ASSCON Condensation Soldering

Transcription

1 ASSCON Condensation Soldering Quality Improvement under Oxidation-free Soldering Conditions ASSCON Systemtechnik GmbH Messerschmittring Königsbrunn

2 What do we expect in future? Miniaturisation of components Massive components, cooling coils for power applications PCB boards up to 84 layers Everything mixed on one board MID (3 dimensional soldering) Voidless soldering Temperature critical components

3 Low Oxidation Reflow Soldering Technology Upon a changeover of the reflow soldering technology to a low oxidation soldering process by introducing vapour phase soldering systems or nitrogen soldering equipment some prevailing physical conditions will change Less oxidation of the paste and the soldered joints due to an oxygen exclusion during the thermal process. The use of minimally activated pastes having few residues is possible. Distinctly better wetting properties of the solder since the surface tension of the solder is reduced by the absence of oxidation. The following quality criteria are significantly enhanced Distinctly better wetting due to a reduced oxidation Very good self-centring of components (Prerequisite: Correct pad design) Reduced bridge formation IMPORTANT: Comparable to nitrogen flow soldering, however, different requirements to an optimum layout will result. This is reflected in the worldwide trend towards distinctly smaller pads, particularly for processing smaller and miniature components.

4 The Idealised Lead-free Standard Profile Basis: Metallurgy Component limits Basic material limits Attention: Despite of this the validity is to be explicitly examined for each product. Particularly maximum temperature gradients may be distinctly lower, e.g. ceramic components. Minimum temperature (min. 10 C over liquidus): Temp. gradient up: Temp. gradient down: Time over liquidus: Time at min. temperature: Max. time from soldering start to the end: Type of profile: 230 C betw. 0,5 and 3 C/sec. betw. 0,5 and 3 C/sec. betw. 30 and 120 sec. min. 10 sec. 5 min. preferred is linear

5 Warpage The reasons of warpage is due to the different heat expansion and contraction coefficients of the materials used in components and PCB s. This effect will be drastically seen by to the current trends towards even complex products. Warpage occurs during heating as well as cooling down. Reparable as well as irreparable defects may occur. Source: AMKOR Reparable: Open soldering joints Bridges Irreparable: Cracks in various sections. Cracks in internal bonding wires of components, etc.

6 Influence of Thermal Expansion Coefficient The graph clearly shows the different thermal expansion rate a component goes through during a thermal reflow cycle. Source: AMKOR These differences in stress behaviour lead to similar changes like in Bimetal, geometrical changes and high internal mechanical stress.

7 Possible cracks due to warpage The following irreparable damages can occur frequently : 1. Cracks in the internal copper layers of the PCB. 2. Cracks in the inter-metallic layers between pad and component connection. 3. Cracks in the inter-metallic layers between component connection and inner pad. 4. Cracks in component internal connections

8 Possible cracks due to warpage Defect 1 Defect 2 Defect 3 Defect 4

9 Warpage issue due to thermal stress Open soldering joints in the corners of a BGA Head and pillow effect Extreme WARPAGE

10 Reducing the risk of warpage effect By taking the following steps WARPAGE effects can be minimised: 1. Avoid steps in the reflow profiles. Each gradient change is negative. Linear profiles or intensely smoothened soak profiles are optimum. 2. Avoid high heating gradients even if this will reduce the throughput. 3. Ensure a Delta T which is as low as possible. 4. Ensure a uniform energy transfer. VP equipment is optimally suitable for this purpose. 5. The cooling gradients need to be significantly lower than users think. The solder joints must already be under liquidus. Otherwise enormous mechanical stress will be generated. 6. Ensure a uniform mass distribution in the design stage of the PCB.

11 Reducing the risk of warpage effect Avoid steps in your reflow profiles. Each gradient change is negative. Linear profiles or intensely smoothened soak profiles are optimum.

12 Reducing the risk of warpage effect The cooling gradients need to be significantly lower than users think. The solder joints are already under liquidus. Enormous mechanical stresses will be generated if cooling is to fast (negative shrinking).

