Since the invention of the transistor in 1947, researchers have been working to make the transistor smaller. Their success has resulted in ever-smaller devices, faster processing speeds, and enhanced performance. As operating frequencies increased, the traditional wirebond package became a limiting factor in performance for large devices. At the same time, chip designers were finding it harder and harder to create high yield, first pass silicon devices containing millions of transistors, and imbedded passives for System on Chip (SOC) radio transceivers. This opened the door for System in Package applications (SiP). SiP integrates surface mount components and silicon devices on a package substrate. SiP started out as a bridge technology to be used between device generations. However, many companies exist today by making SiP devices or “Modules”.

As packaging sub-contractors geared up for the increase of SiP applications, device designers kept pushing on SOC solutions as well as the SiP. To keep pace with System On Chip package size reductions, SiP implementations went from packaged devices integrated with passives, to chip on board with wirebonds and surface mount device (SMD) passives. Today many radio transceivers have gone to flip chip as wirebonding required up to 0.75mm of clean space on each side of the device to enable wirebonding. However, migration to flip chip has some issues. For most low to moderate (<100 I/O) pin count devices, a metal leadframe substrate is used. This results in a low cost per pin package. The metal lead frame is the package of choice for radio chips. Conversely, flip chip devices are packaged on a laminate board. The laminate could be FR4, polyimide, bismaleimide triazine (BT), Duroid, or Liquid Crystal Polymers (LCP). These materials all have mechanical and electrical properties that make them suitable for uses in various market segments.

In a standard leadframe package, a large metal pad is used as the attachment point for the device. In the past, devices were grounded through the backside of the silicon, but patent protection, latch-up issues, and the desire to have separate grounding schemes on one piece of silicon, led most companies to ground devices off pads on the top surface of the device.

Leadframe packages with short wires to ground have about 0.1nH of inductance to ground. A laminate package with one via to ground and a routed line in the laminate may have 0.5-0.8nH of inductance or more. This difference in inductance to ground is not trivial. The performance of a radio transceiver at moderate to high frequencies can be negatively affected by this difference. To mitigate this, the use of a land grid array (LGA) package is an excellent choice. Figure 2-1 shows a typical LGA package, the larger central pads, and I/O pads.

When a device is flipped onto this style of package, one of the central four pads can be taken advantage of as a ground pad. By patterning vias to one of the quad pads that lie under the flip chip ground pad connections, the inductance to ground can be lowered to match the inductance of the metal leadframe. If Electro Magnetic Interference (EMI) is an issue, a drop-in EMI Shield can be added, before injection molding, to provide shielding.

When RF devices go flip chip, a change in the values for components that comprise the radio device are necessary. Typical components for a front-end module are, but not limited to: band pass filters, low pass filters, and baluns. Combinations of capacitors and inductors create these circuits. The filters are designed using the packaged radio devices’ characteristic impedances, and capacitances. An electromagnetic (EM) simulation tool is used to determine the values of inductors and capacitors required to create the passive networks. Trade off studies are made by making slight changes to inductance and capacitance values until the simulation reaches an optimum performance level. Passive surface mount components are provided with tolerance ranges of one to five percent or more. Using components with a wide tolerance such as 10% or 20% will lower the component cost. However, the resultant values for capacitance and inductance will vary and combinations of wide tolerance components may produce a filter that does not perform adequately in real world applications. An additional simulation that examines the variability and relates that to performance over multiple components with varying values is known as a Monte Carlo simulation. This simulation, made at the conclusion of the EM simulations, determines the effects of component variation. This ensures the selected component tolerance will create performance that is suitable for the applications of the product.

When a leadframe and wirebonds are replaced by flip chip and laminate, the device characteristics change enough to require alterations to the filters and passive networks. Acquiring the flip chip device characteristics requires the use of a flip chip test bed and a network analyzer to measure the device impedance during operation.

One of the main advantages of migrating to flip chip from a wire bonded SiP is the reduced number of processing steps. If we use traditional wirebonding and combine surface mount passives, a product flow depicted in figure 2-2a emerges. In figure 2-2b, the process for SiP with flipchip is shown. A clear reduction in the number of process steps required to create the final product occurs when all attachments are made by soldering.

With SiP modules, the most difficult part of the RF integration happens inside the package. A company wishing to use the module need only develop a 50-ohm (or other depending on the device) impedance transmission line to the antenna input. Conversely, the same company selling a single chip, may spend many days helping prospective customers debug their product boards to get optimum performance. From a time to market perspective, modules ease integration headaches, reduce board problems, and lower cost at market entry.

RF modules have made significant gains in the marketplace due to reduced development cost, shorter production cycles, improved technical support, and broader application. There are challenges when evaluating the flip chip device characteristics. Moving a radio frequency device to flip chip must be carefully evaluated based on product application, competition, and market conditions. From a military perspective, standardization at the module level will reduce sustainment expense, improve performance, and lower overall cost.