SMT Component Reliability for RF Applications Print E-mail
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Written by ACI Technologies Inc.   
Monday, 01 March 2010 00:00

Several devices perform well under vibration and temperature cycling testing.

We recently characterized the reliability of surface mount RF components. The RF frequency band of interest was the X band (10.7 to 11.7 GHz). A two-pronged test for printed circuit assembly reliability was designed for both extreme thermal cycling and vibration. The rapid thermal cycling and extreme vibration testing simulates the total stress encountered by the assembly over the life of the product, but accomplishes it in a relatively short period of time. To perform the reliability testing, a test vehicle consisting of a printed circuit board with test structures and components was designed, fabricated and assembled onsite.

The surface mount technology components selected were commonly used and have operating ranges up to the X band of the RF spectrum. A digital attenuator in a quad flatpack, no-lead (QFN) package was used with supporting chip components in 0402 and 0603 sizes. Two surface mount hybrid couplers with different leads were installed: one with L leads and one with castellated leads. Side launch SMA connectors with through-hole ground connections were installed to permit connection to the spectrum analyzer.

The frequency range of the attenuator is up to 13 GHz, while the other RF components are less than 4 GHz, based on their application in the actual circuit.
The test vehicle was designed to simulate a proposed board stackup and permit the mounting of the SMT RF components. Each board has six RF paths that pass through the components (Figure 1). To observe any effects of vibration and thermal cycling on the laminated board, three RF paths were designed with no components to act as controls.

Fig 1

Component manufacturer’s data sheets were used to define the shapes and sizes of both the pads on the CCA and the cutouts for the solder paste stencil. The stencil thickness was 0.005˝ to permit the proper solder volume on the 0402 and 0603 component ends. The larger, castellated lead coupler required a stencil with a “window pane” feature to reduce the volume of solder used to solder the large center ground to the ground plane.

The solder paste selected was the type typically used for military assemblies (SnPb37 with a no-clean flux and a J-STD-004 classification of ROL0). The board layout was programmed into a pick-and-place machine so the QFN and 0402 components could be placed accurately. A double-reflow process was used. All flux residue was removed using an inline cleaner to meet IPC-A-610 Class 3 requirements.

Accelerated testing plans. Component reliability was tested using accelerated temperature cycling based on JEDEC Standard JESD22-A104. The assemblies cycled between +85°C to -40°C for 1,000 cycles of 91 min. each. Breaks for RF testing occurred at 100, 200, 400 and 1,000 cycles to permit more resolution into the possibility of early thermal failures.

Vibration testing also was performed to simulate the stresses of motion on the components over the assembly’s life. The three axes vibration testing was performed for two hours at frequencies from 4Hz to 50Hz per MIL-STD-167 Type 1. The test vehicles were RF tested prior to being sent for vibration and then RF tested again on their return to the EMPF.

Thirty test vehicles were assembled using in-house SMT equipment. Prior to thermal cycling and vibration testing, each of the RF paths on all the assemblies was visually inspected and swept for transmission loss (S21) and insertion loss (S11) to gather baseline data. An Anritsu Spectrum Analyzer was used and data were gathered from 40 MHz to 20 GHz.

Fifteen CCAs were sent for thermal cycling and 15 were sent for vibration testing. After vibration testing, there was no evidence of cracked solder joints or other evidence of stresses between the devices and the board. The RF paths on each of the 15 assemblies were swept, and data showed no significant degradation in the device or path performance (Figure 2).

Fig 2

Fifteen CCAs were run through 1,000 thermal cycles with visual inspection and RF testing performed at established break points. Again, no evidence of damage was apparent on the visual inspections, and no significant degradation in performance was apparent on any of the CCAs after RF testing (Figure 3).

Fig 3

The analysis showed no significant degradation of performance. Visual inspection of the components and solder joints showed no physical damage and almost no degradation in performance through the accelerated life tests (Figures 2 and 3). Although the figures shown are for one specific device through the tests, all other SMT components performed as well.  CA

ACI Technologies Inc. (aciusa.org) is the National Center of Excellence in Electronics Manufacturing, specializing in manufacturing services, IPC standards and manufacturing training, failure analysis and other analytical services. This column appears monthly.

Last Updated on Tuesday, 09 March 2010 13:59
 

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