miniaturising IC packages

Die stacking and miniaturising with Die attach films

With the advance­ments on small pack­ages and mobile devices, 2.5D and 3D pack­ages have become more and more pop­u­lar. As a result we are see­ing more stacked devices, thin­ner wafers and thin­ner dies than ever.

The pack­age trends are towards tight D/P ratios, thin­ner pack­ages that are less than 0.30mm and wafers that are thin­ner than 75um. These devices are meant for high den­si­ty and SiP pack­ages that are lead­ing the way for greater effi­cien­cy in low volt­age applications.

The die attach mar­ket has a lot of dif­fer­ent com­pet­ing tech­nolo­gies. Whether it is die attach pastes, die attach films, sol­ders, sil­ver glass or (semi)sintering pastes, they all fight the same turf war with some advanc­ing more in some fronts than others.

Die attach materials

Die attach pastes are main­ly used for dis­cretes, ICs and MEMS. We use this tried and proven tech­nol­o­gy for low or medi­um pow­er dis­si­pa­tion on small/medium die sizes. This slow, self fil­let­ing solu­tion requires dis­pens­ing and great accu­ra­cy when plac­ing the die, in order to avoid tilt.

Die attach films are for medi­um and large die sizes with the same, medi­um amount of pow­er dis­si­pa­tion. These prod­ucts are main­ly des­tined for mem­o­ry die stack­ing, MEMS and thin dies. They come in con­duc­tive and non con­duc­tive ver­sions that we lam­i­nate on the wafer level.

Final­ly semi-sin­ter­ing, the up and com­ing “trend­ing” tech­nol­o­gy is used for high pow­er dis­si­pa­tion dis­cretes and pow­er modules.

Hon­or­able men­tion to sol­der (nor­mal and eutec­tic) that is in prin­ci­ple used for dis­crete appli­ca­tions and the niche sil­ver glass options that are used for high pow­er mil­i­tary and aero­space appli­ca­tions regard­less of die size.

All of these options have their pros and cons. Suit­able for dif­fer­ent die sizes, warpage tol­er­ances, adhe­sion to var­i­ous sub­strates, con­duc­tiv­i­ty (or lack there­of) and con­trol­ling the flow. 

Why do we want to control the flow?

Minia­tur­i­sa­tion, our sub­ject mat­ter, is the lead­ing fac­tor for con­trol­ling the flow. With small­er and tighter devices it’s eas­i­er to con­t­a­m­i­nate adja­cent wire­bond pads if the adhe­sive bleeds or over­flows on the board. This same issue can hap­pen while die stack­ing and can over­flow and cov­er the chip’s bond­ing pads. Warpage, bond line thick­ness and of course die tilt can also neg­a­tive­ly affect die stacking.

Con­trolled flow plays an impor­tant role when work­ing with thin devices. There might be issues with the fil­let height, the die cov­er­age and suck off induced warpage can poten­tial­ly inter­fere with any stack­ing attempts. Gen­er­al­ly, incon­sis­tent fil­lets reduce the reli­a­bil­i­ty for all the afore­men­tioned reasons.

In prac­tice, con­trol­ling the flow means min­i­mal fil­let size. Or in more sim­ple terms, zero adhe­sive over­flow on the side of the die. In the end, this has three major advantages:

  • Enables the use of thin die,
  • Allows us to use the same die with a small­er footprint, 
  • We can use a larg­er die in the same foot­print as before. 

It gen­er­al­ly saves space that we utilise how­ev­er we deem nec­es­sary. More sil­i­con, more flex­i­bil­i­ty and space for oth­er packages.

Die attach films

Die attach films are the pri­ma­ry solu­tion to con­trol the uni­for­mi­ty of the bond­line thick­ness and to essen­tial­ly elim­i­nate the fil­lets. They use sol­id resins and exhib­it min­i­mal squeeze out and min­i­mal out­gassing upon cure. The BLT remains sta­ble before and after cure and they adhere great across mul­ti­ple sur­faces, mak­ing them a great solu­tion to the dis­pens­ing challenges.

Clas­sic solu­tions all have issues with advanced pack­ag­ing since con­trol­ling the flow is almost impos­si­ble. That’s why films are prac­ti­cal­ly the best way to stack dies and to form pack­ages such as memories.Those con­duc­tive and non con­duc­tive films are applied on the wafer and ensure per­fect fil­lets and uniformity. 

The main appli­ca­tion steps are sim­i­lar for both dic­ing die attach and con­duc­tive (also dic­ing) die attach films:

  1. Lam­i­nate the die attach film on the wafer
  2. Dice the wafer
  3. Remove the dic­ing tape with either UV or PSA peel
  4. Pick up the dies
  5. Check the dies
  6. Place the dies
  7. Cure
  8. ?????
  9. Prof­it

Those films come in either 8″ or 12″ cir­cles that per­fect­ly align with the stan­dard wafer sizes and can be cured or skip cured. 

The typ­i­cal adhe­sive thick­ness starts from 10um but it can also come in thick­er ver­sions. Alter­na­tive­ly mul­ti­ple films can be stacked on top of each oth­er to achieve your desired BLT. There’s no dis­pens­ing, no pres­sure and gen­er­al­ly, from a process point of view, it is a much clean­er process. Final­ly, the dic­ing tape comes with either a UV release or just a pres­sure sen­si­tive adhesive.

How can we help you?

We offer a wide range of Con­duc­tive and non con­duc­tive die attach films such as the indus­try lead­ing, con­duc­tive, low warpage, uni­ver­sal film CDF625P that cov­ers a wide die size range up to 10x10mm. In addi­tion we offer, among oth­ers the non con­duc­tive dic­ing die attach film ATB120U that is used for chip on chip process­es or even glass attach and absorbs the CTE mis­match between substrates.

As men­tioned before, con­trol­ling the flow, attach­ing the die and opti­mis­ing a pack­age is not lim­it­ed to one prod­uct cat­e­go­ry. We offer die attach pastes, semi sin­ter­ing pastes, wafer back­side coat­ings and B‑stageable adhe­sives that can all be used for the same purpose. 

Con­tact us and we will be hap­py to aid you in choos­ing the right die attach method and prod­uct for your application.

About George Kountardas

George is a Jack of all trades with an unappeasable inquiring mind. Obsessed with new products and technologies, he is always pushing forward for better, faster and more efficient applications. Always learning something new.

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