NEWS // FEATURES // NEW PRODUCTS
Re-tinning New Components for Electronics Assembly: Why It’s Done
Alan Cable, President, ACE Production Technologies
assembly. For example, let’s say that I’m building a hi-rel assembly that’s put together using traditional lead-bearing Sn63 alloy. Unfortunately, one of the components that I need to use in that assembly is only available in a RoHS-compatible fi nish, in this case, electroplated tin. What do I do? If the component is plated with tin, shouldn’t it be compatible with tin/lead solder? Well not exactly. The added volume of tin in the plating,
T
however slight, will change the percentage of tin in the solder joint, creating a new alloy that may have undesirable characteristics, such as being more brittle. The other reason for hot solder re-tinning is to prevent the formation of tin whiskers, which tend to grow out of a plated fi nish, but won’t grow out of a fused alloy fi nish applied through dipping in hot solder. One cannot simply solder a component with tin plated leads to a military circuit, because any portion of that lead surface that is not wetted with solder has the potential for tin whisker growth and catastrophic failure of the circuit through shorting. Therefore, what one has to do is remove that pure tin plating and replace it with a fused fi nish of the same solder alloy that is going to be in the connecting solder joint. This is the typical method of converting modern components that were born after
RoHS with lead-free fi nishes that are compatible with
20 EIU
here are a number of good reasons why one might have to re-tin components from the stockroom prior to
tin/lead assembly, usually found only in hi-rel or military applications. Since RoHS, there have been fewer and fewer components available with tin/lead fi nishes as an alternate choice. If not plated with pure tin, these
components may also have a gold plating fi nish on the leads that also must be removed. One does not want gold to be present in the solder joint because it can lead to a phenomenon known as gold embrittlement. In the tinning process, these components,
whether tin or gold plated, are dipped into a pool of solder which washes away the undesired plating. The components are then re-tinned with a tin/lead fi nish. The tinning process basically involves the use of two
solder pots, the fi rst one being the ‘sacrifi cial’ pot that removes the unwanted plating of, gold, tin, or other alloy. The second pot is the ‘virgin’ pot that contains the alloy that is going to be applied to the component so that it can be assembled. Essentially, the component is picked up, dipped in fl ux, then dipped in the fi rst pot to remove the unwanted plating. During this initial dipping, the tinning system should employ some manner of agitation in the pot to produce a ‘scrubbing’ effect for complete removal of the unwanted metallization. The component is then fl uxed again and dipped
into the ‘virgin’ pot to apply the new fused fi nish. The component isn’t dipped fl at, but is tilted, so that one side at a time is dipped. There are a number of reasons for doing this, primarily to prevent icicling, for a clean peel away of the solder. Inerting is also used to minimize icicling, particularly when using lead-free alloys. t’s very important that this entire process is automated.
Reasons for this include the obvious desire for repeatability and consistency of results component to component, but also to take it out of the hands of the operator, to prevent damage to delicate fi ne pitch leads. It’s much better to automate and thoroughly control the process precisely
www.electronicscomponentsworld.com /
www.electronicsproductionworld.com January 14th 2012
Page 1 |
Page 2 |
Page 3 |
Page 4 |
Page 5 |
Page 6 |
Page 7 |
Page 8 |
Page 9 |
Page 10 |
Page 11 |
Page 12 |
Page 13 |
Page 14 |
Page 15 |
Page 16 |
Page 17 |
Page 18 |
Page 19 |
Page 20 |
Page 21 |
Page 22 |
Page 23 |
Page 24 |
Page 25 |
Page 26 |
Page 27 |
Page 28 |
Page 29 |
Page 30 |
Page 31 |
Page 32 |
Page 33 |
Page 34 |
Page 35 |
Page 36 |
Page 37 |
Page 38 |
Page 39 |
Page 40