Preform Method Simplifies Small Volume BGA Ball Attach/Reattach

Introduction
There is a growing need for a low-to-medium volume ball attach method for BGA components. Almost any pcb assembler or test house working with BGAs has a need to put a new array of solder spheres onto a few components. Most options have involved the use of stencils, solder paste and/or loose solder balls. Typically, these methods are quite tedious, inconsistent, and messy.

In the past few years, the need for a small-volume ball attach method has grown as rapidly as the rest of the BGA market. When BGA components are placed on a pcb, a variety of failures can occur that would necessitate the removal of the component. Bad solder joints, bridging, or even improper orientation are all common failure mechanisms. Because many BGAs are relatively expensive, nobody wants to throw away a component where the only problem is a faulty solder ball array. As a result, companies are incorporating reballing techniques into their standard rework line.

The industry also faces a transition period as we move to lead-free soldering (due to the RoHS legislation). During this time, BGA components may only be available with the wrong alloy balls - and with BGAs, "lead" and "lead-free" do not mix, necessitating a component re-ball before assembly.

For large-scale production purposes, an array of solder balls is usually attached using expensive solder ball dispensing machines. The tooling alone for such a machine can cost as much as $10,000 and is typically only good for one package type. In addition, the purchase price of a machine starts at about $100,000. These ball-attach machines work quite well for large-volume production. However, they are simply not very cost-effective and are not a practical solution for low-volume attach or for reballing.

Small Volume Needs Low Cost
Most small-volume processes for placing a new array of solder spheres onto a part involve either stenciling on solder paste or loose solder spheres. Any manual process involving a stencil requires properly alignment and fixing the stencil to the BGA component. Manually screening solder paste through a stencil can be very messy and gives poor volume control. Placing loose solder spheres into a stencil is tedious and time-consuming. One method for using stencils requires a full application of flux to the pad side of the component. With this process, balls tend to migrate and combine at reflow. Another method using stencils calls for screening flux onto pads and then reflowing unconstrained loose balls on the fluxed pads. Since proper cleaning and care of a stencil is extremely important to an effective process, they must be cleaned between each application of flux, and this simply adds to the process time. Finally, stencils do not lend themselves to flexibility. A different stencil will likely be needed for each different package type, further adding to the overall cost of the process.

An answer to this production dilemma is to use BGA preforms such as SolderQuik BGA Preforms. These deliver an easy, clean, cost-effective alternative that is quickly becoming the preferred method for small-volume ball attach or reattach among the leading semiconductor and pcb assembly houses.

The preform is a simple, repeatable, cost-effective, low-to-medium-volume ball attach solution. The patented SolderQuik BGA Preforms, which was originally developed by Raychem, consists of an array of solder spheres embedded in a carrier material. The carrier controls pitch, progression, and alignment. In the case of SolderQuik Preforms, the carrier is made of a water dispersible paper laminate that is easily removed after reflow.

Simple Process
The process to attach an array of solder spheres using a preform is simple. The BGA component is fluxed, placed against the preform, and then they are placed in reflow. Typically an inexpensive fixture is used to properly align the preform to the component. The new array of solder spheres has been attached when the reflow cycle is complete, and one simply needs to remove the carrier material. The water-soluble carrier of SolderQuik Preforms allows for several removal techniques: e.g., let a few drops of DI water soak into the carrier for a few seconds and peel away with tweezers, or it can easily be removed using most aqueous spray rinse or batch-cleaning systems.

A ball attach system using preforms allows for significant advantages over any process using solder paste or loose solder balls. Many reballing methods require the purchase of special tooling, but preforms work with almost any reflow and rework equipment. An inexpensive fixture to control alignment by edge registration is the only tooling needed here. Preforms are extremely flexible and cost-effective with availability in any array pattern, including various pitches, ball sizes, and ball alloys.

When using preforms, flux can simply be brushed onto the entire pad side of the package without any concern about ball migration. The carrier keeps the balls in the proper pitch and progression, so ball migration and bridging are simply not possible. The carrier is disposable so it can be removed and discarded after reflow. There is no stencil to clean after each run.

Fast and Efficient
Using preforms, the time needed to put balls on 20 parts is only a few minutes more than the time to put balls on one part. Using preforms, those parts could also all have different pitches, array patterns, ball diameters, and even ball alloys. Preforms tend to work quite well with almost any convection reflow system, and are well suited to use alongside most rework stations.