3.5.

2. The wiring substrate according to claim 1 , wherein the fluororesin has a melting point of at least 260° C.

3. The wiring substrate according to claim 1 , wherein the at least one type of functional group is a carbonyl group-containing group, and the carbonyl group-containing group is at least one member selected from the group consisting of a group having a carbonyl group between carbon atoms of a hydrocarbon group, a carbonate group, a carboxy group, a haloformyl group, an alkoxycarbonyl group and an acid anhydride residue.

4. The wiring substrate according to claim 1 , wherein the content of the functional groups in the fluororesin is from 10 to 60,000 groups per 1×10 6 carbon atoms in the main chain of the fluororesin.

5. The wiring substrate according to claim 1 , wherein the electrical insulator layer has a dielectric constant of from 2.0 to 3.0.

6. The wiring substrate according to claim 1 , wherein the electrical insulator layer has a linear expansion coefficient of from 0 to 100 ppm/° C.

7. The wiring substrate according to claim 1 , wherein the average particle size of the resin powder to be added to the heat resistant resin layer is from 0.02 to 5 μm, and the volume-based accumulative 90% size (D 90 ) of the resin powder is at most 6 μm.

8. The wiring substrate according to claim 1 , wherein the water droplet contact angle of the electrical insulator layer is from 60° to 100°.

9. The wiring substrate according to claim 1 , wherein the average particle size of the resin powder contained in the electrical insulator layer is from 1 to 7 μm.

10. The wiring substrate according to claim 1 , wherein the rate of change of the resistance of the wiring substrate at 260° C. in the 100th cycle of a thermal shock test to the resistance of the wiring substrate at 260° C. in the first cycle of a thermal shock test is within ±10%.

11. A process for producing the wiring substrate as defined in claim 1 , which comprises:

forming the hole in a laminate comprising the first conductor layer, the electrical insulator layer and the second conductor layer laminated in this order;

applying a pre-treatment to the inner wall surface of the hole, and

forming the plating layer on the inner wall surface of the hole having a pre-treatment applied thereto.

12. A process for producing the wiring substrate as defined in claim 1 , which comprises:

forming the hole in a laminate having the first conductor layer laminated on the first surface of the electrical insulator layer;

applying a pre-treatment to the inner wall surface of the hole,

forming the plating layer on the inner wall surface of the hole having a pre-treatment applied thereto, and

forming the second conductor layer on the second surface of the electrical insulator layer.

13. The process for producing the wiring substrate according to claim 11 , wherein as the pre-treatment, either one or both of a treatment with a permanganic acid solution and a plasma treatment is conducted without conducting an etching treatment using metal sodium.