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WL1801MOD: Optimization of solder process to reduce void content

Part Number: WL1801MOD

Hi team,

Can you have a look at this customer inquiry?

The module is a 2-sided SMD assembled 6-layer multilayer PCB. All vias in the area of the module were sealed airtight. (Filled and Capped according to IPC4761).
The component mix includes 0402 to 1206, 0.5mm pitch BGA, 0.35mm pitch QFN and other standard components.
Heraeus' F640AC SA30C5-89 M40 (lead-free, type 4) was used as solder paste. Both soldering processes were performed by reflow with the attached soldering profile under normal atmosphere (vacuum or nitrogen not possible). The modules were assembled automatically (Mycronic MY200, assembly force is set "low" = <3.5 N). The side of the problematic W-LAN module was processed MSL-compliant in the 2nd process run. The soldering profile used on 29.09 + 30.09 has already been optimized in cooperation with Stannol/Heraeus.

The following results have been achieved in the previous test series:

- Processing on 29.09.2020 with 100µm stencil, lead-free reflow soldered
Result: no short circuits, strong void formation (see X-ray images of 29.09.2020)
The other components on the assembly do not show any abnormally increased void content or other abnormalities.

- Processing on 30.09.2020 with 120µm stencil, lead-free reflow soldering, The pad sizes for the stencil were made according to the TI data sheet
Result: short circuits due to undefined tin migration, void formation (see x-rays from 30.09.2020)

An electrical test of the complete assembly is unfortunately not possible in-house. An adaptation of the printed circuit board design is "hardly" possible anymore, because the product
is already at the end of the product life cycle and several hundred PCBs are in stock. X-ray as an additional inspection method can only be performed by an external service provider.

We ask for support in optimizing our process with regard to reducing the void content in the solder joints and preventing short circuits.

Thank you,

Franz