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DVD-67C ScriptOnce you're satisfied with the alignment, we're ready to select a tip for the soldering iron. Tipselection will be based on the width of the component leads. If a chisel tip will fit - it willtransfer heat faster than a conical tip. The selection of the solder wire will also depend on thewidth of the leads. For this example, we’ll be using a tin-silver-copper alloy that has a meltingtemperature of 217 degrees C.Now we're ready to tack the component in position. It’s helpful to hold the component down withtweezers or a wooden stick - so that it doesn't move when you tack two of the corners. Acommon method of tacking the corners is to add a very small amount of solder to the tip, and thentouch the tip to the junction between the land and the lead.At this point, we’re ready for the soldering process. To improve wetting, it’s usually a good ideato add a compatible type of external flux – even though there is already some flux in the core ofthe solder wire.We begin by resting the tip flush against the top of the lead. Now contact the lead and the landwith the solder wire - away from the tip. This will allow the flux to clean the connection - insteadof burning away on the tip of the iron. You don't want to press down on the leads or lands. Evenlight pressure can bend or skew the lead out of position - or damage the land. You can proceedfrom one joint to the next – until all the joints are soldered.Unless you are using a no clean flux, it's a good idea to clean the flux residue. Your companywill provide you with a compatible cleaning solution. A brush can access between the leads andaround the component - but you need to be careful not to bend the leads with excessive force.There will also be flux residues underneath the component that you may not be able to remove byhand. Reworked assemblies can also be processed through a cleaning machine - to remove thesehidden residues.Now we’ll inspect these joints to make sure they look right. Notice that the tin-silver-copperalloy is grainier in appearance than the tin-lead solder joints you’re used to seeing. Thisgraininess is caused by the higher soldering temperatures and is generally considered a cosmeticissue. We’ll discuss lead free solder joint evaluation more thoroughly in section 3.A faster method of hand soldering rework is by what’s known as “drag soldering.” Althoughdrag soldering is an excellent technique to use during tin-lead solder rework, it is much moredifficult to accomplish successfully with the lead free alloys. There is a tendency for the solder towet poorly and bridge from lead to lead.One method for increasing the probability of a positive outcome when drag soldering in a leadfree process is to preheat the assembly. Preheating will improve the wetting of the solder. Buteven when we preheat, there will be the occasional solder bridge. Solder bridges can be fixed byadding flux and drawing off the excess solder onto the tip of the iron.The choice then becomes whether to fix the solder bridges one at a time – or to solder the leadsone at a time. Some find that by preheating the assembly, drag soldering is faster than point topoint soldering – even when you factor in repairing a solder bridge or two.Our last topic in this section involves touching up unacceptable solder connections. The mostimportant consideration is using compatible lead-free solder and flux -- and setting the solderingtemperature for the higher lead free alloy melting temperature. Whenever a solder joint isrejected for insufficient solder, you’ll need to add a small amount of solder to touch up the joint.4
DVD-67C ScriptTo repair a solder bridge, position a clean solderless tip on top of the bridged leads and reheat thejoints completely. When you remove the tip the excess solder will follow the heat source. Makesure to always melt both of the solder connections completely during the bridge removalprocedure – to avoid creating differences in the internal structure of the solder joints.Lead Free Surface Mount Solder Joint EvaluationThis final section involves the visual inspection of lead free solder joints. We’ll be examiningthe attributes of a typical lead free connection - for chip, gull wing and J-lead components – andcomparing these lead free joints with their tin-lead counterparts. That’s so you’ll be aware ofwhat to expect when evaluating lead free connections. We’ll also be looking at some of thevariations that may or may not be acceptable.This is a good example of a preferred or target rectangular chip component using tin-silvercoppersolder. Notice how the solder feathers onto the land… and up onto the termination. Thesolder fillet is curved inward - or concave. The solder covers all of the land, and the termination.The texture is a little bit grainy. The outline of the termination is visible beneath the solder. Theamount of solder here is just about perfect.Notice the difference in appearance between this solder joint and the tin-lead solder connectionsthat you’re used to seeing. Here are examples of target gull wing and J-lead solder joints. Again,let’s compare them with their tin-lead counterparts.Here’s another chip component - with a little bit less solder… You can see that there is still somegraininess… and the coverage is even. The fillet is concave. This solder joint is perfectlyacceptable - even though there is less solder in the connection.Now here’s another one – with quite a bit more solder. The important thing to notice is the angleof contact between the solder and the metal. Proper wetting is indicated by a solder contact anglethat does not exceed 90 degrees – which is a square or right angle. The lower the contact angle,the better the wetting.As you can see, there is a range of acceptable solder joints. You will be expected to know anacceptable solder joint from a defective one – before you begin evaluating surface mount leadfreeconnections. The requirements for acceptable tin-lead and lead free solder joints are referredto in the <strong>IPC</strong>-A610D standard.Now let’s look at some solder joints that are less than ideal. Here are examples of non-wetting.Notice that the solder has not adhered to either the termination or land. Non-wetting will occur ifthe reflow or wave soldering temperature is too low, or there is poor flux activity.Incomplete reflow occurs when insufficient heat is applied to the assembly during the solderingoperation. This means the solder paste may not have properly wetted the surfaces to be soldered-- and will not form an acceptable electrical and mechanical connection.Tombstoning is a defect that is specific to chip components. Notice how one end of thecomponent termination is completely lifted off the land.5
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