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Reflow troubleshooting guide

How to deal with soldering errors effects and to find solutions
Reflow troubleshooting guide

In general, reflow soldering using convection dominant (forced convection) reflow ovens is a repeatable process. This assumes that the equipment is maintained and that the proper reflow profile was attained. Here we describes some of the common soldering defects and recommend directions to look for a remedy. The user should not be shy about looking upstream – at component and board, paste, printing process and the board design.

Phil Zarrow, ITM Consulting,Durham, NH, USAfor KIC Thermal Profiling

The reflow profile must be matched to the specific requirements of the solder paste as well as the thermal thresholds of the PCB assembly. It is essential that accurate profiling be achieved and thus use of an accurate data recording system as well as a board that has been instrumented properly is imperative. Lead-free solder pastes do not change the basic principles of solder reflow. They do, however, significantly reduce the size of the thermal process window, and hence make reflow a more critical step in the assembly line, affecting total product quality yield.
It should also be noted that most solder defects originate in the solder deposition stage, either through improper printer parameters, damaged or incorrectly designed stencils, or less than adequate solder paste. A legendary study done a number of years ago by Hewlett-Packard revealed as much as 60% of solder defects in PCB assemblies were caused by such means. Subsequent experiences by the author with users is consistent with this figure.
The initial design of the board, particularly pad geometries and solder mask parameters, also have a profound effect on the quality of soldering accomplished. Good design for manufacturability is a fundamental to achieving low solder defects.Finally, the quality of the materials being soldered is also of primary importance. Besides having a high-quality (and qualified) solder paste, parts and boards with good solderability are also required. Control of the procurement and storage of parts and PCBs is essential.
Defect: solder balls
Solder balls are very small fines of solder that have separated from the main body that forms the joint. Their formation is promoted by excessive oxides in the solder paste that inhibit solder coalescence during reflow. Solder balls are probably the most common reflow solder defect, and there are many causes beyond the system that will contribute to their formation.
Process and design-related causes:
• Improper pad design
• Weak solder paste (for ambient con-ditions)
• Expired solder paste
• Misaligned print (overlapping solder mask)
• Misaligned solder mask (overlapping solder pad and print area)
• Flux is too weak for the oxidation level of the board and/or components
• Components have marginal solderability (a degree of contamination)
• Boards have marginal solderability (a degree of contamination)
• Components placed with too much Z-axis downforce by the pick&place machine
• Excessive solder paste slump after printing
• Sphere size of solder paste too large for pitch components being soldered
• Excessive moisture absorbed by solder paste
• Solder paste exposed beyond worklife
• Ambient humidity and temperature beyond solder paste work envelope
Reflow-related causes:
• Too rapid heating, particularly in preheat resulting in splattering. Keep below 4K/sec. per any 20 sec. interval or follow specific parameters specified by solder paste manufacturer
• Profile incompatible with solder paste formulation resulting in drying out paste prematurely. Assure that the preflow soak (if used) is compatible with paste manufacturer’s specifications
• Reduction of oxidation can be achieved by reflow soldering in an inert (nitrogen) atmosphere. Some ultra-low residue, no-clean formulations specify this
Defect: wicking
Wicking is the flow of solder either up the lead of the component or along traces and possibly under insulation and through via holes. It robs solder from the interconnection, and is usually caused by uneven heating between the materials being joined. This is seldom observed in convection dominant reflow and was more a trait in older IR dominant reflow and vapor-phase technologies. Solder mask can be used to control the flow of solder down a trace. A proper heating profile, particularly with a controlled preheat is a remedy to differential heating.
Defect: opensor insufficient solder
The solder joint is incomplete resulting in an open circuit or a weak interconnection.
Process and design-related causes:
• Non-coplanar leads on the component
• Excessive warpage of the PCB or substrate
• Poor wetting
• Insufficient amount of solder due to improper printing parameters
• Skips in the printed solder due to blocked stencil aperture
• Misaligned solder print
• Improper stencil thickness
• Inadequate stencil aperture size
• Excessive pad size
• Via in pad draining solder from interconnection
Reflow-related causes:
• Preheat too aggressive
• Peak reflow (liquidus) temperature not being attained
• Malfunctioning oven which is impeding attenuation of proper temperatures
Tombstoning
A type of open, known by a number of names (including crocodiling, surfboarding, Manhattan effect, drawbridging, Stonehenge effect, billboarding, etc.). This is a soldering defect in which a chip has been pulled into a vertical or near-vertical position with only one side soldered to the PCB. It is typically caused by force imbalances during the reflow soldering process.
