Frequently Asked Questions

  

Assembling EPC Lead Free eGaN® FETs

The following pertain to items specific to the EPC Lead Free eGaN® FETs. For general items related to the assembly of EPC eGaN® FETs, please see Assembling eGaN FETs .

What is the PbF solder bump composition?

EPC uses SAC405 (95.5/4.0/0.5 Sn/Ag/Cu) or 97.5/2.5 Sn/Ag for our PbF solder bumps.

What is the under bump metal (UBM) of the lead free (PbF) solder bump?

The under bump metal layers can be Ti / Ni-Vd / Cu with SAC405 solder bumps or Ti / Cu / Ni with SN97 solder bumps.

What tacky flux can be used for assembling the EPC lead free (PbF) die?

EPC’s eGaN® FETs can be mounted directly onto PC boards without added solder by using a tacky flux to hold the part in place while reflowing the solder. An acceptable PbF process also uses Kester TSF6502 no-rinse flux. This is the same tacky flux that was recommended for the leaded bump parts. However, the use of TSF6502 flux on PbF bumps requires a different reflow profile : Please note that this recommended profile used on the EPC PbF parts shows a peak temperature of 232oC.



Note : Kester recommends the use of TSF-6592LV for a PbF processes. This flux is reflow-able with peak temperatures up to 270oC. (http://www.kester.com ; See Products/ChipAttachFlux/TackyFluxes TSF6502 and TSF-6592-LV)

What lead free (PbF) solder pastes has EPC used?

To date, EPC has used the following solder systems during reliability testing, AIM Solder NC257-2 SAC305 Lead Free No Clean Solder Paste and SMIC Senju Metal Industry Co. Eco Solder M705-GRN360 K-V-Series.

Below are the reflow profiles used for the solder pastes EPC has used.  Please use the vendor recommended reflow guidelines for the solder paste you are using.

AIM Solder NC257-2 SAC305 Lead Free No Clean Solder Paste Temperature profile used :Short profile for low density boards per vendor datasheet.

The following is from the AIM Solder NC257-2 SAC305 Lead Free No Clean Solder Paste 12/10 Rev15 datasheet:

Note: AIM Solder NC257-2 SAC305 Lead Free No Clean Solder Paste 12/10 Rev15; http://www.aimsolder.com

The following is from the SMIC Senju Metal Industry Co. Eco Solder M705-GRN360 K-V-Series Temperature profile used per vendor datasheet

Note: SMIC Senju Metal Industry Co. Eco Solder M705-GRN360 K-V-Series specification sheet TC-P40-3 4F May 2004 “Spec M705-GRN360-K2V.pdf’ ; http://www.senju-m.co.jp/en/product/ecosolder

What does EPC recommend for solder flux being used in assembling lead free (PbF) eGaN® FETs?

EPC recommends the use of no clean flux solders. However, EPC recommends that the no clean flux be cleaned off and removed from the PbF devices. For more information, please see our assembly application note: http://epc-co.com/epc/documents/product-training/Appnote_GaNassembly.pdf

Can a water rinseable solder flux be used in assembling lead free (PbF) eGaN® FETs?

At this time, EPC does not recommend using a water rinseable solder flux for the PbF devices.

What Under fill has EPC used on lead free (PbF) eGaN® FETs?

At this time, EPC does not require that underfill be used, but Shin-Etsu SMC375X7 has been used successfully up to 125oC in reliability testing.

Can a via be placed directly under a bump?

Yes, a via can be placed directly under a bump, but care must be taken to insure that the via is completely filled in order to avoid absorbing any solder from the bumps during assembly. A non-filled via can cause the die to tilt or not connect the bump to the pad properly. The recommended via design is a micro-via with a 6mil hole diameter and an annular ring diameter not to exceed the width of the pad and MUST be filled with either a non-conductive or conductive filler. For general tips on device layout on a PCB, go to Optimzing PCB Layout.

EPC2001 / EPC2015 Die Outline Solder Bar View.

Die Side View (units in um)

EPC2001 / EPC2015 Recommended Solder Mask and Land Pattern (units in um)

The land pattern is solder mask defined. This helps in reducing bump deformation which can cause die tilt.

The solder mask is 10um smaller per side than the bump.

FIDUCIALS – Depending on the accuracy of the pick and place equipment, it may be necessary to have local fiducials. Local fiducials are typically placed diagonally outside of the die.

Alignment legends - can be used as visual indicators for good placement of the die on PCB. Such alignment legends can be copper features on the top layer, or solder mask features, or silkscreen features.

STENCIL DESIGN – The recommended stencil aperture design is shown below. The rectangular shaped apertures are larger than the land pads. The corners of the apertures are rounded at radius of 60 μm. The recom¬mended stencil foil thickness is 100 μm.

EPC2001 / EPC2015 Recommended Stencil Aperture Design (units in um)

References:

1. A. Nakata, E. Abdoulin, J.Cao and Y. Ma, "Assembling eGaN FETs", http://epc-co.com/epc/documents/product-training/Appnote_GaNassembly.pdf

2. AIM Solder NC257-2 SAC305 Lead Free No Clean Solder Paste 12/10 Rev15; http://www.aimsolder.com

3. SMIC Senju Metal Industry Co. Eco Solder M705-GRN360 K-V-Series specification sheet TC-P40-3 4F May 2004 “Spec M705-GRN360-K2V.pdf’ ; http://www.senju-m.co.jp/en/product/ecosolder

4. http://www.kester.com ; See Products/ChipAttachFlux/TackyFluxes TSF6502 and TSF-6592-LV

5. http://www.shinetsu.co.jp/encap-mat/e/product/k_s/smc/index.html Shin-Etsu SMC375X7