by Michael Morrison
Many manufacturers have made great strides in defect reduction through process control and continual improvement. Statistical process control shows that because of inherent variation, defects can be greatly reduced but not completely eliminated. There will always be some percentage of assemblies that do not conform to requirements and must be reworked. The need for rework will vary among manufacturers, products, production equipment, production shifts, and even days of the week, but it will never completely be eliminated.

The advancement of SMT technology in electronics manufacturing has produced many significant benefits. One benefit not often discussed is the ease of repairing and reworking SMT component defects when compared to through-hole technology defects. Twenty years ago, replacing a 16-pin DIP package IC required great skill and ability. Repair technicians not only had to be concerned with damage to component leads, but also to the PCB laminate and plated through-holes. Many technicians resorted to destructive component removal, clean up and repair of the land area, and then installation of the new component.

Today, even fine pitch QFP and BGA IC packages can be replaced by repair technicians with little concern about damage or reliability. Modern hot air and IR rework systems often employ product or part specific thermal profiles which are repeatable and make consistently reliable solder connections. Though SMT has made rework easier, advances in technology have not eliminated rework problems. Component size is decreasing while placement density is increasing, making the possibility of affecting adjacent parts during rework a growing concern.

There are several types of SMT rework equipment available to manufacturers. Hot air convection, IR radiation, and resistance-heated conductive tip systems are the most common. Manufacturers purchasing SMT rework equipment must weigh the pros and cons of these unique systems as they vary greatly in cost, accuracy and ease of use.

In general, conductive tip systems are less expensive but require greater skill and manual dexterity. Hot air convection systems offer good reliability and repeatability but care must be taken in tip and barrier selection to ensure adjacent components are not damaged by air flow. IR systems offer greater control of the thermal foot print through focusing lenses but require greater technical expertise and knowledge because IR absorption rates vary greatly between differing components, and materials.

No matter what rework system or method is employed, the goal is to provide quality solder connections when replacing SMT components without damaging the substrate or adjacent components. The following suggestions are provided as universally accepted guidelines and best practices to help ensure that goal is met.

For All Rework Systems -

1. Minimize the number of times a component is removed and replaced to prevent internal damage to the PCB and to prevent excessive inter-metallic growth which could adversely affect solder joint reliability and wetting.

2. Clean components and PCB as soon as rework is accomplished.

3. Use of high activity flux is highly discouraged. Only no-clean or rosin flux should be used when performing rework or repair.

For Heated Conductive Tip Systems -

4. The temperature of soldering tips should not exceed 700 degrees F (370 degrees C). The potential for PCB or component damage increases exponentially beyond 700 degrees F.

5. When using conductive tips, heat should not be applied to the land area for more than 3 seconds.Research indicates that for tip temperaturesbetween 550 degree F and 850 degree F, it onlytakes 1.5 seconds for eutectic solder to reflow. Remaining on the land area for longer than 3 seconds only multiplies the possibility of thermal damage.

6. Use conductive tips that will heat all of the component leads simultaneously. When all of the solder on all of the leads has reflowed, the part may be removed with tweezers or, in some cases, the tool itself. Special tips are provided by manufacturers of conductive systems to fit a wide rangeof component applications. In the event an in-house tip must be constructed, it should be made of tin plated copper.

7. Investment in the time and effort needed to create an accurate profile will pay huge dividends when installing BGA’s, PLCC's and HD QFP's. Variables induced by humans are virtually eliminated in such systems. Expect longer profile times as these systems provide for pre-heat, soak and reflow slopes similar to large mass reflow ovens.

For Older Hot Air Rework Equipment (not programmable or automatic)-

8. PCB's should be pre-heated before component removal. This will help to create a homogeneous temperature of the repair area and reduce thermal shock during removal.

9. Components should be pre-heated prior to installing into liquidus solder. Cold component leads entering hot liquidus solder will cause the solder to cool and solidify before lead wetting can occur. The solder joint would then have to be driven back into reflow to allow the component leads to properly wet. This creates excessive time under temperature, growing the inter-metallic region, and possibly exhausting
flux agents which may lead to de-wetting.

10. Care should be taken to direct the flow of air in convection systems toward the component to be removed. Time and temperature controls should allow removal of components within 20 seconds, without charring flux or damaging the PCB. Some PLCC's may require heating exposure longer than 20 seconds, and will probably result in some flux charring which is difficult to clean. Thermal profiles running longer than 50 seconds run a high risk of PCB discoloration and damage.

For further information on SMT rework, the EMPF offers detailed skills-based rework and repair courses. Please see the back of this EMPFasis newsletter for a complete list. The EMPF also has many new and cutting edge rework systems manufactured by industry leaders that are available for demonstration.