Abstract

Some of the new handheld communication devices offer real challenges to the paste printing process. Normally, there are very small devices like 01005 chip components as well as 0.3 mm pitch uBGA along with other devices that require higher deposits of solder paste. Surface mount connectors or RF shields with coplanarity issues fall into this category. Aperture sizes for the small devices require a stencil thickness in the 50 to 75 um (2-3 mils) range for effective paste transfer whereas the RF shield and SMT connector would like at least 150 um (6 mils) paste height. Spacing is too small to use normal step stencils.

This paper will explore a different type of step stencil for this application; a "Two-Print Stencil Process" step stencil. Here is a brief description of a "Two-Print Stencil Process." A 50 to 75 um (2-3 mils) stencil is used to print solder paste for the 01005, 0.3 mm pitch uBGA and other fine pitch components. While this paste is still wet a second in-line stencil printer is used to print all other components using a second thicker stencil. This second stencil has relief pockets on the contact side of the stencil any paste was printed with the first stencil.

Design guidelines for minimum keep-out distances between the relief step, the fine pitch apertures, and the RF Shields apertures as well relief pocket height clearance of the paste printed by the first print stencil will be provided.

Introduction

A printing challenge exists when very small devices coexist on a PCB with components requiring either high volume or high paste height. When printing solder paste for 01005 and/or 0.3 mm pitch uBGA a thin stencil, normally (50 to 75 um) thick is preferable to achieve sufficient paste transfer efficiency and low paste volume dispersion. If a 100 um thick stencil is used to print these small devices, the area ratio is less than 0.5 which is lower than the recommended value for both Laser-Cut stencils and E-FAB Electroform stencils. It is very common to print solder paste for RF shields for hand-held devices. Normally, higher solder paste deposits are required because of coplanarity issues with RF shields. To achieve these higher solder paste deposits a thicker stencil is needed. A step stencil(1) is normally used to achieve different solder paste heights. However in many hand-held devices the spacing between the small pitch components and the RF shield is normally very small, in many cases as small as 500 um (20 mils). The design guideline1 for a normal step-down stencil is that the aperture in the step-down region be positioned at least 890 um (35 mils) from the step-up wall for every 25um (1mil) of step-up thickness. Otherwise the squeegee blade (metal or rubber) does not do a very job of depositing paste into the lower level apertures. In the case of RF shields a stencil thickness of 150 to 175 um (6 to 7 mils) may be required to achieve the desired solder paste height. The height difference; 75um (3 mils) thick stencil for the small components and 150um (6 mils) thick stencil, prevents a normal step stencil to be used for this application.

However, a "Two-Print Stencil Process" allows for very small spacing between the apertures of the smaller components and apertures of the RF shield. The objective of this study is to determine the smallest spacing between small component apertures and RF shield apertures for the Two-Print stencil process. Another objective is to determine the minimum clearance between the relief pocket of the 2nd print stencil and the 1st print stencil without paste smearing in the relief pocket.

Two-Print Stencil Test Plan

The Two-Print stencil process consists of two stencils. The 1st print stencil is used to print 0.3 mm pitch uBGA (198 um circular apertures), 0.4 mm pitch uBGA (244 um circular apertures), 01005 chip components (178 um circular apertures), and 0201 chip component apertures (300 um square apertures). Two 1st print stencils were fabricated. Both were E-FAB Electroform stencils; one was 50 um (2 mils) thick and the other was 75 um (3 mils) thick. The 2nd print stencil is used to print RF shields and a SMT connector and some large capacitor apertures. Four 2nd print stencils were manufactured with four different stencil thicknesses; each stencil had a relief pocket etched in the area where solder paste was printed with the 1st stencil. The design is summarized in Table 1.