Volume 1, Issue No. 3 Wednesday, August 18, 1999
Featured Article


BGA REWORK

Earl Moon
Proof Of Design (POD)

INTRODUCTION

This article is written to ensure everyone interested in this vital operation more insight into effective, efficient BGA rework operations. Operations personnel use specified and approved repair and rework equipment, tools, procedures, and attendant process management elements to effect and assure quality rework operations. This is done to assure acceptable solder joint and product quality.

I have written this article using mostly photos from the same procedures I wrote for training and operations at a large OEM-C/M. The reason for this is to more clearly show how BGA rework may be effected in a relatively simple manner. The machine used was the SRT-1000. It is representative, in many ways, of several "higher end" machines including Air-Vac and Conceptronic - both of which I have evaluated, qualified, installed, and used at other companies. This article is not an endorsement for a particular product or machine as most all equipment sold at this level have similar capabilities - some better than others, but most average out in operations.

REWORK PROCESS OVERVIEW

Not matter the tools or equipment, most rework/repair operations use the same principle. A heating nozzle is employed to reflow previously formed solder joints. To do this, it is lowered over a device on a PCB and hot air is forced through it and around the device's solder joints so they are melted.

At the point molten solder is present, a vacuum operated pickup tube (nozzle) descends upon the component and contacts its top. When raised, the tube brings with it the component completing the removal process.

After removal, the component site is inspected to ascertain the heating "profile" so a calculation may be estimated for the replacement and reflow process to follow. After site cleanup, solder paste (most often preferred for highest reliability joints) or paste flux (never liquid as contact compliance required for effective reflow) is applied over each solder termination area (pad). Within a limited time, a replacement BGA is lowered into positioned and aligned with the pads. It is properly placed. Then, it is reflowed in position effecting the rework operation.

Again, this may be done manually (often with comparatively great difficulty). It may be done using semi-automatic equipment with no computerized capability (with some difficulty). It may be done with "automatic" equipment managed by people with considerable training to actively assure process effectiveness. No matter, the principles are the same. Results often differ, even with higher end equipment and processes.

This article deals with "automatic" (equipment manufacturers use this term though not accurate as there is so much human interaction) rework/repair operations. Actually, rework is the proper term as this process means doing it over again when it was not done right the first time. However, this process may be used successfully to place, and/or reflow, first time device types.

Automatic machines require the same basic elements as do manual processes. There is a platform on which all sub-system elements are mounted. These elements consist of a top side heater nozzle and support system and one supporting bottom side heating. There is a vacuum system used with the pickup tube to remove the component from its site after solder melting. A vision system is provided to ensure correct component alignment throughout the entire operation. There is a computer, and its human/machine interface software, to monitor and "recommend" certain processes and profiles for the operation. The following descriptions are provided for the basic process and the selected machine for this article:

MACHINE OVERVIEW

Figure A shows the machine in an operational setting. Figure B provides a view showing some of the machine's internal mechanisms including the X/Y table and bottom side heater.


Figure A
 


Figure B



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