13 Influence of Printing Process on Soldering Defect Rate A stencil design adjusted to the soldering conditions and the layout used as well as an accurate set-up of the stencil on the printer are prerequisites for low soldering defect rates. A poor print will inevitably lead to the known soldering defects. Solder beads (Exception: Superimposed paste having drawn moisture due to deliquescence or overaged paste) Pockets caused by air bubbles embedded during the pressure and population process (this is only a part of the possible causes of pockets) Tombstone effects Bridge formation In case of circuit board layouts to be newly created the application of design rules corresponding to the current state of the art is a prerequisite for a subsequent unproblematic manufacturing process.

14 The Tombstone Effect The causes for tombstones are to be found in: 1. inaccurate layout 2. a faulty stencil geometry 3. bad placement 4. bad surface quality In general the following will apply: The better (the more oxygen-depleted) the soldering method, the more frequently tombstones will occur in the presence of the above circumstances. The high surface tensions in the soldering meniscus may now cause the component to rotate upwards about one side of the component (rotational centre M) in case of a poor soldering joint geometry.

15 Tombstoning Fn Z DM1 = G* Y DM2 = Fn * Z DM3 = Fm * X D: pivot (DM = torque around pivot) F: force surface tension Y: small solder fillet X: large solder fillet If DM3 > (DM1+DM2) then Tombstone

16 Tombstoning Meniscus with less solder paste D X X F2 F1 Meniscus with much solder paste Under the same soldering conditions twice the torque will develop at the component due to the large amount of soldering paste. This effect leads to an increased tombstone formation. Torques acting on the component: Dm1=F1 x X Dm2=F2 x 2X: Legend: F1 = F2 Force, surface tension of the meniscus D = Pivotal point DM1 ; DM2 => Torque about the pivotal point D X= Lever arm on which the force F acts In case of a small paste volume x is included in the calculation of the torques singly, and in case of a high paste volume doubly.

17 Tombstoning Large pad with too much soldering paste for low-oxidation reflow processes.

18 Reduction of Tombstoning Pads cannot always be easily modified. What other measures are there to overcome Tombstoning? 1. A reduction of the amount of paste is imperative. Stencil thickness: from 100 up to 150 µm. A general reduction of 10 % in the stencil to compensate the printing offset (reduction of bridges and solder beads). Further reduction of the stencil aperture of 20 % up to 45 % in case of smaller components (< 08/05) or old pad design. Utilisation of a triangle or preferably stripe print. The stripe print offers a reduced initial wetting capacity since initially not the entire width of the component is wetted by tin. 2. Verify accurate placement, even small offset in x can cause tombstoning. 3. Profile optimisation to achieve a uniform heat distribution. 4. Use of non eutectic ( N2 ) solder paste like 62.5Sn/36.5Pb/1.0Ag. Decelerated melting is reducing forces and reducing tombstoning. Triangle print Comp. Stripe print Comp.

19 Reduction of Tombstoning Old pad design rules 0603 New pad design rules 0603 Cw Pw Cw Pw Pw = Cw + 20%Cw Ch Cm Pl 45 deg Rule: Pl = Ch + Cm mm Pw = Cw Ch Cm Pl 1/2 Rule: Pl = 1/2 * Ch + Cm mm Pl = Pad length Ch = Component height Cm = Component Metallization width Cw = Component width Pw = Pad width The smaller the pads, the lower the risk of tombstones!

20 Incorrect Pad and Stencil Design Board with much too large pads and an unadjusted stencil geometry. Tombstoning is a permanent problem.

21 Stencil Design Improvement (Part 1) Starting point was an old pad design with to big pads compared to the component metallisation. The effect was a to high solder paste volume. -> On every board a huge number of tombstones was produced.

22 Stencil Design Improvement (Part 2) STEP 1: Stencil aperture reduction minus 40%. -> Tombstoning rate immediately was going down minus 95%.

23 Stencil Design Improvement (Part 3) STEP 2: Stencil 10% more reduced. Now only 50% of the pad is printed with paste. -> It is now impossible to create tombstones, even if the temperature profile is increased over the limits. -> The wetting is still going up more then 75% of the vertical metallisation. The minimum recommendet vertical wetting according to the IPC is 30%. Source: Fa. Hekatron, Mr. Reinhold Stark, VP 2000 Dual lane.

