Thermal Management of High Frequency Circuits by Blind Hole Technology

For effective thermal management of high power and dense electronics, the heat sink forms the bulkiest component, and aluminum, owing to its light weight and easy machinability, is the preferred choice for airborne PCB devices used in aerospace and defense telecommunications. In this paper, ASC presents a versatile and robust fabrication method to build circuit boards used for RF and microwave telecommunication devices over a large frequency of interest from MHz up to several tens of GHz range. Typically, 1 mil of copper is plated in the blind holes, followed by the desired final finish applied to it (nickel/gold, tin/lead, chromate). There can be several inner copper layers anchored to the blind holes. Cross sectional examination showed that the connectivity of the inner layer(s) with the barrel wall was very good and no delamination of the hole wall occurred even after 1,000 thermal cycles of (-40°C to +100°C) excursions were carried out in an automated thermal cycling chamber. Resistivity of the blind hole coupons with the 1 mil copper layer (protected by a 0.1 mil tin coating), with 300 mA of current applied, was always less than 0.0006 milli ohm even after 1000 thermal cycles.