It is no secret in the electronics industry that there is an ongoing trend to ever more miniaturised and advanced IC packages which also feature higher performance and provide even more advantages.
One of these distinctive inventions is the plastic-packed QFN (quad flat no-leads) IC, providing a variety of application benefits, especially in the highest frequency/microwave range important for electronics devices such as handheld portable applications (cellphones, PDAs, etc) or any other application where size, weight and performance are issues.
As long as the assembly process is delivered by automated equipment, the reliability of boards populated with QFNs is not impaired. However, as there is no absolutely defect-free production despite all serious efforts, how can defect board assemblies be successfully repaired? Rework specialist company Martin can provide two answers to this question: hot-print prebumping as an effective low-cost concept, and transfer prebump for higher throughput requirements.
In SMT board assembly, the trend goes from the application of SO (small outline) components to the QFN-type packages. We can understand (see basic information on the IC in the sidebar) that the QFN is a fragile, small and leadless component which places very high requirements on board rework, ie, the exchange of such small and delicate semiconductors when they fail. Unlike other SMDs (surface mount devices), QFNs do not feature an array of pad or ball-type terminals provided with a relatively large deposit or amount of solder paste for their assembly.
In case of exchange, the other SMDs typically leave a fairly large amount of solder on the board for the reflow of the new component. However, the QFN parts do not come with pads carrying solder but need first to be provided with solder paste and can only then be soldered to the board assembly with their small pads. Also, the desoldered QFN does not leave enough remaining solder on the PCB for the subsequent reflow procedure of the exchange component. From all these observations it becomes clear that this component places significantly higher requirements on processing and handling in the rework stage of the board than the other SMDs used up until now.
When we regard common hand soldering, this part is also not recommended for this relatively rough procedure due to its delicate design. The perimeter pads at the four IC sides are embedded in the package plastic moulding and if subjected to a typical large soldering tip with much more than 300°C, there is extremely high risk of damage to the plastic package (also by the tip pressure) and the pad seating in the moulded material or even of the die, and consequently to the wire bonds. Eventual delamination of the PCB copper pads is even possible. Numerous defects were experienced and reported in some Asian manufacturing sites by the use of soldering irons.
Just three reflow procedures allowed
We can see from these guidelines and experiences that for the QFN application there is no true control with manual iron rework soldering, which turns out to be widely unreliable and also does not comply with worldwide accepted IPC 610/JEDEC 001 standards. The exchange of QFNs in the repair stage can only be successfully completed with automated and controlled rework process steps according to the specifications of the established board soldering procedure with its tested and preset profile which should not reach more than 260°C. As the soldering process should not be supplied with soldering irons, the use of rework stations delivering a tightly controlled temperature profile (ideally with hot gas) is highly recommended by semiconductor manufacturers.
It must be borne in mind that semiconductor manufacturers specify the QFN component to be exposed to only three reflow cycles. A QFN removed during PCB rework should not be re-used for subsequent assembly. The standard component level qualifications for packages/components specify the survivability to just three solder reflow actions. A package that has been attached to a PCB and then removed has seen two single reflow operations, and if the PCB is double sided, the package has already experienced three reflows processes. Thus the IC is at the end of the tested and qualified span of known survivability and reliable usability. The removed parts should therefore be properly disposed of so that they will not mix with fresh components.
Hotprint prebumping is cost-effective
The new Martin Prebump set 03.1, on which patented Hotprint technology is based, is especially designed for solder paste supply to a small quantity of fresh QFNs placed on a board for the first time. Hotprint delivers the prebumping of QFNs in high quality and reliability down to fine-pitch structures of 0,4 mm, to device dimensions of up to 14 x 14 mm and for an IC pincount of a maximum of 200 pads. Previously, reliable prebumping was only possible down to 0,5 mm pitch structures.
Here, a mini-stencil featuring the appropriate apertures for the QFN is placed and slightly forced on top of the component in a mini-stencil holder, called the Hotprinter. After this step, the paste is supplied and spread across the mini-stencil manually with a small blade or squeegee. In contrast to other similar methods, the stencil will not be removed in the Hotprint process after paste print, but rests on the QFN during reflow. This concept delivers the most effective solution above all other prebumping methods at very low costs with high accuracy. The handling is very easy and can be done by normal staff in electronics assembly lines. Martin supplies stencils for QFNs and frames.
When we look into the offerings from the market we can see that mini-printer, another likely solution, cannot reliably print paste down to structures under 1 mm. Also, the automated lift-off of the mini stencil after reflow in the Hotprinter prebump mechanism is more accurate and maintains higher precision than the manual release seen in other solutions.
The QFN package type has virtually no standoff height, and only a minimal spacing of 50 to 70 microns between the PCB and the component is left. Therefore, post reflow cleaning is difficult. It is recommended to use a no-clean solder paste.
And due to this mini-mum spacing, a solder joint inspection after reflow soldering from the component side also cannot be provided in order to verify the process quality. Conventional visual inspection of the hidden solder joints beneath the body is not possible. Only with costly and time consuming X-ray inspection (AXI) can solder joint verification be completed, but it needs the expertise of a trained inspection operator to evaluate the results. Therefore, for QFNs it is mandatory to deliver a reliable rework process without any flaws, otherwise production costs will grow remarkably due to the investment in sophisticated inspection instruments.
