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Component removal and attachment issues: this month, Les clarifies readers' assumptions concerning underfill material in rework and repair and reflow soldering.

Q: I am using a well-known commercial rework station to rework mobile phones that have a number of microBGAs. I am unable to desolder some of the inoperable or faulty BGAs because they have some type of adhesive or glue on the periphery of the components. After much effort to desolder the components, the glue has made its way under the component. Will anything reduce the melt temperature of the glue so that I can remove the BGAs?

A: The "glue" you see in your application is most likely an underfill material--intentionally placed beneath the ball grid array (BGA) components to absorb stresses from thermal and mechanical shock associated with service environments. Certain environmental stresses can fracture the small solder joints that carry electronic signals. These joints also provide the only attachment for physical strength between the component and the printed circuit board (PCB). The underfill material is more ductile than the solder and can often cushion the device and absorb shock if the phone is dropped or exposed to rapid temperature changes.

Certain rework techniques exist that allow removal of the component through mechanical abrading. However, once the underfill is applied, the component is almost impossible to remove without causing damage to the component and/or the PCB. Using thermal techniques, such as the methods you mention, generally does not solve the problem. Many thermal solutions, to remove the underfill, can also damage the PCB.

In most cases, once the underfill material is applied, rework and repair becomes very difficult. Many sophisticated, time-consuming and costly (and often unsuccessful) techniques have been developed to aid in such rework and repair. Even in situations where rework is possible, the process is costly, often ineffective and sometimes destructive. In many cases, rework can result in less reliable solder connections, which, in turn, may cause early product failure and undesirable business ramifications.

The best option is to understand the importance of solderability in the soldering process, to exercise the use of solderability testing on a regular basis and employ good process control techniques in all operations.

Q: We have double-sided surface-mount assemblies with heavy components, which we glue down, on one side. We have experimented with dispensing the adhesive and the solder paste on this side of the board--followed by placing the components, curing the adhesive and then reflowing the solder paste with one pass through the reflow oven. The profile is set for the solder paste requirements. Some question whether or not this is a good practice. What is your opinion?

A: I am aware of some in the industry who perform this process when assembling and soldering boards individually or in two-up panels, with alternating sides up. The reasons behind this process are generally to prevent heavy components on one side of the assembly from falling off during the second reflow excursion. A perceived productivity gain may be realized from avoiding a separate thermal excursion to cure the adhesive.

In some cases, the older placement and shuttling equipment found in certain facilities is another reason for gluing the parts down. Rough conveyor operation, along with quick starts and stops, can dislodge components secured to the board with only un-reflowed solder paste.

Before embarking on this combined thermal process approach, be sure to consider the compatibility of the temperature required to cure the adhesive and the reflow temperature. The ability of the cured adhesive to withstand exposure to a second-side pass thermal excursion at reflow temperatures should also be considered.

For BGA attachments, engineered materials, such as the combined solder paste, no-flow underfill products, may fulfill the perceived need for an adhesive to hold bottom-side components in place during reflow soldering. High viscosity, epoxy-based tacky fluxes, which will satisfy the requirement for most terminations, are also available.

Send your process, technology or training question to les.hymes@worldnet.att.net. Please type "ASK LES" in the subject line and indicate your name and company or institute affiliation. All questions may not be answered.

Los Hymes is the owner of Los Hymes Associates, Surprise, AZ, (623) 544-4646; e-mail: les.hymes@worldnet.att.net.
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Title Annotation:Ask Les
Author:Hymes, Les
Publication:Circuits Assembly
Geographic Code:1USA
Date:Sep 1, 2002
Words:680
Previous Article:Die skewing: due to the continued growth of flip-chip technology in next-generation products, avoiding die skew is essential.
Next Article:All solder pastes are not created equal: several in-house tests will reveal if you are using the right solder paste for your application.
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