MicroPak™ Evaluation on PC Board Assembly

A major manufacturer of consumer wireless products completed a beta-site evaluation of MicroPak package daisy chain samples in May, 2001. One thousand units were mounted and tested, resulting in 100 % yield for solder joint integrity and electrical isolation. Subsequent temperature cycling performance was significantly better than the criteria for the beta-site. Results are summarized below.

The Beta Test

A matrix of five different PC board pad metalization designs was used to test a range of dimensions for various solder pad, solder stencil, and solder mask combinations. Design No. 4 is used here as an example.

Components
MicroPak packages were assembled with internal connections between every other pair of terminals. Such components allow a board design to be tested by sensing continuity for electrical signals routed serially through every solder joint to the device.

Board Materials
Printed wiring board consisted of a single layer FR4 board 31 mils thick. Board finish was OSP over copper.

Solder/Stencil
A standard eutectic SnPb solder paste was applied through a 5 mil thick laser etched stencil.

Assembly
1000 daisy chain parts were mounted. Since the parts were supplied in waffle paks, mounting was done using a Universal Instruments GSM placement machine with a multihead spindle. Default settings were used including 100 gm placement force. Accuracy of component placement was +/- 2 mils.

Test Results for Assembly:
A perfect outcome was achieved for mounting 1000 devices on the PC boards. No opens were found. Connection integrity was determined by applying continuity tests to all connections for all parts.

Visual inspection after assembly showed no shorting. Since solder joints on the MicroPak package are along the periphery, parts can be visually inspected for quality of the solder joint. An actual photograph of a typical mounted MicroPak package is shown here. (Photo is of a MicroPak package on a board other than that used in the beta site evaluation.)

Solder Separation Between Pads
After tests, with components separated from the board, there was no evidence of solder bridging between any of the terminals for all 1000 parts. The photograph below is from one of the 200 parts mounted on solder pad "design No.4", and shows typical results.

Board Pad Design
No. 4 - Metal Defined.

Temperature Cycling
A metal defined board pad design (Number 4, of the five board pad designs) was used for temperature cycling tests. It had the following dimensions:

Description	Pad	Soldermask	Stencil Opening	Pad to Pad Gap
Metal Defined (4)	0.249 X 0.399	0.401 X 0.551	0.249 X 0.399	0.350

Board pad Design no. 4 is illustrated below. Shown beneath is the layout and terminal pad dimensions on the MicroPak package itself. This board pad layout is slightly different than that shown in our Manufacturing Guidelines.

Test Results for Temperature Cycling
Two hundred (200) daisy chain samples were mounted on FR4 type PC board using the metal defined solder pad design number 4, and subjected to temperature cycling -55 degrees C to +125 degrees C, (Liquid to liquid, 5 minute dwells at extremes). No failures occurred until several hundred cycles beyond the temperature cycle criteria of the beta-site.