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December 2005
library  >  Application Notes  >  Sarang Shidore

Package Substrates


Last time we examined modeling leadframes in leaded packages. In this first month of the new year, we will take a look at package substrates.

A substrate is a key constituent of most area array packages, such as Ball Grid Array (BGA) and Pin Grid Array (PGA). A substrate is essentially an element of the package on which the die is mounted, and which routes the I/Os from the die to the PCB. This is accomplished through a number of traces etched in the substrate. These traces are present in one or more metal layers in the substrate.

Substrates in effect fall under two major categories: Ceramic and Organic.

Ceramic Substrates

Ceramic substrates, present in Ceramic area array packages, have been traditionally used for their superior mechanical and electrical characteristics. Specifically, they -

  • Are hermetic (do not allow moisture to seep in)
  • Have a relatively high thermal conductivity (18 W/mK for the most common ceramic dielectric Alumina)
  • Allow for finer line widths and spaces
  • Have a good CTE match with Silicon die.
  • Allow a large number of metal layers (20 or more)

Figure 1: Ceramic substrate

The dielectric portion of a ceramic substrate is typically made from 94-99% pure Aluminum Oxide, or Alumina. In rare cases Beryllium Oxide or Aluminum Nitride may also be used, their chief advantage being a much higher thermal conductivity (ranging from 200 W/mK upwards). The metal layers in a ceramic substrate are typically made up of Tungsten or Molybdenum.

The ceramic dielectric slabs are placed together and fired in a high temperature oven in a specialized and relatively expensive process.

Organic Substrates

The substrates of plastic area array packages (e.g Plastic Ball Grid Array) are organic. Essentially, they are manufactured using standard PCB technology. For traditional organic substrates, this means an epoxy-glass resin (typically, BT resin) that makes up the dielectric and copper for the metal traces and vias.

Figure 2: Organic substrate

The advantages of organic substrates are:

  • The technology used is nearly identical to that for PCB manufacture. This reduces cost and allows for volume production easily as compared to ceramic substrates.
  • Organic substrates have a good CTE match with the PCB.

The disadvantages of organic substrates are:

  • Higher thermal resistance compared to ceramic substrates. Thus they require thermal enhancements for higher powers.
  • Poor CTE mismatch with silicon die. This can be challenging for flip-chip type interconnections, in which case an underfill must be used.

On the balance though, organic substrates are the choice for a majority of area array packages, unless high reliability and/or high powers are requirements (in which case ceramic substrates hold the advantage).

As organic substrates are more challenging from a modeling standpoint than ceramic substrates, let us look at the structure of these substrates in greater detail.

A traditional organic substrate is typically a 2-layer or 4-layer. The number of layers refers to the number of metal layers.

Two-layer substrates:

A 2-layer substrate has a signal trace on the top and another metal layer on the bottom, primarily composed of metal lands for the 2nd level interconnect (e.g. solder balls).

Figure 3: A 2-layer organic substrate

Signal vias connect the traces on the top layer to the corresponding lands on the bottom layer. In addition, a number of vias are also present below the die whose function is strictly thermal. These are known as thermal vias, and are usually grounded. They provide a path for the heat to flow more directly from the die to the solder balls.

Four-layer substrate:

A 4-layer substrate is similar to a 2-layer substrate except that there are two additional metal layers. These are typically power and ground planes, and not signal trace layers. Because the copper coverage for power/ground planes is high (usually > 90%), a 4-layer substrate has a significantly smaller thermal resistance than a 2-layer substrate.

Figure 4: 4-layer organic substrate

Built-up substrates:

Over the last two or three years a different type of organic substrate technology has gained prominence, especially for flip-chip type packages. This is called the built-up substrate technology (also known as HDI, or High Density Interconnect technology). A built-up substrate is different from the traditional organic substrate, as it is manufactured by building it up, one layer at a time. Vias are not drilled mechanically, but using lasers. Thus the vias are significantly smaller in diameter (~ 4 mils) and can also be blind or buried. A built up substrate usually has a core region that is essentially a traditional substrate structure, flanked by built-up portions above and below this core region.

Figure 5: Flip-chip package with built-up substrate

Next Time

This month we have tried to understand what constitutes some of the most common substrate technologies in electronics packaging. Next month, we will examine how to model these various substrates from a thermal standpoint.

About Sarang Shidore:

Sarang Shidore obtained engineering degrees from IIT Madras (India), Texas A & M University (College Station), and University of Texas (Austin). Since 1995 he has worked at Flomerics Inc. in various roles in engineering and product management with a special focus on package-level thermal modeling and analysis, a field in which he has authored several papers and articles. Sarang is currently Director of the Web Business Division at Flomerics Inc. at their Austin office. He welcomes comments and questions at sarang@flomerics.com.

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