Multi-Chip Module-Laminated (MCM-L), based on organic laminate technology. Each type of MCM has its place in the technological hierarchy. MCM-L is derived from conventional PCB technology and makes more use of enhance materials and processes, high-definition photolithographic techniques, and strict process control to allow the fabrication of small feature sizes and the more accurate placement of components than are customarily found with PCBs. MCM-L component assembly procedures differ dramatically from PCB assembly practices; most noticeable is the replacement of standard packaged-part soldering processes associated with surface mount technology (SMT), with bare die interconnected with wire bonding or flip chip processes. In this respect, MCM-L is similar to what once was called hybrid microelectronics with two notable exceptions:

  1. The substrate is a laminated organic structure rather than ceramic or silicon.
  2. The module package need not be hermetically sealed.

MCM-L provides the lowest density of the three major (MCM-D, MCM-C) MCM technologies and is usually the least expensive to implement. MCM-L is also known as chip-on-board (COB). While some people make a distinction between COB and MCM-Ls, the differences are minor.

MCM-L technology is derived from organic PCB production and uses copper metal traces separated by fiberglass-reinforced organic laminates (the most common), with plated through-holes or vias interconnecting the layers. Compared with conventional PCBs, advanced processing techniques result in smaller feature size and allow the incorporation of vias that extend only part way through the board (blind vias) as well as those that connect between layers entirely within the board (buried vias). The use of blind and buried vias greatly increases the wiring density of components that can be connected on the board.

The label of MCM-L refers specifically to the technology used to manufacture the wiring platform. The platform can be used to interconnect ICs, as I a pure stand-alone MCM, or to build up a complex circuit board containing many different components, avtive and passive. Once populated, the MCM-L uses organic coatings to protect the chips and their bonds (glob top), as well as the components and the entire board. Some complex boards may even have other MCMs attached to them.

There are three types of laminated (MCM-L) substrates: rigid, flex, and rigid-flex. Rigid MCM-L substrates are typically made of epoxy-based polymer resin dielectrics, reinforced with fiberglass. Other reinforcing materials include quartz, Kevlar, and Aramid. Different resin systems, include polyimide, Teflon (PTFE), and PEEK (polyether ether ketone.) Polyimide provides excellent stability and a low dielectric constant, thus assuring excellent signal propagation and high frequency performance. There are two types of dielectric layers in typical MCM-L construction: cores and prepregs. Core material is fully cured when delivered from the manufacturer and is usually clad on both sides with copper. The as-received prepreg material is partially cured and is used between core layers as a bonding agent to hold, or glue, them together. Both the cores and prepregs are typically fiber reinforced. If the dielectric layers have no reinforcing fibers, the resulting laminate in thin layers is quite flexible, leading to the name flexible or flex circuits. Combining both types of laminates into a single board structure yields and MCM-L substrate called rigid-flex.

MCM-L substrate process. There are five basic process steps in the manufacture of MCM-L substrates:

  1. Selecting the appropriate core and prepreg layers. The layer thicknesses are chosen to meet the circuit electrical and mechanical performance criteria.
  2. Photolithographic patterning and etching of the copper conductors on the core layers.
  3. Drilling of vias (blind, buried, or full through-holes).
  4. Lamination of the cores to each other using the prepreg layers. Multiple laminations are usually required to from blind and buried vias. If the board only has plated through-holes, then a single lamination step is usually possible with post-lamination via drilling.
  5. Plating of drilled holes in single layers (buried vias), partially through several laminated layers (blind vias), and holes going all the way through the board (plated through-holes as used in PCB manufacturing).

 

Copper is the typical conductor material used in MCM-Ls and PCBs. The copper comes laminated to the core material. The thickness of the copper is specified by an arcane measure of its weight, in ounces, rather than its direct thickness.

 

  • Inner Layer Processing: The copper surfaces are cleaned in preparation for pattern processing. Photoresist is applied to the copper surfaces by laminating (heat and pressure) of a dry film resist material or by dipping in a liquid photoresist bath and drawing the copper material out at a uniform rate. Other techniques for resist application have been used including spray coating, screen printing, and/or electrophoretic deposition. Liquid resist typically allow finer line definition than the dry film used in PCB fabrication process. Following setting of the resist, the pattern is exposed through a mask by UV light, developed to remove the unwanted resist areas, and hardened by baking against the etch. The copper foil is etched in an ammonia-based alkaline system or by using such materials as ferric chloride. The photoresist is then chemically removed, or stripped, and the copper surface is treated to form an oxide layer that promotes the adhesion of the overlaying dielectric layer during lamination.
  • Via Formation: PCBs utilize holes drilled through the entire board to electrically connect the desire metal planes, and the patterned conductor layers that intersect the hole. After drilling, the holes are copperplated, a combination of electroless plating followed by electroplating, to make the required electrical nation of electroless plating followed by electroplating, to make the required electrical connections, called plated through-holes (PTHs). Blind vias extend from one surface to the desired internal plane while buried vias only interconnect internal planes. Both blind and buried vias are then copper plated as above to affect the electrical interconnection. The use of blind and buried vias is considered expensive by many experts, due to the difficulties encountered in handling the thin core and prepreg layers with the multiple drilling, plating, and lamination operations required. However, the use of blind and buried vias in place of plated through-holes greatly increases board wiring density.
  • Surface Finish: the treatment of copper surface layers defines a major difference between PCBs and MCM-Ls. Conventional PCBs are made for surface mounting or the through-hole soldering of leaded packaged components. The surface pads are typically solder plated with tin-lead solder, which is suitable for mass surface mount reflow soldering or the wave solering of through-hole devices. MCM-L substrate potentially require the selective plating of several metals, including gold for wire bonding, tin-lead solder for chip components and TAB leas, and hard wear-resistant gold for edge connectors. When gold is plated, it must be segregated from the copper by use of a nickel layer. Since these surface layers are usually required to be plated after patterning, electroless deposition processes must be used. Gold for wire bonding must be of medium hardness (<=80Kg/m2 on the Knoop Hardness Scale) and at least 0.5μm in thickness for good bonding results.