Why is Auto Tin, Alloy Conversion and Alloy Refresh required?

Auto Tin

This process is aimed at the “high reliability”  (Hi-rel) industries such as aerospace and defence. If your iPhone reboots itself once a week it’s an inconvenience, if you’re a pilot and your Boeing 737 engine shuts down and reboots in the air once a week, it’s a far bigger problem.

Our services are then aimed at making the electronics in these sectors more reliable.

For many years, up till 2006, the electronics industry used a certain type of solder to connect components on PCBs. This was 63/37 solder (tin/lead), comprising of a mix of metals that was 63% tin and 37% lead. This mix was the best and most suitable for use – it melted at a lowish temperature ensuring the components weren’t at risk from overheating, it formed a strong and stable joint.

Then the European Commission decided that lead content in solder was bad for the environment, as when electronics goods reached end of life, they would be scrapped and the “dangerous” lead could contaminate the environment. When the lead was removed, the solder then became almost 100% tin, some variants have small amounts of other metals, eg 1% silver. However this form of solder, known as SAC alloy, does not perform as well as 63/37 and has one major flaw, it reflows at a higher temp, subjecting the electronics to a higher danger of thermal damage. It also isn’t so good at thermal cycling.

The major flaw though is it can result in “tin whiskers”. These are thin strands of tin (they look like cats whiskers) that can grow uncontrollably from IC terminations covered in pure tin. No one knows why, it’s suspected they can be caused where the leg is under strain from being bent, but no one really knows, when and how they will start. They are a huge problem though, as a tin whisker can grow from one termination till it touches an adjacent termination and cause a short circuit in the IC, damaging it.

There are two popular ways of eliminating the chance of tin whiskers occurring :

  • You can use SAC alloy on the PCB, but then spray on a type of clear varnish. This doesn’t stop tin whiskers forming, but it stops them growing to touch other terminations. It doesn’t always work though, and causes other issues eg you cant then rework the PCB
  • You can reintroduce lead to the solder alloy, as the presence of lead stops tin whiskers forming

Hirel applications (eg aerospace) have three issues over consumer applications (eg iphones) :

  • Hirel products have to last many years longer than a consumer product, eg an aeroplane is in service for at least 25 years, whereas a PC may only be used for a few years
  • Hirel products are built in lower volumes than consumer products, which gives an oxidisation (rust) risk
  • Hirel products can be subject to extreme temps and vibration, (eg aeroplane, oil rig, car) whereas a PC isn’t.

So when lead was banned from solder, certain hirel industries were exempted from eliminating lead, until the long term effects of SAC alloy were known

So hirel industries were and continue to be allowed to use 63/37 solder on the terminations of their electronics. However they don’t use high volume. Boeing may be a huge corporation, but they perhaps make 2 planes per week, so don’t buy electronics in high volume. So they cant demand that IC manufacturers make them components in tin/lead solder, they don’t have the buying power. So all components are made lead free to suit the high volume buyers (laptop, phone etc).

So how can auto tin thru Retronix help the hirel industries?

Our auto tin is a process of dipping the terminations of a component into a solder bath to either :

  • Change the alloy on the termination (eg from SAC to tin/lead)
  • Refresh the alloy to eliminate oxidisation (rust)

Alloy Conversion

Hi-Rel companies buy lead free (SAC) alloy components from the manufacturers and send them to us. Retronix converts new SAC alloy components to tin/lead (to be used in hi-rel applications) by dipping them in the correct solder. We generally dip them twice, once in tin/lead scavenging pot to change the alloy, and again in a tin/lead pot to ensure the termination has been converted.

This ensures that long term tin whisker issues are eliminated. It also eliminates oxidisation risk.

Double Dip

For applications where it is known the electronics will be in an environment where they will be exposed to extreme heat and vibration, a “belt and braces” option to ensure the solder joints will be strong exists, developed by Halliburton, the oil company. Halliburton introduced this to great success in oil rig applications. This involves double dipping component terminations in alloy to completely eliminate oxidisation and ensure a solid solder joint.

Alloy Refresh

One of the issues of hirel is low volume assembly. So a manufacturer could buy a reel of eg 3k components and use only 200 of them. The reel is then open, and the component terminations are exposed to the atmosphere until the next batch of components are used, which could be a year. Like most metals, over time this leads to the risk of oxides (rust) forming on the termination. If this is not removed, it can create a barrier between the component termination and the solder joint which can weaken the joint. This may not be an issue in eg a PC where there is no vibration or heat on the solder joints. However an aeroplane might be sitting on the tarmac in dubai airport at 40 deg, then goes thru extreme vibration during take off, and 10 mins later is at -60 deg at 30k feet. So this could be an issue and needs eliminated

Retronix avoid this by dipping the terminations in the same alloy, which burns off any oxidisation (which cant be seen) and ensures solder joint will be good

Alloy conversion achieves the same result, when you change the alloy you remove any oxidisation at the same time.