Features Articles
A selective soldering machine can be used to fix misaligned parts.
OK, I confess. Sometimes we do make mistakes. As much as I hate to admit it, it does happen. For example, we’ve seen headers not fully seated to the board. They’re a little crooked, or one edge is touching the PCB, while the other edge is so far up in the air that you can’t even see the pin protruding through the other side of the board. Other times, an edge connector needs to protrude through a cover panel and there’s no room for play. The connector must be perfectly flat and perfectly square. If you find yourself in this situation, your selective soldering machine can be your best friend.
Warning! On some machines, you may be required to bypass security features. Please consult your manufacturer before bypassing anything designed to protect you from a dangerous machine.
If the connector is larger than the size of the nozzle, program the machine to “walk” back and forth over the pins of the connector (Figure 1). There’s a limit to how long a connector could reasonably be reworked using this method. We’ve been able to fix 2"-long connectors on four-layer boards. The key was to make sure the nozzle could dwell a little bit on the pins that were connected to ground.
Figure 1. A selective soldering machine can be programmed to move back and forth over the pins of a large connector.
Every connector is different. Some connectors will become damaged after much exposure to heat. The plastics are not designed to handle it. For these connectors, we find it’s best to remove the entire connector and insert a new one. Other connectors, however, hold up well under the heat, and you’ll be able to apply slight pressure with your hand to push them back through the board. Be careful, however, as in many cases, pushing the pins down will also push the solder down, and you’ll have no topside fillet.
Make sure to preheat the board. You don’t want to hit this cold board with a bunch of molten solder. The thermal shock could shorten the lifetime of the assembly. Make sure it’s nice and hot before you begin.
Chris Denney is chief technology officer at Worthington Assembly (worthingtonassembly.com); cdenney@worthingtonassembly.com.
Poorly maintained baths or improper wave settings can lead to whiskers.
A solder whisker or a dross short is surface contamination found after soldering due either to a poorly maintained solder bath or incorrect setting of a wave. If a solder bath is poorly maintained, dross can be present in the ducting below the solder surface. When the wave operates, any dross can emerge randomly in the wave and contact the board and form micro shorts.
In the case of the back flow on a lambda style wave, the solder should flow at the same speed as the board or slightly faster. The board displaces any surface oxides on the surface of the wave before it can contact the base of the board. If the wave stops flowing before the board exits the wave, the board will contact the surface oxide, leading to what is sometimes referred to as “snail trails.” This can also be seen on waves running nitrogen where only the surrounding areas of the wave are inserted.
Similar micro shorts have been seen during wave soldering in a Pb-free process where the copper level is excessive and the temperature has permitted SnCu needles to form and flow in the wave. The same is true if the surface of the tank becomes eroded, permitting SnCuFe needles in the bath. Both these contamination issues can lead to very fine needle-like solder shorts. The excess copper can be overcome by balancing the copper levels with tin. SnCuFe is a real problem not only to the system but also the extremely expensive alloy, which will need to be changed.
These are typical defects shown in the National Physical Laboratory’s interactive assembly and soldering defects database. The database (http://defectsdatabase.npl.co.uk), available to all this publication’s readers, allows engineers to search and view countless defects and solutions, or to submit defects online. To complement the defect of the month, NPL features the “Defect Video of the Month,” presented online by Bob Willis. This describes over 20 different failure modes, many with video examples of the defect occurring in real time.
Figure 1. Solder whisker bridging two conductors.
Chris Hunt is with the National Physical Laboratory Industry and Innovation division (npl.co.uk); chris.hunt@npl.co.uk. His column appears monthly.
We started this year wanting to stress the innovation that is occurring in electronics, often below the radar of the masses. Twelve months later, we find even ourselves a bit stunned at the depth and breadth of what we uncovered.
