Page 22 - EngineerIT January 2022

ICT – RADIO COMMUNICATION

Reliable PCB assembly of land grid

array packages in planar phased

array antennas

By Joel Dobler, Staff Product Applications Engineer, and Eamon Nash, Applications Director

igh frequency planar phased arrays often require a
complexity from their beam-forming ICs and transmit/
Hreceive (T/R) modules that force the devices into high pin
count land grid array (LGA) packages, so that all the RF circuits fit
into a λ/2 lattice spacing. LGA packages have recessed pins located
on their bottom side. As a result, PCB solder mask variation and
solder volume invariance both contribute to difficult PCB assembly
if the LGA footprint is not designed correctly. This article gives
guidelines for proper LGA footprint design that leads to high yield
PCB assembly and gives a practical LGA footprint design example
using the large, high pin count ADAR1000 beamformer IC.

Planar phased array systems
Phased array technology uses an arrangement of antenna elements
where the relative phase of each element is varied to steer the
focused radiation pattern, also known as a beam. The beam can be
aimed in various directions electronically, overcoming the limited
speed and reliability problems of a mechanically steered antenna.
A phased array system significantly reduces size, weight and power Figure 2: A typical LGA package, with a high pin count and dual rings of pins.
(SWaP), making it an attractive technology for defence applications
such as radar, communications, space and electronic warfare. The multichannel functionality of these beam formers,
In addition to eliminating moving mechanical parts, along with support and control circuitry (for example, receiver
implementing a flat panel radar front end - where patch antennas and transmitter RF traces, amplifier bias control lines, and T/R
and RF electronics populate the opposite sides of the same PCB control lines between the beam former and the T/R module)
- has obvious appeal as a size reducer. For analogue and hybrid forces the beam former ICs into high pin count packages. This
(analogue/digital) beam forming systems, the 4-channel beam migration is also driven by the need to conform to a lattice
former has become the form factor of choice. This arrangement spacing of lambda over two (15 mm at 10 GHz). As a result,
consists of a series of cells, with the quad beam former at the centre LGA packages have become a popular choice for multichannel
surrounded by four T/R modules (see Figure 1). beam former ICs. The ADAR1000 shown in Figure 2 is a prime
example of a typical beam former IC in an LGA package.

The soldering process
Automated PCB assembly is a multi-step process. It begins
with the application of solder paste to the exposed metal
on the PCB. This is done by placing a solder stencil on top
of the PCB and aligning the holes in the solder stencil with
the exposed PCB metal underneath. Solder paste is then
applied. The stencil is then removed, and the surface-
mount components are placed on the board, resting on
the deposited solder paste. The board, solder paste and
components are then placed in a reflow oven, where the
solder paste melts and bonds to the PCB and the pads of the
components. In this process there are multiple variables at
Figure 1: A planar phased array front end with electronics and patch
antenna elements on opposite sides of the same printed circuit board (PCB). play, which can lead to problems.

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