Friday, November 28, 2008

Jet dispensing technology for capillary underfiller

Press release, November 28, 2008

by Anton Knupfer, product manager, Essemtec AG

Capillary underfiller gained more and more in importance during recent years. The increased use of BGA and flip chip components, improved underfill materials and the use of high precision dispensing systems led from an everyday physical effect to a reproducible high tech process.

In Electronic Assembly underfilling is defined as a process where under large and/or sensitive components a layer is inserted, that combines this component flat with the printed circuit board (PCB). Such a procedure is most common for BGA components (Ball Grid Array). The connections of these components, designated as a ball grid array structure, lie under the component (Fig. 1).

Fig.1

By this design the connections consist of solder balls that later on during the reflow process connects these leads with the PCB. So a high number of leads in a small area is possible.

To get a higher mechanical stability and to protect the component against environmental influences after the reflow process a material is inserted under the component. This non-conductive synthetic material furthermore has to show a permanent elasticity. Temperature changes and associated different heat expansions of PCB and component should not produce mechanical tensions that could destroy the component.

Capillary effect produces high tensile forces
Nowadays there are three types of underfilling systems. Capillary underfills, no-flow underfills and corner-bond underfills. All three have special fields of operations, but capillary underfills record the largest market share. This system uses the so-called capillary force, drawing the underfiller under the component by itself. This capillary effect for example draws up water a little bit into a straw against gravity. It also enables the writing with a fountain pen. This effect is caused by an adhesion force in the boundary layer between fluid and solid surface. Many have experienced how strong these forces are with wet microscope glass slides that could not be separated again. One can calculate that a liquid film of 1 µm between two windowpanes causes a pressure of 2.8 bar (28 m water depth).

The capillary effect is used to fill completely the thin space under the BGA component with an underfiller after soldering. The tensile force of the capillary and the flow characteristics depend on many factors. If all parameters are correct, it is enough to apply the underfill material along at one of the two sides of the component. The rest will be done by the capillary force. Simultaneously the material flowing in, displaces all air out of the cavity.

Dispensing technology for underfill
There are different possibilities for depositing a fluid. The mostly used method is dispensing with a dosing needle. The underfill medium was led to the edge of the component (Fig. 2).

Fig. 2

Then the capillary effect draws the fluid into the gap between component and PCB. Precondition is a very small distance between needle and component. The fluid must completely wet the gap to start the capillary effect.
Getting more and more common and widely spread is dispensing without a dosing needle with the contactless JET technology (Fig 3).

Fig. 3

Here the liquid medium is ejected as small drops by a pestle that is moving with high speed into a specially formed cylinder. These drops fly with relatively high speed to the PCB surface.

Also a small lateral distance between component and flying drops are very important to initiate the capillary effect that draws the underfill medium under the component. The advantages of this method are obvious. Because there is no dispensing needle the system becomes insensitive against height changes of the components. In this way possible damages of components by the needle are prevented.

A further advantage of JET technology is that the underfiller can be shot into small gaps. Regarding high integration density this is the only possibility for dispensing the material secure and precise between two components (Fig. 4)

Fig. 4

Process control by controlling temperature
The properties of many commercial underfill materials can be controlled via temperature. In production this is used optimizing processes. During the process the PCB is heated up. By controlling the temperature flow characteristics can be accelerated and improved.

To reach a shorter process duration the component will be heated up one step before the dispense position. This is called pre-heating zone. Also after dispensing one finds usually a heating zone, which supports finally the flow characteristics of the underfilling. This heating zone is called post-heating zone.

The simplest form is the contact heating: a heated up metal plate will be pressed at the bottom side of the PCB. Work pieces with components on both sides cannot be heated up by this method. In this case a convection heating with hot circulating air can be used. A third possibility is an infrared heater to warm up complex forms.

The A and O: positioning
As mentioned above the accuracy of positioning the needle, respectively the landing zone of the drops plays an important role dispensing underfill materials. Especially with products where several components are placed in small distances.

Tolerances in placement and reflow process are normal practice. A dispensing system for the underfill process must be equipped with all kinds of high tech to fit the required accuracy. An exemplary system is the automatic dispensing machine CDS6200 from Essemtec (Fig. 5)

Fig.5

The dosing head has an integrated vision system that can exactly determine position and angular position of components. The movement of the dosing head will be adjusted in the way that the underfill material is dispensed perfectly along the edge of the component. Furthermore the dispenser has an integrated surface scanner. This device scans the topography of the PCB in advance and corrects strong height differences automatically.

The dispensing systems of Essemtec are modular and very flexible. They can be delivered with various dispensing valves, depending on process. Substrate mounts and transport systems are standard equipment, but also customer and product specific handling systems can be integrated.

The Swiss based manufacturer Essemtec is a worldwide leader in the range of flexible production systems for industry. Since 1991 Essemtec developed, produced and distributed devices and machines for all electronic production processes: printing systems, dispenser, pick and place machines and soldering systems. Manual, semi-automatic and automatic systems are available. The product portfolio includes conveyer and storage systems, also software solutions in the production for planning, simulation, optimization and documentation. All Essemtec’s systems are optimized for maximum flexibility. So the customer can switch as soon as possible from one product to the next taking optimally advantage of production capacity. Essemtec – be more flexible.