24 Influence of the Paste Alloy on Tombstoning To examine the influence of the alloy on the tombstoning rate respectively 10 test boards respectively comprising 100 components were tested with different alloys. The reference is the tin lead alloy which has been known for years. Tombstoning rate in dependence on the alloys used. SnAgCu and SnPb pastes were tested at 240 C and 210 C in vapour phase soldering systems. A significant difference in the defect rates can be observed. A high SN content combined with a high AG content will result in drastically increased defect rates.

25 Influence of the Paste Alloy on Tombstoning The alloys tested were measured with respect to the resulting surface tensions by means of a wetting scale. The graphics, in turn, show a clear correlation between the surface tension and the tombstoning rate. A low surface tension will result in high tombstoning rates. Influence of the surface tension of SnAgCu alloys on the tombstoning rate. The following finding can be derived in the reverse conclusion : Pastes having an excellent wetting behaviour will lead to high defect rates in case of an unadjusted layout. Since the wetting capacity is significantly increased in low-oxidation soldering systems this does also explain the resulting higher soldering defect rates.

26 Influence of the Paste Alloy on Tombstoning The alloys were tested with respect to their melting behaviour by means of the DSC method. It is significant that alloys with a very narrow melting range tend to have high defect rates. Analysis of the melting ranges of the SAC alloys by means of DSC (Differential Scanning Calometry). Pastes with a wide melting range therefore cause a slower increase of the wetting capability. Therefore a better thermal equalisation is possible between the two sides of a component whereby the tendency towards tombstoning is minimised.

27 Summary Tombstoning Behaviour Stencil optimisation: Stencil thickness up to max mm. General reduction 10 % continuously to compensate a printing offset. In case of component sizes of 08/05 or less and an unadjusted layout 20 to 50 % additional reduction depending on the quality of the layout. Use of a stripe print for the reduction of the soldering paste volume. Use of a drainage print if a knife support is required. Paste optimisation: The use of suitable pastes having a pronounced melting range will significantly reduce the tombstone defect rate. Layout optimisation: It would be optimum to generally adjust the layout to the requirements of low-oxidation reflow processes.

28 Stencil Optimisation to Reduce Voiding In case of large surfaces to be printed the knife needs support for preventing it from sinking into the breakout of the stencil and to scoop out paste. This is often the reason for largely dimensioned voids. In case of falsely positioned supports large amounts of air are enclosed under the components after the assembly. These effects can be significantly minimised by a change of the stencil by which scooping is minimised. Air inclusion caused by scooped paste

29 Stencil Optimisation to Reduce Voiding Bad: Entirely plane print, dots, stripes, etc. (air inclusion after the assembly due to pressing the component into the paste. Soldering paste dots are pressed flat. In the centre between respectively four dots air will then be enclosed) Good: Cross print, also referred to as drainage print. Air inclusions escape laterally during the assembly Optimum: Crossprint rotated by 45. In this case, additionally, a clearly enhanced self centring behaviour is obtained Print bad Print unaceptable Crossprint good Crossprint 45 optimum

30 Popcorn Effect The destruction of components by the development of an excessive internal pressure is referred to as popcorn effect. The internal pressure is generated during the evaporation of moisture present in the component. Moisture occurs due to the hygroscopic behaviour of components (water is embedded in the atomic grid structure) or non-polymerised constituents of compounds. Said moisture is released when the components are heated, cannot escape during the brief soldering times and therefore generates the interior pressure. Frequently affected components : BGAs, large processors.

31 Popcorn Effect Possible Enhancements: Purchase of corresponding component qualities Adequate storage and drying of the components Keeping soldering times as short as possible (linear profiles) Selecting soldering temperatures which are as low as possible!the use of a vacuum step will not result in damages in components due to popcorning! Component before soldering at ambient temperature. Moisture inside of the component evaporates and blasts the component, bonding wires are destroyed.