The Martin Prebump-03.1 mechanism, the Hotprinter, is mounted in the Hot-Reball-03 controller unit with compact dimensions of 140 x 290 x 83 mm and a heating power of 50 to 500 W for process temperatures from 210°C to 260°C. A K-type sensor precisely controls the temperature, and programs for eleven different applications are already prepared. A typical prebump procedure takes about one minute. It is noteworthy that the same controller unit can also accept the Martin Reball-03.1 mechanism. This equipment is designed for the reballing of BGAs and CSPs with a pitch of 1,27 mm. With this rework equipment concept, users can kill two birds with one stone. It is a highly cost-effective solution for manufacturing companies with low demand for board rework.
Transfer prebump of multiple components
The other method is the transfer prebumping developed by Martin just a few months ago. It is a fast and accurate method delivering excellent results also down to 0,4 mm pitch (even 0,35 mm and 0,30 mm are possible) which is based on the use of the Rework Dispenser 05.6 equipment. The innovative and practical process presents the solution in just three steps. First, the dispenser deposits the small dots for the QFN structures on a special ceramic transfer plate. The dots can be as small as 0,013 mm in diameter with a negligible tolerance of ±3% – up to nine QFN structures can be dispensed in one stroke. The ceramic plate functions as the transfer surface of the mirrored dispensed QFN pad array.
In the second step, all the solder paste structures dispensed will be perfectly reflowed to the pads of the previously placed QFNs with the Martin Expert 09 in one run, according to the tested and programmed temperature profile for these components. Then, in the third step, the prebumped QFNs can be used in rework processes easily and reliably. In contrast to the Hotprint method described above, the transfer technique delivers a significantly higher throughput. But due to the required equipment it demands a higher investment. From this point of view it is a technique ideal for small regular production volumes or higher demand in board rework numbers.
Conclusion
With the new and patented QFN prebumping technologies developed by the experts of Martin, fine-pitch structures down to 0,4 mm can be reliably handled, and even 0,35/0,30 mm are possible. Where the Hotprint method is focused on a lower rework demand, the transfer prebumping with its higher throughput is the answer to the requirements of a greater number of boards. In contrast to other solutions on the market, these prebumping methods are more accurate and therefore the results are excellent and reliable. No inspection equipment is necessary for verification of the results.
The QFN and its design and application advantages
The QFN (quad flat no-leads) or from other companies called MLF (micro lead frame) is an often employed integrated circuit (IC) package to be placed on SMT (surface mount technology) board assemblies. This kind of package is similar to the quad flat package (QFP), but in contrast to this, the leads do not protrude from the sides of the package. This is a near-CSP (chip scale plastic) encapsulated IC made up with a planar copper lead frame and wire-bonded I/Os.
The perimeter lands (tiny pads on all four sides) on the bottom of the package deliver the electrical contacts to the other board circuitry. Some versions of the package feature an uncovered thermal pad which supports thermal enhancement by having the die exposed to the PCB surface, providing a better heat transfer path when soldered directly to the PCB. A direct heat dissipation path is achieved by via holes to the electrically appropriate thermal PCB pad, so the silicon die is discharged from excess heat and thus the thermal performance is increased. This heatsink design improvement also enables stable ground by the use of down bonds or by electrical connection through a conductive die-attach material.
In principle, the body design consists of the silicon die, an attached thermal lead pad, a lead frame made of copper, and a thermally conductive adhesive is used for attaching the silicon die to the exposed heat pad. The I/Os (four up to a maximum of 164) of the die are attached to the frame by thin gold wire bonding technology. To date, there are two general types of QFN packages in use: an air-cavity QFN featuring air within the package, and a fully encapsulated plastic-moulded QFN with minimal air voids and no cavity within the package. Each package type has its own benefits.
Due to its inner structure, the fully plastic-moulded QFN is typically limited to RF applications up to about 3 GHz. It is usually composed of just two single elements – a plastic compound and a copper lead frame – and does not need a lid. In contrast, the air-cavity QFN is usually made up of three elements: a copper lead frame, a plastic-moulded body (open, not sealed) and either a ceramic or plastic lid. This version is usually more expensive due to its design, and can be used for highest frequency/microwave applications up to about 25 GHz.
In general, the QFN component offers a variety of benefits including reduced lead inductance, a remarkably small outline, very thin profile and low weight. Maximum height is from 0,6 to 1,5 mm and body size from 1 to 12 mm. The results are significant: thin profile and superior die-to-body size ratio. Since it uses perimeter I/O pads, the PCB trace routing is made easy, and the exposed copper die-pad technology offers good thermal and electrical performance. However, the part comes with a small lead-free solder deposit just sufficient to place and reflow solder it once; for rework and repair, fresh solder alloy is needed. Its engineering and economical features make the QFN an ideal choice for many applications where size, weight and thermal and electrical performance matter. And in relation to CSP, for example, pricing is a benefit. Multichip package and stacked die configurations can also be designed based on this IC technology.
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