Introduction to Ball Grid Array Technology

All text Copyright Bob Willis, EPS

Ball Grid Array (BGA) Technology is certainly starting to capture the imagination of both design engineers and process engineering staff. Based on the published assembly yield data of <10PPM defect levels it is now the process engineers that are pushing design departments for its introduction and not the other way round. 

Inevitably the suppliers of materials and equipment are also starting to inundate the market with BGA related products, “its surface mount all over again”. Over the last six months I have conducted ten ball grid array workshops in Europe & America for both suppliers and potential users of the technology. In most cases design issues are at the top of every ones agenda.

Existing footprint design used by many companies consists of a combination of via, track and 0.020”-0.025” surface pads. The solder resist is positioned on the pad surface and used to minimise movement and paste flow during reflow. This simple layout allows tracking to take place on both board surfaces. Test access may be achieved on the underside of the board through via hole connection from the topside of the board or to selected test pads.

If just surface tracking is required then the via is omitted from the design foot print. It is recommended however that standard design rules be used for pads with a solder mask left free of the pad surface with the same 0.020”-0.025” pad. It generally accepted that resist on the pad will reduce the surface contact with the stencil during paste printing and may lead to poor printing. 

A resist tolerance of 0.003” around the pad may be extended along the track. This allows the track to wet with solder paste during reflow which provides a different shape to the ball termination during x-ray inspection. One problem with inspection is that a poorly wetted joint still looks like a solid round ball after reflow. The projection from the round fillet can confirm that the solder has wet the track and that all terminations have reflowed properly. During x-ray inspection the joint area from above will look like a frying pan with a short handle.

BGA devices may be soldered directly to pads with via holes in the centre provide the via is the correct size. The via pad should also be reduced to provide the correct stand off after soldering. This technique may require higher degrees of placement accuracy due to the reduction in pad size. With the parts on a 0.050" pitch this should not be a problem with modern assembly equipment.

Direct via attachment provides benefits to the design engineer in increasing the opportunity to rout surface tracking and internal connections. It can be a major benefit with larger BGA devices. Theoretically the method may also provide better heat dissipation from the device to the board. This is due to the direct contact with the solder slug in the board.

The use of direct via attachment may also allow the possibility of blind vias to be used as a surface pad. Again this gives the possibility of increased routing and simplifies the operation. One potential problem with blind via mounting is outgassing. During paste reflow the volatile parts of the paste flux may slump into the blind via and gas back producing voiding.

If direct soldering of BGAs to via holes is considered during design it may also provide benefits to the rework department. If traditional vias are used which extend through the board direct contact may be made with a soldering iron. This will allow the solder in the via to be reflowed allowing rework of individual ball locations under the BGA. This is not possible with any traditional pad design.

If the via in pad approach is taken then a solder fountain could also be used to reflow BGA parts during rework provided complete access is available underneath the part. A solder fountain is traditionally used for conventional through hole parts. The system may be used for both removal and replacement. The solder resist on the board would also need to be left clear of the via to allow direct solder contact.

Limited trials have been conducted on the use of direct via connection of BGA parts. The pads were 0.030" with a 0.020" via hole.. The trials consisted of solder paste stencil printing using 0.006" stencil. After placement and reflow the board was microsectioned to examine the joints in each case the via completely filled with solder. The solder balls formed joints which were only slightly smaller than the joints obtained using the standard pads.

Via hole mounting of BGA parts is a new consideration and only currently used by one company in America. The company has only used the design technique with 90/10 high temperature lead/tin balls. This has prevented the ball collapsing into the via hole. As this a comparatively new technique trials should be undertaken along with reliability testing.

Positioning corner marks on each BGA edge is necessary during design. Each of the three marks are positioned 0.010” apart which allows any misplacement of the device to be assessed visually. The marks are etched into the copper on the surface of the board so that accurate alignment is maintained with the pad artwork. This form of mark or a single mark needs to be included on any board to aid inspection for misplacement, alignment during hand placement or during rework. See fig 3

If no mark is included in the design more sophisticated placement and rework machines are required to reposition the parts on the board. If not included manual assembly is impossible.

For further information on EPS services Email Bob Willis or visit his Internet Home Page.

Email: eps@bobwillis.co.uk
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