Section 1: Solder Reflow Basics - tch.es

1 Page 1 Research InternationalSolder Reflow Technology HandbookSection 1: Solder Reflow BasicsIntroductionElectronics assembly is the general term forjoining electrical components to printedcircuit boards (PCB). Surface mounttechnology (SMT) is the electronicsassembly process where electricalcomponent leads are joined to the PCB viaindividual pad connections located on theboard basic SMT process consists of thefollowing steps:1. Solder paste is applied to the PCB usinga screen printer. Stencils designed withholes over individual pads control thesolder application to the Electronic components are positioned onthe PCB using placement equipment(pick-and-place machines, chip shooters,etc.). Component leads are placed indirect contact with the The Solder paste is heated until liquidus(reflowed) then cooled until the solderhardens and creates permanentinterconnection between the componentleads and the PCB.

2 This process isperformed in a Solder After Reflow , the assembled circuitboard can be cleaned, tested orassembled into a final SMT lines use automatedequipment to perform these steps. Theselines can typically produce a completedcircuit board in less than 20 seconds, withplacement machines that can positionupwards of 40,000 components per hour onthe PCB. However, once the Solder paste isapplied and the components are placed onthe board, the only way to create afunctioning circuit board is an effectivesolder Reflow process. A high-volume SMT assembly Know-How: Prevent and ResolveSMT Process ProblemsThe Reflow oven is the key to the solderingprocess. Properly working ovens should be invisible to line operators. Yet if processproblems occur during board production, thereflow oven is often the first placemanufacturing engineers look to findanswers. Knowing the Reflow process andways the oven effects soldering results iscritical to consistent SMT purpose of the Reflow TechnologyHandbook is to explain the Reflow processin a way that answers basic solderingquestions.

3 The handbook also helps qualifythe performance factors you should considerto find a Reflow oven to best suit yourprocess InternationalPage 2 Solder Reflow Technology HandbookSolder AlloysThe vast majority of electronicinterconnections are accomplished with tin-lead alloy tin is a soft, shiny metal that is mostoften found in ore form rather than metallicform. It is easily shaped and moldedwithout essence of the Solder process is theability of molten tin to dissolve nearly anyother metal. Copper is one of those metals,and copper is used extensively in themanufacture of printed circuit boards. In thenext subsection we will discuss the role ofcompounds formed when surface copper isdissolved by , when exposed to air, has a dull grayappearance. Like tin, lead is an easy metalto work with because it is both soft does very little to aid the bonding ofmetal to metal during soldering. However,when lead is combined with tin at a ratio of63% tin to 37% lead, the melting point ofthe resultant alloy becomes lower than thatof either pure tin or pure lead.

4 Theabbreviation for tin in chemistry is Sn andfor lead is Pb, so this alloy is usually type of alloy is called a eutecticcomposition. Pure tin melts at 232 C(449 F) and pure lead melts at 327 C(621 F). The melting temperature ofSn63Pb37, however, is 183 C (361 F). Atthis temperature, the alloy goes from acompletely solid to a completely liquid statewithout going through a pasty stage. Thisis called the eutectic eutectic tin/lead is heated to atemperature higher than 183 C it is said tobe in its liquidus stage. The relatively lowmelting temperature has made eutectictin/lead the Solder alloy of choice for printedcircuit board assembly because circuitboards made of FR4 substrate or similarmaterials can be safely processed at addition, the low melting temperaturemeans that the equipment that solders theboards does not have to operate at highertemperatures. Finally, a benefit mostmanagers and accountants appreciate is thelow cost of tin/lead compared with CompoundsAs mentioned above, molten tin dissolvesmost metals.

5 During the Solder process, theprimary metals in the Solder formcompounds with the metals in componentleads and circuit board pads at boundariesbetween the Solder and the pad or lead.(Figure 1-1).Sn63\Pb37 solderCopperpadOne to twomicronintermetalliclayer: Cu3 Snand Cu6Sn5 Figure 1-1. The intermetallic layer at theboundary of the Solder and copper 3 Research InternationalSolder Reflow Technology HandbookThese intermetallic compounds form aboundary layer that is extremely strongwhen it is no thicker than one to twomicrons. At this thickness the naturalbrittleness of the intermetallic layer is not the intermetallic layer thickens, itbecomes more susceptible to cracking. Thecracking is caused by expansion andcontraction of the circuit board substrate asit heats and cools during normal operation ofthe electrical device in which it thickness of the intermetallic layer iscontrolled primarily by controlling theliquidus time of the Solder .

