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Summary of Multilayer Board (BUM-PCB)

writer: G May 08, 2019

1.BUM-PCB development history

In the early 1990s, in Japan's Yasu branch, a high-density circuit board was developed by coating a photosensitive resin on a core board, forming a conductive microvia interconnect by photo-forming, and performing a line-forming process by addition. After the new method, this high-density circuit board was successfully used in a large number of Thinkpad notebook computers. In 1991, the new technology was first published, called Surface Laminar Circuit, SLC (Surface Laminated Circuit Board). Because this technology has created an unprecedented high-density Interconnect, HDI (high-density interconnect) new ideas, Japanese electronics manufacturers have developed, and since then opened the BUM era in the history of PCB development.

BUM adapts to the requirements of electronic products to lighter, smaller, thinner and shorter, and meets the needs of next-generation electronic packaging technologies (such as BGA, CSP, MCM, FCP, etc.), so it developed very rapidly throughout the 1990s. It is mainly used in portable electronic products such as laptops, mobile phones, digital cameras and MCM package substrates.

Corresponding to the Japanese first proposed BUM (multilayer laminate), European and American countries later proposed HDI (high density interconnect). In fact, the conceptual connotations of these two nouns are almost the same. According to IPC, the definition of HDI includes the following: non-mechanical drilling aperture is less than 0.15mm (6mil), and most of them are buried holes. The annular ring (pad or land) has a ring diameter of less than 0.25mm. (6 mil), a hole that satisfies this condition is called a micmovia; a PCB with a micropore has a contact density of 130 points/inch 2 or more, and a wiring density (a gorge width channel of 50 mils) at 117 inches/inch 2 The above is called an HDI-type PCB, and its line width/line spacing (L/S) is 3 mil / 3 mil or less. It can be seen that the most essential feature of the multilayer method is the high density interconnect (HDI).

2. BUM-PCB manufacturing process

The main difference between BUM and traditional PCB manufacturing process is the hole forming method. The key technologies of BUM mainly include the dielectric materials used in the laminated insulation layer; the microporous hole forming technology; and the hole metallization technology.

laminated dielectric material

Due to the difference between the insulating layer material and the interconnected micropore processing method, dozens of different BUM manufacturing methods have appeared, but they are representative and compared according to the different insulating dielectric materials used in various BUM manufacturing processes. Mature processes can be mainly divided into resin-coated copper foil (RCC) process, thermosetting resin (dry film or liquid) process and photosensitive resin (dry film or liquid) process. These three processes are now in use. The latter two processes generally require metallization and line formation through an additive process, which is highly demanding on materials and corresponding technologies.

The RCC process uses the subtractive process to complete the circuitization, does not require a large equipment investment (the main investment is laser driller), and adapts to the traditional multilayer PCB manufacturing process, and the finished BUM board has good performance and reliability. There are more and more manufacturers of BUM boards using RCC technology, and the demand for RCC is increasing. Taking the RCC process as an example, the more the number of layers, the worse the flatness of the board surface. Due to this limitation, the build layer of the BUM on the core board generally does not exceed 4 layers.

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