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Using Deliberate "tombstoning" to Place Parts.

IP.com Disclosure Number: IPCOM000004580D
Original Publication Date: 2001-Feb-22
Included in the Prior Art Database: 2001-Feb-22
Document File: 4 page(s) / 47K

Publishing Venue

Motorola

Related People

Marcel Lowe Ching: AUTHOR

Abstract

This publication describes a novel manufacturing technique that enables standard double-sided reflow factories to place vertical or thin components with standard vacuum nozzle and reflow equipment. The two step procedure is based on the idea of not placing compo-nents in their final orientation.

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Using Deliberate "tombstoning" to Place Parts.

By

Marcel Lowe Ching

ABSTRACT This publication describes a novel manufacturing technique that enables standard double-sided reflow factories to place vertical or thin components with standard vacuum nozzle and reflow equipment. The two step procedure is based on the idea of not placing components in their final orientation.

I. Background

Surface-mount technology has been widely used for soldering components to circuit boards. In the traditional process, placement machines are used to pick tape and reel parts from the tape carrier and place them onto the printed circuit board (PCB). The populated board is then sent through a reflow oven where the components are soldered to the PCB.

For double-sided boards (boards with components on each side) this is performed once for each side. The first side of the board is populated and soldered, the board is allowed to cool, then flipped over and the second side is populated and soldered. While the second side of the board is being soldered, solder surface tension and/or glue keeps the parts soldered on the first side from moving or falling off.

Throughout this process, the parts are kept in their final desired orientation, meaning that the parts arrive in the tape carrier in the same orientation that they are in when placed and soldered.

Popular placement machines employ a small vacuum nozzle to pick up a component by its horizontal surface. Other placement machines hold parts by using finger-like grabbers.

"Tombstoning" is a term used in the electronic circuit board manufacturing industry to describe a traditionally undesired phenomenon where components are placed horizontally and "stand up" during the reflow soldering process so that they resemble tombstones. This can happen to a small surface-mount capacitor if it is placed on its two solder pads and the solder paste on one of the solder pads liquefies before the other and creates surface tension that is sufficient to lift the capacitor up off its other pad. The result is an unconnected vertical capacitor that is soldered to the board by only one of its terminals.

II. Problem

Parts that are to be placed using standard vacuum nozzles must have a horizontal flat surface. Thin vertical components cannot be placed in their final orientations using standard vacuum nozzles because they do not have a substantial horizontal surface for a vacuum nozzle to hold. An example of a thin vertical part is a single shielding wall designed to block Radio Frequency (RF) energy, as shown in Figure 1.

Even though it is possible to place tall or thin parts by using placement machines with "grabbers", these components will tend to fall over during the reflow soldering process and could benefit from the manufacturing technique described below.

III. Solution Procedure

The solution to placing tall or thin parts u...