Process and design-related causes:
• Improper pad design
• Improper pad trace
• Improper solder mask application
• Via in pad draining solder from connection
• Trace with solder mask over it running under component causing a fulcrum effect on the component
Reflow-related causes:
• Improper pad design may be aggravated by use of nitrogen. The nitrogen increases surface tension of the metals to the extent that a previously marginal problem becomes distinct.
Bridging
A bridge is comprised of solder that spans across two conductors that should not be electrically connected thus causing an electrical short.
Process and design-related causes:
• Excessive volume of solder due to improper pad dimension
• Excessive volume of solder due to improper stencil aperture
• Improper application of solder mask
• Excessive slump of solder paste
Reflow-related causes:
• Improper reflow profile for solder paste. Check against manufacturer’s specifications.
Dewetting
Dewetting is a condition that results when molten solder has coated a surface and then receded, leaving irregularly shaped mounds of solder separated by areas covered with a thin solder film and the base metal is not exposed.
Process and design-related causes:
• Solder paste flux not aggressive enough for level of oxidation present on part or PCB
• PCB pad contaminated
• Component lead contaminated
• Excessive moisture absorbed by paste
• Paste exposed beyond worklife
• Ambient humidity and temperature beyond paste work envelope
• A palladium lead finish which requires higher reflow (liquidus) temperature
Reflow-related causes:
• Peak reflow (liquidus) temperature not being attained
• Malfunctioning oven which is impeding attenuation of proper temperatures
Nonwetting
Nonwetting is a condition whereby a surface has contacted molten solder but has had part or none of the solder adhere to it. Nonwetting is recognized by the fact that the base metal is visible. It is usually due to the presence of contamination on the surface to be soldered.
Process and design-related causes:
• Solder paste flux not aggressive enough for level of oxidation present on part or PCB
• PCB pad contaminated
• Component lead contaminated
• Excessive moisture absorbed by paste
• Paste exposed beyond worklife
• Ambient humidity and temperature beyond paste work envelope
• A palladium lead finish which requires higher reflow (liquidus) temperature
Reflow-related causes:
• Peak reflow (liquidus) temperature not being attained
• Malfunctioning oven which is impeding attenuation of proper temperatures
• Flux activity too weak for the level of oxidation present on the PCB and/or components
Voids
Voids are cavities or air pockets inside the solder joint formed by gases that are released during reflow or by flux residues that fail to escape from the solder before it solidifies.
Process and design-related causes:
• Improper solder volume due to improper land design
• Improper solder volume due to blocked stencil aperture
• Improper solder volume due to improper stencil design
• Paste viscosity too low
• Paste metal content too low
• Bad or expired solder paste
• Ambient humidity too high for solder paste work envelope
Reflow-related causes:
• Preheat too aggressive for flux – adjust to manufacturer’s recommendations
• Improper profile for solder paste
Delamination
Delamination is a separation between any of the layers of the base material or between the laminate and the conductive foil, or both.
Process and design-related causes:
• Improper fabrication of the PCB
• Improper packaging of PCBs during shipping
• Improper storage of PCBs resulting in excessive absorbed moisture. (Some practitioners pre-bake components and boards to drive out moisture. This is not encouraged and should be a last resort as baking tends to build intermetallics as well as degrade the solderability by increasing oxidation levels. Shipping and storing suspect plastic molded components in nitrogen is preferred.)
Reflow-related causes:
• Preheat portion of profile and rate of heating too aggressive (should not exceed 4K/sec.)
Component cracking
Cracking of multilayer ceramic capacitors has been attributed to excessive heating rates during reflow. This may be somewhat mythical and what little cracking is still observed usually tends to be due to defects in the manufacturing process of the capacitor. Cracking of plastic-molded packages, such as QFPs and BGAs, sometimes called the popcorn effect is sometimes due to absorbed moisture in the thermoset plastic body but may also be due to a flaw in the design and/or manufacture of the component.
Process and design-related causes:
• Improper fabrication of the component
• Improper design of the component
• Improper packaging of component during shipping
• Improper storage of components at incoming and on the manufacturing floor resulting in excessive absorbed moisture
Reflow-related causes:
• Preheat portion of profile and rate of heating too aggressive (should not exceed 4K/sec. per 20 sec. interval.
