Milano, 28-29 Maggio 2013 Centro Congressi Milanofiori Electronics Day Automotive Roberto Tiziani Powertrain Technologies Manager Magneti Marelli PCB design for reliability on power components Organizzato da
PCB Design for reliability on power components". " Name R.Tiziani R&D PowerTrain 20 Novembre, 2010
PCB Design for SMT power components Surface Mount Technology took over the mass production of many automotive designs" TH is limited to some big capacitors or connectors" Power devices are housed in special packages with massive copper slug or exposed pad which are intended to dissipate the heat by PCB" Slug and exposed pad are soldered to PCB copper area " PCB bottom surfaces are in thermal contact with external metal case that is dissipating by natural or forced air or in some case to the chassis by means of screws"
Typical Assembly Structure Heat Source Power Component Copper Sluig Solder Paste Printed Circuit Board Thermal Interface Material Aluminium alloy shell case Ambient sink Common Packaging of ECU
Rth(junction-to-housing) PCB layout" Vias layout" Solder pattern" TIM type" Tj Rth(j-housing) Tcase
Rth(junction-to-housing) improvement TIM type" Thermal Pad for dry thermal contact" Adhesive on one side" Sized according to the layout" Cheap for low thermal conductivity (1W/mC)" Thermal Grease or Gap Filler" Paste dispensed by CNC" Dispensing pattern by CAD" High thermal conductivity > 2 W/mC" Wet Contact, low thermal contact resistance"
PCB layout for Power Components Printed Circuit Board has low thermal conductivity, 0.3 W/mC" To improve the thermal transfer to bottom side, vias are designed in the solder pad" Solder pad has arrays of vias and solder lands for package heat sink attachment"
PCB layout for Power Components Component Placement on PCB can push out the solder paste into vias" Paste definition by stencil Paste after component placement
PCB layout for Power Components After Reflow, solder is filling the adiacent vias randomly" Solder bump can happen on bottom side"
PCB layout for Power Components Vias Constrains for lead free finishing" Most used finishing for PCB lead free is pure tin electroless" Solder resist viscosity is not enough to cover the vias top/ bottom for the most used drilled vias 0.4-0.6 mm diameter" To avoid the risk of tin chemicals residuals entrapped in the vias and not removed by final rinsing, vias must be open" Solder resist definition
PCB layout for Power Components Vias Solder leakage and bumps examples "
PCB layout for Power Components Vias Solder leakage and bumps examples " Solder bumps on bottom side Cross section of solder bumps
PCB Layout for Power Components Solder paste cause voids in the attach layer"
PCB Layout for Power Components Voids in solder attach increase the Rth(j-housing)" Impact of solder voids is done by thermal modelling" Detailed Package-PCB-Vias model with FlowTherm software has been simulated on PwSO-20 case" Impact of solder voids on Rth(j-to-housing) is given" Distribuited voids" Concentrated voids"
Materials and Structure Thermal Conductivity ( W/mK)" Si : 135" Solder SAC: 53" Solder Die Attach PbSn: 25" Cu: 390" Thermal Grease : 2.3" Aluminum: 210 " PCB: 0.3" Stack layers (from top)" Silicon Die 4x4x0.22 mm " Solder die attach ( PbSnAg 97.5/2/2.5, 30 um )" Copper slug, 1.25 mm thick" Solder paste (SAC, 80 um )" PCB 1.6 mm" Thermal Grease layer 0.175 mm" Al case, 1.5 mm thick"
Rth(junction-to-housing) modelling by Flotherm Tj Rth(j-housing) Tcase=0
Rth(j-housing) vs pad layout Rth (j-housing)" Effective PCB solder area" Std Layout" Vias 0.6 mm" 3.5 C/W" 54 mm 2"
Rth(j-housing) vs solder voids: Case 1: distribuited voids Rth(j-housing) C/W 4,5 4 3,5 3 4,37 4 3,71 3,5 3,51 3,54 0 10 20 30 40 50 60 70 80 90 100 Voids percentage (%) std layout
Rth(j-housing) vs solder voids: Case 2: concentrated voids Rth(j-housing) C/W 7,5 7 6,5 6 5,5 5 4,5 4 3,5 3 2,5 2 3,5 1 2 3 3,9 4,62 7,18 Central Slot n.2 missing : 12 mm 2 solder voids area Lateral slots n.1, n.3 missing : 24 mm 2 solder voids area n.1,n.2,n.3 slots missing : 36 mm 2 solder voids area 0 5 10 15 20 25 30 35 40 voids area ( mm2)
PCB Layout for Power Components How to overcome to the assembly issues of solder voids under power components and solder leakage?" New layout concepts are coming based on plugged vias" Low cost plugging are becoming available based on dedicated ink by screen printing"
IPC 4761 specification for vias filling/plugging 7 types of vias filling are listed" Most common is the type VI"
Vias Plugging Scheme
PCB Layout for Power Components Vias plugging allows to make free layout under power components using normal drilling design rules" Matrix design is the most common used"
Rth(j-housing) vs pad layout Std Layout" Vias 0.6 mm" Array" Vias 0.4 mm" Rth (j-housing)" 3.5 C/W" 2.4 C/W" Effective PCB solder area" 54 mm 2" 58 mm 2"
Rth(j-housing) vs solder voids: Case 1: distribuited voids Rth(j-housing) C/W 4,5 4 3,5 3 2,5 2 3,5 3,51 3,54 3,71 2,4 2,41 2,44 2,5 4 4,37 2,89 2,67 0 10 20 30 40 50 60 70 80 90 100 Voids percentage (%) std layout matrix vias
Rth(j-housing) vs solder voids: Case 2: concentrated voids Rth(j-housing) C/W 7,5 7 6,5 6 5,5 5 4,5 4 3,5 3 2,5 2 3,5 2,4 3,9 2,9 4,62 3,07 7,18 3,24 std layout matrix vias 0 5 10 15 20 25 30 35 40 voids area ( mm2)
Rth(j-housing) vs solder voids: Case 2: concentrated voids LEGENDA Central Slot n.2 missing : 12 mm 2 solder voids area Lateral slots n.1, n.3 missing : 24 mm 2 solder voids area n.1,n.2,n.3 slots missing : 36 mm 2 solder voids area 1 2 3 Central Slot n.2 missing : 12 mm 2 solder voids area Lateral slots n.1, n.3 missing : 24 mm 2 solder voids area n.1,n.2,n.3 slots missing : 36 mm 2 solder voids area 1 2 3
Matrix plugged vias Solder voids are reduced by means of dedicated stencil design and better flux outgassing"
Conclusions PCB layout for power components with matrix plugged vias can improved the reliability as:" Better thermal resistance" Tj is improving" Solder voids under power components are strongly reduced" More stable thermal resistance vs solder voids" No solder leakage in the vias"