Transcript - IPC Training Home Page

DVD-67C ScriptLead Free Surface Mount ReworkThere are two basic types of rework. The first involves removing the component, preparing thelands, then repositioning and resoldering the component. The second involves simply touchingup a solder connection that doesn’t meet quality and reliability standards.We’ll discuss component removal and replacement first. There are a number of ways to remove acomponent, prepare the lands and to resolder the connections. The method you’ll choose has todo with the type of component being reworked and your company’s policy.For a comprehensive demonstration of the various techniques available, please refer to IPC’sSurface Mount Rework Series, as well as our training DVD on Rework Stations. This video willconcentrate on the differences between tin-lead and lead free rework processes – and we won’tget into each of the soldering processes for removing and replacing components.Component removal is pretty much the same whether you’re reworking a tin-lead solder joint, ora connection made with a lead free alloy. The main difference is the amount of heat you’ll needto melt the solder joints. The lead free alloys require a higher temperature.Let’s watch a common technique for removing an SOIC component using a soldering iron fittedwith a special tip. Before beginning, it’s important to clean and tin the tip of the iron. Now,place the tip over the component. Once all the solder joints have melted, lift the iron and removethe component. Then set the hot component on a heat resistant surface until it cools down. Let’swatch another component removal procedure using hot air.At this point, the solder that remains on the lands needs to be removed so that the component canbe properly repositioned and resoldered. Land preparation is typically done with a solder braid.Again, higher soldering temperatures are used to “wick” the lead free solder onto the braid.You’ll use a fine point soldering iron tip to accomplish this operation. Using a compatibleexternal flux will improve the wetting characteristics and allow the solder to wick onto the braidquicker. This decreases the possibility of thermal damage to the circuit board.Another method is using continuous vacuum extraction. After the tip heats up, we'll apply a lightcoating of solder right on the tip. Again, you may also want to apply an external flux over all ofthe lands. Both of these steps will help speed up the heat transfer process. Now we position theflat end of the tip over the first few lands.The vacuum should now be activated, and allowed to run on a continuous basis. The surge of airthrough the orifice will tend to cool the tip. One advantage of the larger tips is they can maintaintheir operating temperature better than the thinner tips - since there is a larger mass through whichthe heat is transferred. Notice how we move the tip along the length of the lands - to remove all ofthe solder. It's very important not to put any downward pressure on the lands. Once the solderhas been removed from the lands, any flux residue should be cleaned.Now, we’ll reposition and solder the component. The easiest and safest way to pick up andposition this SOIC component is with a vacuum pickup tool. You can usually position an SOICcomponent onto the lands without the aid of a microscope. Just make sure that all of the leadsare centered evenly - from side to side. You also need to make sure that the heel and toe of eachlead is centered evenly in the lengthwise direction.3

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 IPC-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

DVD-67C ScriptThis joint has excessive solder. The fillet is not concave, and there is solder flowing over theedge of the land. Too much solder paste was deposited on this land and it is not possible to tellwhether proper wetting has occurred.This condition is called solder bridging. A bridge is an electrical path, or short circuit that wasnot intended.And these are solder balls. They can create electrical shorts - by lodging in between twoconductors or component leads.There are several types of cosmetic imperfections – such as fillet lifting, cooling lines and tearsthat are more likely to occur in a lead free process. It’s important to understand that these typesof conditions typically won’t affect the strength of the mechanical and electrical connection of thesolder joint.Although some of these conditions are always unacceptable - many of these imperfections can beallowed - in various degrees - depending on what type of product they would be used in. Forexample, we’d all expect that any aircraft or medical electronics would be highly reliable… butwe may be able to live with some minor cosmetic imperfections in a portable CD player - as longas the product works.The IPC A-610 has three classes of requirements, based on the final use of the end product. Class1 is for consumer products, Class 2 is for business and computer products, and Class 3 is for highreliability electronics - where failure is not an option. The specification and the class ofrequirements that your company uses to inspect solder joints will be clearly explained to you.Some companies build a wide range of products with different requirements. Other companiesbuild only one class of product. Part of your job is to make sure to always apply the right criteriato each assembly.The program has presented the essential information required to successfully use lead free solderalloys in surface mount rework operations. We discussed the reason for the switch to lead freeand the characteristics of the specific alloys.Then we showed what’s involved in lead free hand soldering rework – for surface mountcomponents. And we concluded with a brief explanation of the acceptance criteria used toevaluate lead free surface mount solder joints.The more time you spend working with and observing the results of lead free alloys, the moreconfident you’ll become about reworking and evaluating reliable solder joints. And you’ll behelping the environment too.6