32 Wicking Effect An ascend of the liquid solder on the component s leg preventing the formation of a solder joint with the pad is referred to as wicking effect. Reasons: Extreme mismatch of the wetting behaviour of circuit board and component metallization. If the wetting behaviour of the component is much better than on the pad surface the solder will only rise on the component. Remedy: Purchase of a corresponding material quality. Good solder joint. Solder forms connection between component leg and pad Wicking effect. Solder rises on the component s leg. No connection is established between component and pad

33 Experiences With Lead-free Components Frequently an excessive residual moisture in components > Cracking of the housings during the soldering process. Remedy: Tempering, preferably drying to produce no further oxidation. Quality of the metallization frequently not acceptable due to poor electroplating. Electrolytic aluminium capacitors and capacitor films are over-proportionally conspicuous. Reason: Excessively low boiling temperatures of the electrolytes used. Use of really lead free capable components required. Defect rates upon soldering are otherwise comparable to the values obtained to date. High peak temperatures cause component defect. Never cool components too rapidly. Risk: High mechanical stresses up to conductor track brakeage.

34 Solder Balling In case of solder balling solder splashes occur on the circuit board. Reasons: Excessive moisture absorption in the paste due to an unsuitable storage or an excessive air humidity during processing in connection with long lay times of the printed boards. Poor solder paste printing and/or poor stencil layout. Paste is printed onto the solder mask. Insufficient cleaning of the stencil bottom side before printing. Poor assembly. Insufficient activation of the paste or expired decay date (Optically the solder beads will then rather look like mist - consisting of finest granules).

35 Experiences with Lead-free Pastes Storage of the originally sealed containers in a refrigerator. See to a clean heat penetration before processing. Open containers may NEVER be kept in a refrigerator. Never mix residual amounts from old cans into new containers. Close cans immediately after use and store them at ambient temperature. Solder joints are less shiny -> readjustment of AOI systems may be required. Voiding rates are slightly higher than in case of lead-containing pastes. Printing behaviour is unproblematic. Soldering equipment is contaminated more rapidly. Cleaning of the circuit boards often more difficult since residues of new soldering flux formulations behave differently. Cleaners and/or process management have to be adjusted.

36 Experiences With Lead-free Circuit Boards Surfaces are partly still problematic. HAL and nickel gold are presently preferred. Drying of the solder mask is frequently poor -> Wetting problems due to residues. Poor electroplating by the circuit board manufacturer leads to soldering problems. Residual moisture in circuit boards is frequently too high. Drying is imperative. Preferably drying in vacuum driers due to low drying cycles. In case of a high residual moisture delaminating and extreme void formation will occur. Avoid vias under component connections in any case, even if capped. Storage of used packing units under controlled conditions (moisture and temperature). Gas release rates will increase enormously in case of poor qualities. Cleaning intervals of soldering equipment will partly decrease dramatically. In general a poor board quality is very often reason for significant failure rates.

37 Requirements Relating to a Repair Process Problem: Due to higher liquidus temperatures and readily oxidising solders the conventional repair methods can only be deployed on a limited scope. Defects such as delaminating, thermal damages of components and assemblies, mechanical stresses including the risk of, e.g., conductor track breakage are to be expected. Desoldered components are often not reusable. Problematic components such as large-area, metal shielded BGAs, power components, multiway connectors can no longer be desoldered. Even if the unsoldering process is successful a soldering is no longer possible since the permitted load limits of the assemblies (cumulative time over liquidus and oxidation) are exceeded.

38 Requirements Relating to a Repair Process Heating of the entire assembly often required for repair Repair under exclusion of oxygen Accumulated damage potential for 5 heating processes (production of sides A and B, wave soldering, unsoldering, re-soldering) has to remain within the permitted limits. Components should be reusable Simple handling of the repair equipment => The repair process in the vapour phase with the corresponding unsoldering and setting manipulators does satisfy all requirements. Disadvantage: Since the entire assembly is again subjected to reflow temperatures subsequently assembled, temperature-sensitive components have to be previously removed. => Deployment only adequate in case of expensive assemblies or direct after reflow soldering.

39 Thank you very much for your attention Reference: Ning-Cheng Lee Ph.D., Indium research center T. Taguchi, R. Katoh, O. Munekata, Y. Toyoda ASSCON Development Documentation Lecture Documentation European Electronic Assembly Summit 2008 (KIK on Board) Information Material AMKOR Hekatron, Mr. Reinhold Stark