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Solderability and traceability: les addresses surface finish concerns and component identity tracking.

Q: Our assemblies will soon be incorporating 0.75 mm and 0.5 mm micro-ball grid arrays (BGAs) on FR-4 printed wiring boards (PWBs). Will a hot-air solder leveling (HASL) surface finish on the boards be flat enough for reproducible results with these components? We are currently using HASL for assemblies with 0.5 mm fine pitch quad flat packs (QFPs) with reasonable success, though we have occasional problems with excess solder on pads.

As alternatives to HASL, we have experimented with an immersion tin finish with 0.8 mm BGAs and 0.4 mm QFPs but were not impressed with the solderability and are concerned about shelf life. We are considering using the nickel/gold immersion coating but are concerned about black pad. Comments and suggestions would be appreciated.

A: You are right with some of your observations The concern for black pad is well founded, and an effective nickel/gold immersion coating requires diligent process control to avoid the problem.

HASL coating might work, particularly if it is applied horizontally. As you are aware, however, the coating is often not uniform. Organic solder protectants (OSPs) can work well, but multiple passes may be problem--particularly with no-lead solder alloys.

Immersion tin is widely used with success, and immersion silver is gaining popularity over other finishes. However, material cost concerns may be an issue for some applications. With any coating, solderability testing of the PWB and components should be carried out, and the soldering process and solder paste flux should be compatible. The total process needs to be well controlled.

Be assured you are not alone with your dilemma!

Q: How do well-controlled assembly shops track components populated on a specific printed circuit assembly serial number? Often, there are several alternate sources for components and reels of components may be changed at any time during an assembly run. We want to define which specific vendor's parts and date codes are on any given assembly.

A: Specific component sources or date codes can be associated with serialized PWBs during the manufacture by following specific steps. The procedures in a particular company may vary, but the principle and purpose are the same: to aid in detecting and tracking component performance issues and limit the proliferation of quality issues.

The component manufacturer's name or designate number, the P.O. number and the date received at the assembly shop are stored in the assembler's database. Component date codes are valuable to have when troubleshooting is required and should be recorded available.

At receiving, each package is assigned a bar-code label containing a package number, which appears on the component package and on the packing slip. The number is used throughout the manufacturing process to track that specific group of components. Component should not be issued to the production floor without package number. If a partial release is issued to the fleet a new package number must be assigned to the partial release package. With a partial release, the lowest package number must be used on the floor to maintain a firs in/first out (FIFO) inventory.

After components are issued to the floor, the department to which the components are issued is responsible for maintaining tracking and traceability. of the components.

When the components are used on the line, the package number and the associated components ant quantity used are stored in the component tracking system. Before assembly of a lot begins, the component package numbers that are needed to start manufacturing are entered into the system. As the product is assembled, the serial number on the unit is scanned. The components used to assemble that PWB are then associated with the serial number of that unit through the tracking system.

The basis of any component tracking system is the maintenance of component date code identities associated with specific board assembly serial numbers and production dates. Such maintenance requires control systems at the component manufacturer, the assembly shop and the final customer. The ISO 9000 document outline some recommended procedures fur material and quality control.

Les Hymes is the owner of Les Hymes Associates, Surprise, AZ; (623) 544-4646; e-mail:
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Title Annotation:Ask Les
Author:Hymes, Les
Publication:Circuits Assembly
Geographic Code:1USA
Date:May 1, 2003
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