6 Most soldermanufacturers recommend a liquidus timebetween 45 and 60 seconds. (See Section 3 Profiling for additional information.)Other Solder CompoundsSome special applications require alloysother than eutectic tin/lead Solder . Acommon application for other soldercompounds is double sided surface mountassembly. To Solder on both sides of theboard requires bottom side components besoldered with an alloy that has a meltingtemperature higher than eutectic , the board is flipped over and top sidecomponents are soldered with an alloy thathas a melting temperature lower than that ofeutectic tin/lead. The higher meltingtemperature allows the Solder used for thebottom side components to stay solid duringthe top side Reflow , reducing the risk ofintermetallic and Low Temperature SolderingThe addition of bismuth to tin/lead solderreduces the melting temperaturesignificantly without adding serioussolderability problems.

7 A typicaltin/bismuth/lead alloy is Sn43/Bi14 alloy has a pasty stage from 143 C-163 C (289 F-325 F). The pasty stagemeans the Solder is no longer completelysolid at 143 C, but not yet completely liquiduntil it reaches 163 lower melting temperature of this andsimilar alloys means that the flux used in thesolder paste must also be different from thatused in eutectic tin/lead. This may changethe shape of the thermal profile. Consultyour Solder paste supplier for therecommended thermal profile for theparticular low temperature Solder you matter to consider when you uselow temperature Solder is that the meltingtemperature of the Solder is affected by anymetals dissolved from leads or pads thatwere plated to prevent oxidation. Usuallythese contaminant metals raise the meltingpoint of the Solder . If this becomes aproblem in your process, it might benecessary to strip the plating just prior to thelow temperature soldering printed circuit boards havecomponents with silver plated leads.

8 Asnoted in the Section on tin above, molten tindissolves most metals, including the silver insilver plated way to reduce the amount of platedsilver that dissolves in the molten tin/lead isto add a small quantity of silver to the solderitself. The alloy Sn62/Pb36/Ag02 (62% tin,36% lead, 2% silver) is good for popular silver alloy Sn96/Ag04melts at 221 C (430 F) and is sometimesused for double sided InternationalPage 4 Solder Reflow Technology HandbookLead free SolderAmong the most discussed topics in the late-1990s is the effort to remove lead fromelectronic products. Concerns about groundwater lead contamination are forcing thedebate. Many governments are demandingthat Tin/Lead alloys be phased out andreplaced with lead- free solders. Solder pastemanufacturers are working to create newalloys to meet what is likely to become anew regulatory requirement for to Phil Zarrow of ITMC onsulting, the most promising lead-freealloys are in the Tin-Silver family withvariations incorporating relatively lowamounts of copper of bismuth.

9 (CircuitsAssembly August 1999). These alloys costabout the same as Sn63/Pb37, but the 221 Cmelting temperature means that electroniccomponents must be exposed to reflowtemperatures as high as 240 C for Reflow alloy being considered Adding bismuthresults in a melting range between 208 Cand 215 C (Phil Zarrow: Circuits Assembly August 1999).Lead- free alloys are being evaluated toverify their performance to standard Tin-Lead compounds. Anyone involved in SMTproduction should be prepared to converttheir lines to lead- free soldering by the in the air combines with metal oncircuit board pads and surface mountcomponent leads and forms oxides. Thisoxidation blocks the molecular attractionbetween the Solder and the metal at thesurface of the pad or removes oxides from pad and leadsurfaces, as well as from the surfaces of thesolder particles themselves. Molten solderthen can flow evenly over the surface ofpads and leads and form intermetallic bondswith Tension and WettingSurface tension is the attraction thatmolecules at the surface of a drop of liquidhave for each other.

10 If that attraction isgreater than the attraction for the materialwhich the liquid touches, the liquid will notspread, but will remain in drop tension explains why a drop ofrainwater on a waxed car stays in drop wax surface is intended to repel waterand keep it from bonding to the car earth s gravity works against surfacetension and tries to flatten the raindrop intoan oval. If the attraction of the molecules ofrainwater for each other is greater than thepull of gravity and the attraction of the car ssurface, the drop will remain somewhatround and self contained. If gravity isstronger and the wax is worn down, the dropwill is the word used to describe theextent that Solder flows over the surfaces tobe bonded. Poor wetting is the result ofsolder particles in the paste bonding tothemselves to form a sphere, so the edges ofa poorly wetted Solder joint are roundedrather than is measured by the angle madewhere the edge of a layer of Solder pastemeets the pad on a circuit board.