Intermetallic growth
Where the tin-lead solder adheres to the copper of the lead (from plating) and to the pad (from hot air leveling or other board solder-coating method), an intermetallic of tin-copper forms. This is typically Cu6Sn5 or Cu5Sn6 and this intermetallic is what causes the solder to adhere to the copper pads and leads. During reflow, with the addition of tin-lead solders, the intermetallic grows. With other board metal finishes, other metallics form, such as AuSn when soldering components to boards with gold-flash finish (and tin-lead solder).
Over time, an intermetallic layer grows. The speed at which it grows is governed by the thermal excursions as well as the ambient temperature. One of the unfortunate properties of the intermetallic is that it is far more brittle than the tin-lead solder or the copper (or other metals involved). Over time, as this intermetallic layer grows thicker, it becomes the Achilles Heal of the solder joint in terms of integrity. Depending upon the stresses imposed on the assembly and the solder joints, this is what usually ultimately leads to joint cracking and failure.
Typically we measure intermetallic growth over months and years. However, during the reflow process, the intermetallic growth is somewhat accelerated due to the high thermal excursions (the highest the joint will ever see) and the fact that the tin-lead is in a molten state. With double-sided boards, there is additional intermetallic growth as the side A becomes liquidus again. If there is repair, the heat of the repair system can induce accelerated intermetallic growth in the joints being reworked and adjacent joints. (This is one of the key reasons we never perform touch-up for purely cosmetic reasons.)
While excessive intermetallic growth is difficult to measure and quantify on the shop floor, it is ultimately a cause of defects. Such growth can be as much as several microns during reflow. Therefore it is imperative that liquidus dwell time -the interval that the interconnection is above the melting temperature of the solder alloy, be kept as short as possible. It is also imperative that the board not be heated any higher than absolutely necessary and that peak temperatures are kept as close to T1 as possible.
Dull solder joints
Dull (non-shiny) solder joints are usually the effect of coarse grain structure in the solid solder joint (though there can be other causes). The more slowly a joint cools, the coarser the grain growth and, conversely, the faster the joint cools, the finer the grain growth and the shinier the joint. Dull or grainy joints are not a big concern – the grain structure will grow in a joint over time anyway. But having a dull joint is giving this eventuality a “head start”. However, under no circumstances, should a joint be touched up by applying a soldering iron (or other means) simply for the cosmetics of a shinier joint. The damage you may be causing by inducing accelerated intermetallic growth will far offset any gains in joint strength through a finer grain structure.
Process and design-related causes:
• Impurity in the component plating
• Impurity in the circuit board finish
• Bad or expired solder paste
Reflow-related causes:
• Cooling too slow inducing coarse grain structure and a dull finish. Accelerate cooling by use of cooling modules. It is recommended that a cooling rate of 4K/sec. over any 20 sec. Interval is not exceeded
Zusammenfassung
Der Lötprozeß steckt voller Tücken, dabei betrifft es gar nicht so sehr das Löten als solches, sondern zumeist die Randbedingungen. Das sind das Leiterplattendesign, die Lötbarkeit der Bauteile, der Fluxer, das Material der Komponenten inklusive LP, das Lötprofil usw. Die Litanei ist ziemlich lang. Als Hilfe zur Selbshilfe, wenn es wieder mal nicht optimal klappen sollte, hier ein praktischer Leitfaden darüber, wie häufig vorkommende Störungen zu beheben sind.
Résumé
Le processus de brasage comporte nombre de problèmes qui ne concernent pas à proprement parler le brasage par lui même mais bien plus les conditions «marginales» : la conception des circuits imprimés, l’aptitude au brasage des composants, le poste de fluxage, les matériaux des composants y compris les CI, le profil de brasage, etc. La liste est encore longue. A tire d’aide si les opérations ne se déroulent pas de la manière prévue, voici un guide pratique permettant de remédier aux problèmes fréquemment rencontrés.
Sommario
Il processo di brasatura nasconde molte insidie, che non riguardano la brasatura vera e propria ma più che altro le condizioni corollarie in cui essa avviene. Si intendono qui il design delle schede dei circuiti, la saldabilità dei componenti, il fluxer, il materiale dei componenti incluso quello delle schede, il profilo di brasatura etc. e chi più ne ha più ne metta. Per trovare aiuto in caso di problemi ecco una pratica guida su come eliminare le più frequenti e conosciute difficoltà.
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